diff options
Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perlfunc.pod')
-rw-r--r-- | Master/tlpkg/tlperl/lib/pods/perlfunc.pod | 9088 |
1 files changed, 0 insertions, 9088 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perlfunc.pod b/Master/tlpkg/tlperl/lib/pods/perlfunc.pod deleted file mode 100644 index c9307f342fc..00000000000 --- a/Master/tlpkg/tlperl/lib/pods/perlfunc.pod +++ /dev/null @@ -1,9088 +0,0 @@ -=head1 NAME -X<function> - -perlfunc - Perl builtin functions - -=head1 DESCRIPTION - -The functions in this section can serve as terms in an expression. -They fall into two major categories: list operators and named unary -operators. These differ in their precedence relationship with a -following comma. (See the precedence table in L<perlop>.) List -operators take more than one argument, while unary operators can never -take more than one argument. Thus, a comma terminates the argument of -a unary operator, but merely separates the arguments of a list -operator. A unary operator generally provides scalar context to its -argument, while a list operator may provide either scalar or list -contexts for its arguments. If it does both, scalar arguments -come first and list argument follow, and there can only ever -be one such list argument. For instance, splice() has three scalar -arguments followed by a list, whereas gethostbyname() has four scalar -arguments. - -In the syntax descriptions that follow, list operators that expect a -list (and provide list context for elements of the list) are shown -with LIST as an argument. Such a list may consist of any combination -of scalar arguments or list values; the list values will be included -in the list as if each individual element were interpolated at that -point in the list, forming a longer single-dimensional list value. -Commas should separate literal elements of the LIST. - -Any function in the list below may be used either with or without -parentheses around its arguments. (The syntax descriptions omit the -parentheses.) If you use parentheses, the simple but occasionally -surprising rule is this: It I<looks> like a function, therefore it I<is> a -function, and precedence doesn't matter. Otherwise it's a list -operator or unary operator, and precedence does matter. Whitespace -between the function and left parenthesis doesn't count, so sometimes -you need to be careful: - - print 1+2+4; # Prints 7. - print(1+2) + 4; # Prints 3. - print (1+2)+4; # Also prints 3! - print +(1+2)+4; # Prints 7. - print ((1+2)+4); # Prints 7. - -If you run Perl with the B<-w> switch it can warn you about this. For -example, the third line above produces: - - print (...) interpreted as function at - line 1. - Useless use of integer addition in void context at - line 1. - -A few functions take no arguments at all, and therefore work as neither -unary nor list operators. These include such functions as C<time> -and C<endpwent>. For example, C<time+86_400> always means -C<time() + 86_400>. - -For functions that can be used in either a scalar or list context, -nonabortive failure is generally indicated in scalar context by -returning the undefined value, and in list context by returning the -empty list. - -Remember the following important rule: There is B<no rule> that relates -the behavior of an expression in list context to its behavior in scalar -context, or vice versa. It might do two totally different things. -Each operator and function decides which sort of value would be most -appropriate to return in scalar context. Some operators return the -length of the list that would have been returned in list context. Some -operators return the first value in the list. Some operators return the -last value in the list. Some operators return a count of successful -operations. In general, they do what you want, unless you want -consistency. -X<context> - -A named array in scalar context is quite different from what would at -first glance appear to be a list in scalar context. You can't get a list -like C<(1,2,3)> into being in scalar context, because the compiler knows -the context at compile time. It would generate the scalar comma operator -there, not the list construction version of the comma. That means it -was never a list to start with. - -In general, functions in Perl that serve as wrappers for system calls ("syscalls") -of the same name (like chown(2), fork(2), closedir(2), etc.) return -true when they succeed and C<undef> otherwise, as is usually mentioned -in the descriptions below. This is different from the C interfaces, -which return C<-1> on failure. Exceptions to this rule include C<wait>, -C<waitpid>, and C<syscall>. System calls also set the special C<$!> -variable on failure. Other functions do not, except accidentally. - -Extension modules can also hook into the Perl parser to define new -kinds of keyword-headed expression. These may look like functions, but -may also look completely different. The syntax following the keyword -is defined entirely by the extension. If you are an implementor, see -L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such -a module, see the module's documentation for details of the syntax that -it defines. - -=head2 Perl Functions by Category -X<function> - -Here are Perl's functions (including things that look like -functions, like some keywords and named operators) -arranged by category. Some functions appear in more -than one place. - -=over 4 - -=item Functions for SCALARs or strings -X<scalar> X<string> X<character> - -=for Pod::Functions =String - -C<chomp>, C<chop>, C<chr>, C<crypt>, C<fc>, C<hex>, C<index>, C<lc>, -C<lcfirst>, C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>, -C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> - -C<fc> is available only if the C<"fc"> feature is enabled or if it is -prefixed with C<CORE::>. The C<"fc"> feature is enabled automatically -with a C<use v5.16> (or higher) declaration in the current scope. - - -=item Regular expressions and pattern matching -X<regular expression> X<regex> X<regexp> - -=for Pod::Functions =Regexp - -C<m//>, C<pos>, C<qr//>, C<quotemeta>, C<s///>, C<split>, C<study> - -=item Numeric functions -X<numeric> X<number> X<trigonometric> X<trigonometry> - -=for Pod::Functions =Math - -C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, -C<sin>, C<sqrt>, C<srand> - -=item Functions for real @ARRAYs -X<array> - -=for Pod::Functions =ARRAY - -C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values> - -=item Functions for list data -X<list> - -=for Pod::Functions =LIST - -C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack> - -=item Functions for real %HASHes -X<hash> - -=for Pod::Functions =HASH - -C<delete>, C<each>, C<exists>, C<keys>, C<values> - -=item Input and output functions -X<I/O> X<input> X<output> X<dbm> - -=for Pod::Functions =I/O - -C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, -C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, -C<readdir>, C<readline> C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, -C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, -C<truncate>, C<warn>, C<write> - -C<say> is available only if the C<"say"> feature is enabled or if it is -prefixed with C<CORE::>. The C<"say"> feature is enabled automatically -with a C<use v5.10> (or higher) declaration in the current scope. - -=item Functions for fixed-length data or records - -=for Pod::Functions =Binary - -C<pack>, C<read>, C<syscall>, C<sysread>, C<sysseek>, C<syswrite>, C<unpack>, -C<vec> - -=item Functions for filehandles, files, or directories -X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink> - -=for Pod::Functions =File - -C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, -C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, -C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>, -C<umask>, C<unlink>, C<utime> - -=item Keywords related to the control flow of your Perl program -X<control flow> - -=for Pod::Functions =Flow - -C<break>, C<caller>, C<continue>, C<die>, C<do>, -C<dump>, C<eval>, C<evalbytes> C<exit>, -C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>, -C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray> - -C<break> is available only if you enable the experimental C<"switch"> -feature or use the C<CORE::> prefix. The C<"switch"> feature also enables -the C<default>, C<given> and C<when> statements, which are documented in -L<perlsyn/"Switch Statements">. The C<"switch"> feature is enabled -automatically with a C<use v5.10> (or higher) declaration in the current -scope. In Perl v5.14 and earlier, C<continue> required the C<"switch"> -feature, like the other keywords. - -C<evalbytes> is only available with with the C<"evalbytes"> feature (see -L<feature>) or if prefixed with C<CORE::>. C<__SUB__> is only available -with with the C<"current_sub"> feature or if prefixed with C<CORE::>. Both -the C<"evalbytes"> and C<"current_sub"> features are enabled automatically -with a C<use v5.16> (or higher) declaration in the current scope. - -=item Keywords related to scoping - -=for Pod::Functions =Namespace - -C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use> - -C<state> is available only if the C<"state"> feature is enabled or if it is -prefixed with C<CORE::>. The C<"state"> feature is enabled automatically -with a C<use v5.10> (or higher) declaration in the current scope. - -=item Miscellaneous functions - -=for Pod::Functions =Misc - -C<defined>, C<formline>, C<lock>, C<prototype>, C<reset>, C<scalar>, C<undef> - -=item Functions for processes and process groups -X<process> X<pid> X<process id> - -=for Pod::Functions =Process - -C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, -C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>, -C<setpriority>, C<sleep>, C<system>, -C<times>, C<wait>, C<waitpid> - -=item Keywords related to Perl modules -X<module> - -=for Pod::Functions =Modules - -C<do>, C<import>, C<no>, C<package>, C<require>, C<use> - -=item Keywords related to classes and object-orientation -X<object> X<class> X<package> - -=for Pod::Functions =Objects - -C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, -C<untie>, C<use> - -=item Low-level socket functions -X<socket> X<sock> - -=for Pod::Functions =Socket - -C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, -C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, -C<socket>, C<socketpair> - -=item System V interprocess communication functions -X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message> - -=for Pod::Functions =SysV - -C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, -C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> - -=item Fetching user and group info -X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd> - -=for Pod::Functions =User - -C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, -C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, -C<getpwuid>, C<setgrent>, C<setpwent> - -=item Fetching network info -X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service> - -=for Pod::Functions =Network - -C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, -C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, -C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, -C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, -C<setnetent>, C<setprotoent>, C<setservent> - -=item Time-related functions -X<time> X<date> - -=for Pod::Functions =Time - -C<gmtime>, C<localtime>, C<time>, C<times> - -=item Non-function keywords - -=for Pod::Functions =!Non-functions - -C<and>, C<AUTOLOAD>, C<BEGIN>, C<CHECK>, C<cmp>, C<CORE>, C<__DATA__>, -C<default>, C<DESTROY>, C<else>, C<elseif>, C<elsif>, C<END>, C<__END__>, -C<eq>, C<for>, C<foreach>, C<ge>, C<given>, C<gt>, C<if>, C<INIT>, C<le>, -C<lt>, C<ne>, C<not>, C<or>, C<UNITCHECK>, C<unless>, C<until>, C<when>, -C<while>, C<x>, C<xor> - -=back - -=head2 Portability -X<portability> X<Unix> X<portable> - -Perl was born in Unix and can therefore access all common Unix -system calls. In non-Unix environments, the functionality of some -Unix system calls may not be available or details of the available -functionality may differ slightly. The Perl functions affected -by this are: - -C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, -C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, -C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, -C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>, -C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, -C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>, -C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, -C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, -C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, -C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, -C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, -C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, -C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, -C<shmwrite>, C<socket>, C<socketpair>, -C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>, -C<times>, C<truncate>, C<umask>, C<unlink>, -C<utime>, C<wait>, C<waitpid> - -For more information about the portability of these functions, see -L<perlport> and other available platform-specific documentation. - -=head2 Alphabetical Listing of Perl Functions - -=over - -=item -X FILEHANDLE -X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> -X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> - -=item -X EXPR - -=item -X DIRHANDLE - -=item -X - -=for Pod::Functions a file test (-r, -x, etc) - -A file test, where X is one of the letters listed below. This unary -operator takes one argument, either a filename, a filehandle, or a dirhandle, -and tests the associated file to see if something is true about it. If the -argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. -Unless otherwise documented, it returns C<1> for true and C<''> for false, or -the undefined value if the file doesn't exist. Despite the funny -names, precedence is the same as any other named unary operator. The -operator may be any of: - - -r File is readable by effective uid/gid. - -w File is writable by effective uid/gid. - -x File is executable by effective uid/gid. - -o File is owned by effective uid. - - -R File is readable by real uid/gid. - -W File is writable by real uid/gid. - -X File is executable by real uid/gid. - -O File is owned by real uid. - - -e File exists. - -z File has zero size (is empty). - -s File has nonzero size (returns size in bytes). - - -f File is a plain file. - -d File is a directory. - -l File is a symbolic link. - -p File is a named pipe (FIFO), or Filehandle is a pipe. - -S File is a socket. - -b File is a block special file. - -c File is a character special file. - -t Filehandle is opened to a tty. - - -u File has setuid bit set. - -g File has setgid bit set. - -k File has sticky bit set. - - -T File is an ASCII text file (heuristic guess). - -B File is a "binary" file (opposite of -T). - - -M Script start time minus file modification time, in days. - -A Same for access time. - -C Same for inode change time (Unix, may differ for other platforms) - -Example: - - while (<>) { - chomp; - next unless -f $_; # ignore specials - #... - } - -Note that C<-s/a/b/> does not do a negated substitution. Saying -C<-exp($foo)> still works as expected, however: only single letters -following a minus are interpreted as file tests. - -These operators are exempt from the "looks like a function rule" described -above. That is, an opening parenthesis after the operator does not affect -how much of the following code constitutes the argument. Put the opening -parentheses before the operator to separate it from code that follows (this -applies only to operators with higher precedence than unary operators, of -course): - - -s($file) + 1024 # probably wrong; same as -s($file + 1024) - (-s $file) + 1024 # correct - -The interpretation of the file permission operators C<-r>, C<-R>, -C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode -of the file and the uids and gids of the user. There may be other -reasons you can't actually read, write, or execute the file: for -example network filesystem access controls, ACLs (access control lists), -read-only filesystems, and unrecognized executable formats. Note -that the use of these six specific operators to verify if some operation -is possible is usually a mistake, because it may be open to race -conditions. - -Also note that, for the superuser on the local filesystems, the C<-r>, -C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 -if any execute bit is set in the mode. Scripts run by the superuser -may thus need to do a stat() to determine the actual mode of the file, -or temporarily set their effective uid to something else. - -If you are using ACLs, there is a pragma called C<filetest> that may -produce more accurate results than the bare stat() mode bits. -When under C<use filetest 'access'> the above-mentioned filetests -test whether the permission can(not) be granted using the -access(2) family of system calls. Also note that the C<-x> and C<-X> may -under this pragma return true even if there are no execute permission -bits set (nor any extra execute permission ACLs). This strangeness is -due to the underlying system calls' definitions. Note also that, due to -the implementation of C<use filetest 'access'>, the C<_> special -filehandle won't cache the results of the file tests when this pragma is -in effect. Read the documentation for the C<filetest> pragma for more -information. - -The C<-T> and C<-B> switches work as follows. The first block or so of the -file is examined for odd characters such as strange control codes or -characters with the high bit set. If too many strange characters (>30%) -are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file -containing a zero byte in the first block is considered a binary file. If C<-T> -or C<-B> is used on a filehandle, the current IO buffer is examined -rather than the first block. Both C<-T> and C<-B> return true on an empty -file, or a file at EOF when testing a filehandle. Because you have to -read a file to do the C<-T> test, on most occasions you want to use a C<-f> -against the file first, as in C<next unless -f $file && -T $file>. - -If any of the file tests (or either the C<stat> or C<lstat> operator) is given -the special filehandle consisting of a solitary underline, then the stat -structure of the previous file test (or stat operator) is used, saving -a system call. (This doesn't work with C<-t>, and you need to remember -that lstat() and C<-l> leave values in the stat structure for the -symbolic link, not the real file.) (Also, if the stat buffer was filled by -an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>). -Example: - - print "Can do.\n" if -r $a || -w _ || -x _; - - stat($filename); - print "Readable\n" if -r _; - print "Writable\n" if -w _; - print "Executable\n" if -x _; - print "Setuid\n" if -u _; - print "Setgid\n" if -g _; - print "Sticky\n" if -k _; - print "Text\n" if -T _; - print "Binary\n" if -B _; - -As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file -test operators, in a way that C<-f -w -x $file> is equivalent to -C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use -the return value of C<-f $file> as an argument to another filetest -operator, no special magic will happen.) - -Portability issues: L<perlport/-X>. - -To avoid confusing would-be users of your code with mysterious -syntax errors, put something like this at the top of your script: - - use 5.010; # so filetest ops can stack - -=item abs VALUE -X<abs> X<absolute> - -=item abs - -=for Pod::Functions absolute value function - -Returns the absolute value of its argument. -If VALUE is omitted, uses C<$_>. - -=item accept NEWSOCKET,GENERICSOCKET -X<accept> - -=for Pod::Functions accept an incoming socket connect - -Accepts an incoming socket connect, just as accept(2) -does. Returns the packed address if it succeeded, false otherwise. -See the example in L<perlipc/"Sockets: Client/Server Communication">. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptor, as determined by the -value of $^F. See L<perlvar/$^F>. - -=item alarm SECONDS -X<alarm> -X<SIGALRM> -X<timer> - -=item alarm - -=for Pod::Functions schedule a SIGALRM - -Arranges to have a SIGALRM delivered to this process after the -specified number of wallclock seconds has elapsed. If SECONDS is not -specified, the value stored in C<$_> is used. (On some machines, -unfortunately, the elapsed time may be up to one second less or more -than you specified because of how seconds are counted, and process -scheduling may delay the delivery of the signal even further.) - -Only one timer may be counting at once. Each call disables the -previous timer, and an argument of C<0> may be supplied to cancel the -previous timer without starting a new one. The returned value is the -amount of time remaining on the previous timer. - -For delays of finer granularity than one second, the Time::HiRes module -(from CPAN, and starting from Perl 5.8 part of the standard -distribution) provides ualarm(). You may also use Perl's four-argument -version of select() leaving the first three arguments undefined, or you -might be able to use the C<syscall> interface to access setitimer(2) if -your system supports it. See L<perlfaq8> for details. - -It is usually a mistake to intermix C<alarm> and C<sleep> calls, because -C<sleep> may be internally implemented on your system with C<alarm>. - -If you want to use C<alarm> to time out a system call you need to use an -C<eval>/C<die> pair. You can't rely on the alarm causing the system call to -fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to -restart system calls on some systems. Using C<eval>/C<die> always works, -modulo the caveats given in L<perlipc/"Signals">. - - eval { - local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required - alarm $timeout; - $nread = sysread SOCKET, $buffer, $size; - alarm 0; - }; - if ($@) { - die unless $@ eq "alarm\n"; # propagate unexpected errors - # timed out - } - else { - # didn't - } - -For more information see L<perlipc>. - -Portability issues: L<perlport/alarm>. - -=item atan2 Y,X -X<atan2> X<arctangent> X<tan> X<tangent> - -=for Pod::Functions arctangent of Y/X in the range -PI to PI - -Returns the arctangent of Y/X in the range -PI to PI. - -For the tangent operation, you may use the C<Math::Trig::tan> -function, or use the familiar relation: - - sub tan { sin($_[0]) / cos($_[0]) } - -The return value for C<atan2(0,0)> is implementation-defined; consult -your atan2(3) manpage for more information. - -Portability issues: L<perlport/atan2>. - -=item bind SOCKET,NAME -X<bind> - -=for Pod::Functions binds an address to a socket - -Binds a network address to a socket, just as bind(2) -does. Returns true if it succeeded, false otherwise. NAME should be a -packed address of the appropriate type for the socket. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -=item binmode FILEHANDLE, LAYER -X<binmode> X<binary> X<text> X<DOS> X<Windows> - -=item binmode FILEHANDLE - -=for Pod::Functions prepare binary files for I/O - -Arranges for FILEHANDLE to be read or written in "binary" or "text" -mode on systems where the run-time libraries distinguish between -binary and text files. If FILEHANDLE is an expression, the value is -taken as the name of the filehandle. Returns true on success, -otherwise it returns C<undef> and sets C<$!> (errno). - -On some systems (in general, DOS- and Windows-based systems) binmode() -is necessary when you're not working with a text file. For the sake -of portability it is a good idea always to use it when appropriate, -and never to use it when it isn't appropriate. Also, people can -set their I/O to be by default UTF8-encoded Unicode, not bytes. - -In other words: regardless of platform, use binmode() on binary data, -like images, for example. - -If LAYER is present it is a single string, but may contain multiple -directives. The directives alter the behaviour of the filehandle. -When LAYER is present, using binmode on a text file makes sense. - -If LAYER is omitted or specified as C<:raw> the filehandle is made -suitable for passing binary data. This includes turning off possible CRLF -translation and marking it as bytes (as opposed to Unicode characters). -Note that, despite what may be implied in I<"Programming Perl"> (the -Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>. -Other layers that would affect the binary nature of the stream are -I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the -PERLIO environment variable. - -The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the -form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to -establish default I/O layers. See L<open>. - -I<The LAYER parameter of the binmode() function is described as "DISCIPLINE" -in "Programming Perl, 3rd Edition". However, since the publishing of this -book, by many known as "Camel III", the consensus of the naming of this -functionality has moved from "discipline" to "layer". All documentation -of this version of Perl therefore refers to "layers" rather than to -"disciplines". Now back to the regularly scheduled documentation...> - -To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>. -C<:utf8> just marks the data as UTF-8 without further checking, -while C<:encoding(UTF-8)> checks the data for actually being valid -UTF-8. More details can be found in L<PerlIO::encoding>. - -In general, binmode() should be called after open() but before any I/O -is done on the filehandle. Calling binmode() normally flushes any -pending buffered output data (and perhaps pending input data) on the -handle. An exception to this is the C<:encoding> layer that -changes the default character encoding of the handle; see L</open>. -The C<:encoding> layer sometimes needs to be called in -mid-stream, and it doesn't flush the stream. The C<:encoding> -also implicitly pushes on top of itself the C<:utf8> layer because -internally Perl operates on UTF8-encoded Unicode characters. - -The operating system, device drivers, C libraries, and Perl run-time -system all conspire to let the programmer treat a single -character (C<\n>) as the line terminator, irrespective of external -representation. On many operating systems, the native text file -representation matches the internal representation, but on some -platforms the external representation of C<\n> is made up of more than -one character. - -All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use -a single character to end each line in the external representation of text -(even though that single character is CARRIAGE RETURN on old, pre-Darwin -flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other -systems like OS/2, DOS, and the various flavors of MS-Windows, your program -sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the -two characters C<\cM\cJ>. That means that if you don't use binmode() on -these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on -input, and any C<\n> in your program will be converted back to C<\cM\cJ> on -output. This is what you want for text files, but it can be disastrous for -binary files. - -Another consequence of using binmode() (on some systems) is that -special end-of-file markers will be seen as part of the data stream. -For systems from the Microsoft family this means that, if your binary -data contain C<\cZ>, the I/O subsystem will regard it as the end of -the file, unless you use binmode(). - -binmode() is important not only for readline() and print() operations, -but also when using read(), seek(), sysread(), syswrite() and tell() -(see L<perlport> for more details). See the C<$/> and C<$\> variables -in L<perlvar> for how to manually set your input and output -line-termination sequences. - -Portability issues: L<perlport/binmode>. - -=item bless REF,CLASSNAME -X<bless> - -=item bless REF - -=for Pod::Functions create an object - -This function tells the thingy referenced by REF that it is now an object -in the CLASSNAME package. If CLASSNAME is omitted, the current package -is used. Because a C<bless> is often the last thing in a constructor, -it returns the reference for convenience. Always use the two-argument -version if a derived class might inherit the function doing the blessing. -SeeL<perlobj> for more about the blessing (and blessings) of objects. - -Consider always blessing objects in CLASSNAMEs that are mixed case. -Namespaces with all lowercase names are considered reserved for -Perl pragmata. Builtin types have all uppercase names. To prevent -confusion, you may wish to avoid such package names as well. Make sure -that CLASSNAME is a true value. - -See L<perlmod/"Perl Modules">. - -=item break - -=for Pod::Functions +switch break out of a C<given> block - -Break out of a C<given()> block. - -This keyword is enabled by the C<"switch"> feature: see -L<feature> for more information. You can also access it by -prefixing it with C<CORE::>. Alternately, include a C<use -v5.10> or later to the current scope. - -=item caller EXPR -X<caller> X<call stack> X<stack> X<stack trace> - -=item caller - -=for Pod::Functions get context of the current subroutine call - -Returns the context of the current subroutine call. In scalar context, -returns the caller's package name if there I<is> a caller (that is, if -we're in a subroutine or C<eval> or C<require>) and the undefined value -otherwise. In list context, returns - - # 0 1 2 - ($package, $filename, $line) = caller; - -With EXPR, it returns some extra information that the debugger uses to -print a stack trace. The value of EXPR indicates how many call frames -to go back before the current one. - - # 0 1 2 3 4 - ($package, $filename, $line, $subroutine, $hasargs, - - # 5 6 7 8 9 10 - $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) - = caller($i); - -Here $subroutine may be C<(eval)> if the frame is not a subroutine -call, but an C<eval>. In such a case additional elements $evaltext and -C<$is_require> are set: C<$is_require> is true if the frame is created by a -C<require> or C<use> statement, $evaltext contains the text of the -C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, -$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that -each C<use> statement creates a C<require> frame inside an C<eval EXPR> -frame.) $subroutine may also be C<(unknown)> if this particular -subroutine happens to have been deleted from the symbol table. -C<$hasargs> is true if a new instance of C<@_> was set up for the frame. -C<$hints> and C<$bitmask> contain pragmatic hints that the caller was -compiled with. The C<$hints> and C<$bitmask> values are subject to change -between versions of Perl, and are not meant for external use. - -C<$hinthash> is a reference to a hash containing the value of C<%^H> when the -caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values -of this hash, as they are the actual values stored in the optree. - -Furthermore, when called from within the DB package in -list context, and with an argument, caller returns more -detailed information: it sets the list variable C<@DB::args> to be the -arguments with which the subroutine was invoked. - -Be aware that the optimizer might have optimized call frames away before -C<caller> had a chance to get the information. That means that C<caller(N)> -might not return information about the call frame you expect it to, for -C<< N > 1 >>. In particular, C<@DB::args> might have information from the -previous time C<caller> was called. - -Be aware that setting C<@DB::args> is I<best effort>, intended for -debugging or generating backtraces, and should not be relied upon. In -particular, as C<@_> contains aliases to the caller's arguments, Perl does -not take a copy of C<@_>, so C<@DB::args> will contain modifications the -subroutine makes to C<@_> or its contents, not the original values at call -time. C<@DB::args>, like C<@_>, does not hold explicit references to its -elements, so under certain cases its elements may have become freed and -reallocated for other variables or temporary values. Finally, a side effect -of the current implementation is that the effects of C<shift @_> can -I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a -reference to C<@_> has been taken, I<and> subject to the caveat about reallocated -elements), so C<@DB::args> is actually a hybrid of the current state and -initial state of C<@_>. Buyer beware. - -=item chdir EXPR -X<chdir> -X<cd> -X<directory, change> - -=item chdir FILEHANDLE - -=item chdir DIRHANDLE - -=item chdir - -=for Pod::Functions change your current working directory - -Changes the working directory to EXPR, if possible. If EXPR is omitted, -changes to the directory specified by C<$ENV{HOME}>, if set; if not, -changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the -variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If -neither is set, C<chdir> does nothing. It returns true on success, -false otherwise. See the example under C<die>. - -On systems that support fchdir(2), you may pass a filehandle or -directory handle as the argument. On systems that don't support fchdir(2), -passing handles raises an exception. - -=item chmod LIST -X<chmod> X<permission> X<mode> - -=for Pod::Functions changes the permissions on a list of files - -Changes the permissions of a list of files. The first element of the -list must be the numeric mode, which should probably be an octal -number, and which definitely should I<not> be a string of octal digits: -C<0644> is okay, but C<"0644"> is not. Returns the number of files -successfully changed. See also L</oct> if all you have is a string. - - $cnt = chmod 0755, "foo", "bar"; - chmod 0755, @executables; - $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to - # --w----r-T - $mode = "0644"; chmod oct($mode), "foo"; # this is better - $mode = 0644; chmod $mode, "foo"; # this is best - -On systems that support fchmod(2), you may pass filehandles among the -files. On systems that don't support fchmod(2), passing filehandles raises -an exception. Filehandles must be passed as globs or glob references to be -recognized; barewords are considered filenames. - - open(my $fh, "<", "foo"); - my $perm = (stat $fh)[2] & 07777; - chmod($perm | 0600, $fh); - -You can also import the symbolic C<S_I*> constants from the C<Fcntl> -module: - - use Fcntl qw( :mode ); - chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; - # Identical to the chmod 0755 of the example above. - -Portability issues: L<perlport/chmod>. - -=item chomp VARIABLE -X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol> - -=item chomp( LIST ) - -=item chomp - -=for Pod::Functions remove a trailing record separator from a string - -This safer version of L</chop> removes any trailing string -that corresponds to the current value of C<$/> (also known as -$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total -number of characters removed from all its arguments. It's often used to -remove the newline from the end of an input record when you're worried -that the final record may be missing its newline. When in paragraph -mode (C<$/ = "">), it removes all trailing newlines from the string. -When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is -a reference to an integer or the like; see L<perlvar>) chomp() won't -remove anything. -If VARIABLE is omitted, it chomps C<$_>. Example: - - while (<>) { - chomp; # avoid \n on last field - @array = split(/:/); - # ... - } - -If VARIABLE is a hash, it chomps the hash's values, but not its keys. - -You can actually chomp anything that's an lvalue, including an assignment: - - chomp($cwd = `pwd`); - chomp($answer = <STDIN>); - -If you chomp a list, each element is chomped, and the total number of -characters removed is returned. - -Note that parentheses are necessary when you're chomping anything -that is not a simple variable. This is because C<chomp $cwd = `pwd`;> -is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as -C<chomp( $cwd = `pwd` )> which you might expect. Similarly, -C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than -as C<chomp($a, $b)>. - -=item chop VARIABLE -X<chop> - -=item chop( LIST ) - -=item chop - -=for Pod::Functions remove the last character from a string - -Chops off the last character of a string and returns the character -chopped. It is much more efficient than C<s/.$//s> because it neither -scans nor copies the string. If VARIABLE is omitted, chops C<$_>. -If VARIABLE is a hash, it chops the hash's values, but not its keys. - -You can actually chop anything that's an lvalue, including an assignment. - -If you chop a list, each element is chopped. Only the value of the -last C<chop> is returned. - -Note that C<chop> returns the last character. To return all but the last -character, use C<substr($string, 0, -1)>. - -See also L</chomp>. - -=item chown LIST -X<chown> X<owner> X<user> X<group> - -=for Pod::Functions change the ownership on a list of files - -Changes the owner (and group) of a list of files. The first two -elements of the list must be the I<numeric> uid and gid, in that -order. A value of -1 in either position is interpreted by most -systems to leave that value unchanged. Returns the number of files -successfully changed. - - $cnt = chown $uid, $gid, 'foo', 'bar'; - chown $uid, $gid, @filenames; - -On systems that support fchown(2), you may pass filehandles among the -files. On systems that don't support fchown(2), passing filehandles raises -an exception. Filehandles must be passed as globs or glob references to be -recognized; barewords are considered filenames. - -Here's an example that looks up nonnumeric uids in the passwd file: - - print "User: "; - chomp($user = <STDIN>); - print "Files: "; - chomp($pattern = <STDIN>); - - ($login,$pass,$uid,$gid) = getpwnam($user) - or die "$user not in passwd file"; - - @ary = glob($pattern); # expand filenames - chown $uid, $gid, @ary; - -On most systems, you are not allowed to change the ownership of the -file unless you're the superuser, although you should be able to change -the group to any of your secondary groups. On insecure systems, these -restrictions may be relaxed, but this is not a portable assumption. -On POSIX systems, you can detect this condition this way: - - use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); - $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); - -Portability issues: L<perlport/chmod>. - -=item chr NUMBER -X<chr> X<character> X<ASCII> X<Unicode> - -=item chr - -=for Pod::Functions get character this number represents - -Returns the character represented by that NUMBER in the character set. -For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and -chr(0x263a) is a Unicode smiley face. - -Negative values give the Unicode replacement character (chr(0xfffd)), -except under the L<bytes> pragma, where the low eight bits of the value -(truncated to an integer) are used. - -If NUMBER is omitted, uses C<$_>. - -For the reverse, use L</ord>. - -Note that characters from 128 to 255 (inclusive) are by default -internally not encoded as UTF-8 for backward compatibility reasons. - -See L<perlunicode> for more about Unicode. - -=item chroot FILENAME -X<chroot> X<root> - -=item chroot - -=for Pod::Functions make directory new root for path lookups - -This function works like the system call by the same name: it makes the -named directory the new root directory for all further pathnames that -begin with a C</> by your process and all its children. (It doesn't -change your current working directory, which is unaffected.) For security -reasons, this call is restricted to the superuser. If FILENAME is -omitted, does a C<chroot> to C<$_>. - -Portability issues: L<perlport/chroot>. - -=item close FILEHANDLE -X<close> - -=item close - -=for Pod::Functions close file (or pipe or socket) handle - -Closes the file or pipe associated with the filehandle, flushes the IO -buffers, and closes the system file descriptor. Returns true if those -operations succeed and if no error was reported by any PerlIO -layer. Closes the currently selected filehandle if the argument is -omitted. - -You don't have to close FILEHANDLE if you are immediately going to do -another C<open> on it, because C<open> closes it for you. (See -L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line -counter (C<$.>), while the implicit close done by C<open> does not. - -If the filehandle came from a piped open, C<close> returns false if one of -the other syscalls involved fails or if its program exits with non-zero -status. If the only problem was that the program exited non-zero, C<$!> -will be set to C<0>. Closing a pipe also waits for the process executing -on the pipe to exit--in case you wish to look at the output of the pipe -afterwards--and implicitly puts the exit status value of that command into -C<$?> and C<${^CHILD_ERROR_NATIVE}>. - -If there are multiple threads running, C<close> on a filehandle from a -piped open returns true without waiting for the child process to terminate, -if the filehandle is still open in another thread. - -Closing the read end of a pipe before the process writing to it at the -other end is done writing results in the writer receiving a SIGPIPE. If -the other end can't handle that, be sure to read all the data before -closing the pipe. - -Example: - - open(OUTPUT, '|sort >foo') # pipe to sort - or die "Can't start sort: $!"; - #... # print stuff to output - close OUTPUT # wait for sort to finish - or warn $! ? "Error closing sort pipe: $!" - : "Exit status $? from sort"; - open(INPUT, 'foo') # get sort's results - or die "Can't open 'foo' for input: $!"; - -FILEHANDLE may be an expression whose value can be used as an indirect -filehandle, usually the real filehandle name or an autovivified handle. - -=item closedir DIRHANDLE -X<closedir> - -=for Pod::Functions close directory handle - -Closes a directory opened by C<opendir> and returns the success of that -system call. - -=item connect SOCKET,NAME -X<connect> - -=for Pod::Functions connect to a remote socket - -Attempts to connect to a remote socket, just like connect(2). -Returns true if it succeeded, false otherwise. NAME should be a -packed address of the appropriate type for the socket. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -=item continue BLOCK -X<continue> - -=item continue - -=for Pod::Functions optional trailing block in a while or foreach - -When followed by a BLOCK, C<continue> is actually a -flow control statement rather than a function. If -there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or -C<foreach>), it is always executed just before the conditional is about to -be evaluated again, just like the third part of a C<for> loop in C. Thus -it can be used to increment a loop variable, even when the loop has been -continued via the C<next> statement (which is similar to the C C<continue> -statement). - -C<last>, C<next>, or C<redo> may appear within a C<continue> -block; C<last> and C<redo> behave as if they had been executed within -the main block. So will C<next>, but since it will execute a C<continue> -block, it may be more entertaining. - - while (EXPR) { - ### redo always comes here - do_something; - } continue { - ### next always comes here - do_something_else; - # then back the top to re-check EXPR - } - ### last always comes here - -Omitting the C<continue> section is equivalent to using an -empty one, logically enough, so C<next> goes directly back -to check the condition at the top of the loop. - -When there is no BLOCK, C<continue> is a function that -falls through the current C<when> or C<default> block instead of iterating -a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>. -In Perl 5.14 and earlier, this form of C<continue> was -only available when the C<"switch"> feature was enabled. -See L<feature> and L<perlsyn/"Switch Statements"> for more -information. - -=item cos EXPR -X<cos> X<cosine> X<acos> X<arccosine> - -=item cos - -=for Pod::Functions cosine function - -Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, -takes the cosine of C<$_>. - -For the inverse cosine operation, you may use the C<Math::Trig::acos()> -function, or use this relation: - - sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } - -=item crypt PLAINTEXT,SALT -X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password> -X<decrypt> X<cryptography> X<passwd> X<encrypt> - -=for Pod::Functions one-way passwd-style encryption - -Creates a digest string exactly like the crypt(3) function in the C -library (assuming that you actually have a version there that has not -been extirpated as a potential munition). - -crypt() is a one-way hash function. The PLAINTEXT and SALT are turned -into a short string, called a digest, which is returned. The same -PLAINTEXT and SALT will always return the same string, but there is no -(known) way to get the original PLAINTEXT from the hash. Small -changes in the PLAINTEXT or SALT will result in large changes in the -digest. - -There is no decrypt function. This function isn't all that useful for -cryptography (for that, look for F<Crypt> modules on your nearby CPAN -mirror) and the name "crypt" is a bit of a misnomer. Instead it is -primarily used to check if two pieces of text are the same without -having to transmit or store the text itself. An example is checking -if a correct password is given. The digest of the password is stored, -not the password itself. The user types in a password that is -crypt()'d with the same salt as the stored digest. If the two digests -match, the password is correct. - -When verifying an existing digest string you should use the digest as -the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used -to create the digest is visible as part of the digest. This ensures -crypt() will hash the new string with the same salt as the digest. -This allows your code to work with the standard L<crypt|/crypt> and -with more exotic implementations. In other words, assume -nothing about the returned string itself nor about how many bytes -of SALT may matter. - -Traditionally the result is a string of 13 bytes: two first bytes of -the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only -the first eight bytes of PLAINTEXT mattered. But alternative -hashing schemes (like MD5), higher level security schemes (like C2), -and implementations on non-Unix platforms may produce different -strings. - -When choosing a new salt create a random two character string whose -characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.', -'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of -characters is just a recommendation; the characters allowed in -the salt depend solely on your system's crypt library, and Perl can't -restrict what salts C<crypt()> accepts. - -Here's an example that makes sure that whoever runs this program knows -their password: - - $pwd = (getpwuid($<))[1]; - - system "stty -echo"; - print "Password: "; - chomp($word = <STDIN>); - print "\n"; - system "stty echo"; - - if (crypt($word, $pwd) ne $pwd) { - die "Sorry...\n"; - } else { - print "ok\n"; - } - -Of course, typing in your own password to whoever asks you -for it is unwise. - -The L<crypt|/crypt> function is unsuitable for hashing large quantities -of data, not least of all because you can't get the information -back. Look at the L<Digest> module for more robust algorithms. - -If using crypt() on a Unicode string (which I<potentially> has -characters with codepoints above 255), Perl tries to make sense -of the situation by trying to downgrade (a copy of) -the string back to an eight-bit byte string before calling crypt() -(on that copy). If that works, good. If not, crypt() dies with -C<Wide character in crypt>. - -Portability issues: L<perlport/crypt>. - -=item dbmclose HASH -X<dbmclose> - -=for Pod::Functions breaks binding on a tied dbm file - -[This function has been largely superseded by the C<untie> function.] - -Breaks the binding between a DBM file and a hash. - -Portability issues: L<perlport/dbmclose>. - -=item dbmopen HASH,DBNAME,MASK -X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm> - -=for Pod::Functions create binding on a tied dbm file - -[This function has been largely superseded by the -L<tie|/tie VARIABLE,CLASSNAME,LIST> function.] - -This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a -hash. HASH is the name of the hash. (Unlike normal C<open>, the first -argument is I<not> a filehandle, even though it looks like one). DBNAME -is the name of the database (without the F<.dir> or F<.pag> extension if -any). If the database does not exist, it is created with protection -specified by MASK (as modified by the C<umask>). To prevent creation of -the database if it doesn't exist, you may specify a MODE -of 0, and the function will return a false value if it -can't find an existing database. If your system supports -only the older DBM functions, you may make only one C<dbmopen> call in your -program. In older versions of Perl, if your system had neither DBM nor -ndbm, calling C<dbmopen> produced a fatal error; it now falls back to -sdbm(3). - -If you don't have write access to the DBM file, you can only read hash -variables, not set them. If you want to test whether you can write, -either use file tests or try setting a dummy hash entry inside an C<eval> -to trap the error. - -Note that functions such as C<keys> and C<values> may return huge lists -when used on large DBM files. You may prefer to use the C<each> -function to iterate over large DBM files. Example: - - # print out history file offsets - dbmopen(%HIST,'/usr/lib/news/history',0666); - while (($key,$val) = each %HIST) { - print $key, ' = ', unpack('L',$val), "\n"; - } - dbmclose(%HIST); - -See also L<AnyDBM_File> for a more general description of the pros and -cons of the various dbm approaches, as well as L<DB_File> for a particularly -rich implementation. - -You can control which DBM library you use by loading that library -before you call dbmopen(): - - use DB_File; - dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") - or die "Can't open netscape history file: $!"; - -Portability issues: L<perlport/dbmopen>. - -=item defined EXPR -X<defined> X<undef> X<undefined> - -=item defined - -=for Pod::Functions test whether a value, variable, or function is defined - -Returns a Boolean value telling whether EXPR has a value other than -the undefined value C<undef>. If EXPR is not present, C<$_> is -checked. - -Many operations return C<undef> to indicate failure, end of file, -system error, uninitialized variable, and other exceptional -conditions. This function allows you to distinguish C<undef> from -other values. (A simple Boolean test will not distinguish among -C<undef>, zero, the empty string, and C<"0">, which are all equally -false.) Note that since C<undef> is a valid scalar, its presence -doesn't I<necessarily> indicate an exceptional condition: C<pop> -returns C<undef> when its argument is an empty array, I<or> when the -element to return happens to be C<undef>. - -You may also use C<defined(&func)> to check whether subroutine C<&func> -has ever been defined. The return value is unaffected by any forward -declarations of C<&func>. A subroutine that is not defined -may still be callable: its package may have an C<AUTOLOAD> method that -makes it spring into existence the first time that it is called; see -L<perlsub>. - -Use of C<defined> on aggregates (hashes and arrays) is deprecated. It -used to report whether memory for that aggregate had ever been -allocated. This behavior may disappear in future versions of Perl. -You should instead use a simple test for size: - - if (@an_array) { print "has array elements\n" } - if (%a_hash) { print "has hash members\n" } - -When used on a hash element, it tells you whether the value is defined, -not whether the key exists in the hash. Use L</exists> for the latter -purpose. - -Examples: - - print if defined $switch{D}; - print "$val\n" while defined($val = pop(@ary)); - die "Can't readlink $sym: $!" - unless defined($value = readlink $sym); - sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } - $debugging = 0 unless defined $debugging; - -Note: Many folks tend to overuse C<defined> and are then surprised to -discover that the number C<0> and C<""> (the zero-length string) are, in fact, -defined values. For example, if you say - - "ab" =~ /a(.*)b/; - -The pattern match succeeds and C<$1> is defined, although it -matched "nothing". It didn't really fail to match anything. Rather, it -matched something that happened to be zero characters long. This is all -very above-board and honest. When a function returns an undefined value, -it's an admission that it couldn't give you an honest answer. So you -should use C<defined> only when questioning the integrity of what -you're trying to do. At other times, a simple comparison to C<0> or C<""> is -what you want. - -See also L</undef>, L</exists>, L</ref>. - -=item delete EXPR -X<delete> - -=for Pod::Functions deletes a value from a hash - -Given an expression that specifies an element or slice of a hash, C<delete> -deletes the specified elements from that hash so that exists() on that element -no longer returns true. Setting a hash element to the undefined value does -not remove its key, but deleting it does; see L</exists>. - -In list context, returns the value or values deleted, or the last such -element in scalar context. The return list's length always matches that of -the argument list: deleting non-existent elements returns the undefined value -in their corresponding positions. - -delete() may also be used on arrays and array slices, but its behavior is less -straightforward. Although exists() will return false for deleted entries, -deleting array elements never changes indices of existing values; use shift() -or splice() for that. However, if all deleted elements fall at the end of an -array, the array's size shrinks to the position of the highest element that -still tests true for exists(), or to 0 if none do. - -B<WARNING:> Calling delete on array values is deprecated and likely to -be removed in a future version of Perl. - -Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to -a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash -or array may not necessarily return anything; it depends on the implementation -of the C<tied> package's DELETE method, which may do whatever it pleases. - -The C<delete local EXPR> construct localizes the deletion to the current -block at run time. Until the block exits, elements locally deleted -temporarily no longer exist. See L<perlsub/"Localized deletion of elements -of composite types">. - - %hash = (foo => 11, bar => 22, baz => 33); - $scalar = delete $hash{foo}; # $scalar is 11 - $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 - @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) - -The following (inefficiently) deletes all the values of %HASH and @ARRAY: - - foreach $key (keys %HASH) { - delete $HASH{$key}; - } - - foreach $index (0 .. $#ARRAY) { - delete $ARRAY[$index]; - } - -And so do these: - - delete @HASH{keys %HASH}; - - delete @ARRAY[0 .. $#ARRAY]; - -But both are slower than assigning the empty list -or undefining %HASH or @ARRAY, which is the customary -way to empty out an aggregate: - - %HASH = (); # completely empty %HASH - undef %HASH; # forget %HASH ever existed - - @ARRAY = (); # completely empty @ARRAY - undef @ARRAY; # forget @ARRAY ever existed - -The EXPR can be arbitrarily complicated provided its -final operation is an element or slice of an aggregate: - - delete $ref->[$x][$y]{$key}; - delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; - - delete $ref->[$x][$y][$index]; - delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; - -=item die LIST -X<die> X<throw> X<exception> X<raise> X<$@> X<abort> - -=for Pod::Functions raise an exception or bail out - -C<die> raises an exception. Inside an C<eval> the error message is stuffed -into C<$@> and the C<eval> is terminated with the undefined value. -If the exception is outside of all enclosing C<eval>s, then the uncaught -exception prints LIST to C<STDERR> and exits with a non-zero value. If you -need to exit the process with a specific exit code, see L</exit>. - -Equivalent examples: - - die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; - chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" - -If the last element of LIST does not end in a newline, the current -script line number and input line number (if any) are also printed, -and a newline is supplied. Note that the "input line number" (also -known as "chunk") is subject to whatever notion of "line" happens to -be currently in effect, and is also available as the special variable -C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">. - -Hint: sometimes appending C<", stopped"> to your message will cause it -to make better sense when the string C<"at foo line 123"> is appended. -Suppose you are running script "canasta". - - die "/etc/games is no good"; - die "/etc/games is no good, stopped"; - -produce, respectively - - /etc/games is no good at canasta line 123. - /etc/games is no good, stopped at canasta line 123. - -If the output is empty and C<$@> already contains a value (typically from a -previous eval) that value is reused after appending C<"\t...propagated">. -This is useful for propagating exceptions: - - eval { ... }; - die unless $@ =~ /Expected exception/; - -If the output is empty and C<$@> contains an object reference that has a -C<PROPAGATE> method, that method will be called with additional file -and line number parameters. The return value replaces the value in -C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >> -were called. - -If C<$@> is empty then the string C<"Died"> is used. - -If an uncaught exception results in interpreter exit, the exit code is -determined from the values of C<$!> and C<$?> with this pseudocode: - - exit $! if $!; # errno - exit $? >> 8 if $? >> 8; # child exit status - exit 255; # last resort - -The intent is to squeeze as much possible information about the likely cause -into the limited space of the system exit -code. However, as C<$!> is the value -of C's C<errno>, which can be set by any system call, this means that the value -of the exit code used by C<die> can be non-predictable, so should not be relied -upon, other than to be non-zero. - -You can also call C<die> with a reference argument, and if this is trapped -within an C<eval>, C<$@> contains that reference. This permits more -elaborate exception handling using objects that maintain arbitrary state -about the exception. Such a scheme is sometimes preferable to matching -particular string values of C<$@> with regular expressions. Because C<$@> -is a global variable and C<eval> may be used within object implementations, -be careful that analyzing the error object doesn't replace the reference in -the global variable. It's easiest to make a local copy of the reference -before any manipulations. Here's an example: - - use Scalar::Util "blessed"; - - eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; - if (my $ev_err = $@) { - if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { - # handle Some::Module::Exception - } - else { - # handle all other possible exceptions - } - } - -Because Perl stringifies uncaught exception messages before display, -you'll probably want to overload stringification operations on -exception objects. See L<overload> for details about that. - -You can arrange for a callback to be run just before the C<die> -does its deed, by setting the C<$SIG{__DIE__}> hook. The associated -handler is called with the error text and can change the error -message, if it sees fit, by calling C<die> again. See -L<perlvar/%SIG> for details on setting C<%SIG> entries, and -L<"eval BLOCK"> for some examples. Although this feature was -to be run only right before your program was to exit, this is not -currently so: the C<$SIG{__DIE__}> hook is currently called -even inside eval()ed blocks/strings! If one wants the hook to do -nothing in such situations, put - - die @_ if $^S; - -as the first line of the handler (see L<perlvar/$^S>). Because -this promotes strange action at a distance, this counterintuitive -behavior may be fixed in a future release. - -See also exit(), warn(), and the Carp module. - -=item do BLOCK -X<do> X<block> - -=for Pod::Functions turn a BLOCK into a TERM - -Not really a function. Returns the value of the last command in the -sequence of commands indicated by BLOCK. When modified by the C<while> or -C<until> loop modifier, executes the BLOCK once before testing the loop -condition. (On other statements the loop modifiers test the conditional -first.) - -C<do BLOCK> does I<not> count as a loop, so the loop control statements -C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. -See L<perlsyn> for alternative strategies. - -=item do SUBROUTINE(LIST) -X<do> - -This form of subroutine call is deprecated. SUBROUTINE can be a bareword, -a scalar variable or a subroutine beginning with C<&>. - -=item do EXPR -X<do> - -Uses the value of EXPR as a filename and executes the contents of the -file as a Perl script. - - do 'stat.pl'; - -is just like - - eval `cat stat.pl`; - -except that it's more efficient and concise, keeps track of the current -filename for error messages, searches the C<@INC> directories, and updates -C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for -these variables. It also differs in that code evaluated with C<do FILENAME> -cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the -same, however, in that it does reparse the file every time you call it, -so you probably don't want to do this inside a loop. - -If C<do> can read the file but cannot compile it, it returns C<undef> and sets -an error message in C<$@>. If C<do> cannot read the file, it returns undef -and sets C<$!> to the error. Always check C<$@> first, as compilation -could fail in a way that also sets C<$!>. If the file is successfully -compiled, C<do> returns the value of the last expression evaluated. - -Inclusion of library modules is better done with the -C<use> and C<require> operators, which also do automatic error checking -and raise an exception if there's a problem. - -You might like to use C<do> to read in a program configuration -file. Manual error checking can be done this way: - - # read in config files: system first, then user - for $file ("/share/prog/defaults.rc", - "$ENV{HOME}/.someprogrc") - { - unless ($return = do $file) { - warn "couldn't parse $file: $@" if $@; - warn "couldn't do $file: $!" unless defined $return; - warn "couldn't run $file" unless $return; - } - } - -=item dump LABEL -X<dump> X<core> X<undump> - -=item dump - -=for Pod::Functions create an immediate core dump - -This function causes an immediate core dump. See also the B<-u> -command-line switch in L<perlrun>, which does the same thing. -Primarily this is so that you can use the B<undump> program (not -supplied) to turn your core dump into an executable binary after -having initialized all your variables at the beginning of the -program. When the new binary is executed it will begin by executing -a C<goto LABEL> (with all the restrictions that C<goto> suffers). -Think of it as a goto with an intervening core dump and reincarnation. -If C<LABEL> is omitted, restarts the program from the top. - -B<WARNING>: Any files opened at the time of the dump will I<not> -be open any more when the program is reincarnated, with possible -resulting confusion by Perl. - -This function is now largely obsolete, mostly because it's very hard to -convert a core file into an executable. That's why you should now invoke -it as C<CORE::dump()>, if you don't want to be warned against a possible -typo. - -Portability issues: L<perlport/dump>. - -=item each HASH -X<each> X<hash, iterator> - -=item each ARRAY -X<array, iterator> - -=item each EXPR - -=for Pod::Functions retrieve the next key/value pair from a hash - -When called on a hash in list context, returns a 2-element list -consisting of the key and value for the next element of a hash. In Perl -5.12 and later only, it will also return the index and value for the next -element of an array so that you can iterate over it; older Perls consider -this a syntax error. When called in scalar context, returns only the key -(not the value) in a hash, or the index in an array. - -Hash entries are returned in an apparently random order. The actual random -order is subject to change in future versions of Perl, but it is -guaranteed to be in the same order as either the C<keys> or C<values> -function would produce on the same (unmodified) hash. Since Perl -5.8.2 the ordering can be different even between different runs of Perl -for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). - -After C<each> has returned all entries from the hash or array, the next -call to C<each> returns the empty list in list context and C<undef> in -scalar context; the next call following I<that> one restarts iteration. -Each hash or array has its own internal iterator, accessed by C<each>, -C<keys>, and C<values>. The iterator is implicitly reset when C<each> has -reached the end as just described; it can be explicitly reset by calling -C<keys> or C<values> on the hash or array. If you add or delete a hash's -elements while iterating over it, entries may be skipped or duplicated--so -don't do that. Exception: In the current implementation, it is always safe -to delete the item most recently returned by C<each()>, so the following -code works properly: - - while (($key, $value) = each %hash) { - print $key, "\n"; - delete $hash{$key}; # This is safe - } - -This prints out your environment like the printenv(1) program, -but in a different order: - - while (($key,$value) = each %ENV) { - print "$key=$value\n"; - } - -Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold -reference to an unblessed hash or array. The argument will be dereferenced -automatically. This aspect of C<each> is considered highly experimental. -The exact behaviour may change in a future version of Perl. - - while (($key,$value) = each $hashref) { ... } - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.012; # so keys/values/each work on arrays - use 5.014; # so keys/values/each work on scalars (experimental) - -See also C<keys>, C<values>, and C<sort>. - -=item eof FILEHANDLE -X<eof> -X<end of file> -X<end-of-file> - -=item eof () - -=item eof - -=for Pod::Functions test a filehandle for its end - -Returns 1 if the next read on FILEHANDLE will return end of file I<or> if -FILEHANDLE is not open. FILEHANDLE may be an expression whose value -gives the real filehandle. (Note that this function actually -reads a character and then C<ungetc>s it, so isn't useful in an -interactive context.) Do not read from a terminal file (or call -C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such -as terminals may lose the end-of-file condition if you do. - -An C<eof> without an argument uses the last file read. Using C<eof()> -with empty parentheses is different. It refers to the pseudo file -formed from the files listed on the command line and accessed via the -C<< <> >> operator. Since C<< <> >> isn't explicitly opened, -as a normal filehandle is, an C<eof()> before C<< <> >> has been -used will cause C<@ARGV> to be examined to determine if input is -available. Similarly, an C<eof()> after C<< <> >> has returned -end-of-file will assume you are processing another C<@ARGV> list, -and if you haven't set C<@ARGV>, will read input from C<STDIN>; -see L<perlop/"I/O Operators">. - -In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to -detect the end of each file, whereas C<eof()> will detect the end -of the very last file only. Examples: - - # reset line numbering on each input file - while (<>) { - next if /^\s*#/; # skip comments - print "$.\t$_"; - } continue { - close ARGV if eof; # Not eof()! - } - - # insert dashes just before last line of last file - while (<>) { - if (eof()) { # check for end of last file - print "--------------\n"; - } - print; - last if eof(); # needed if we're reading from a terminal - } - -Practical hint: you almost never need to use C<eof> in Perl, because the -input operators typically return C<undef> when they run out of data or -encounter an error. - -=item eval EXPR -X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute> -X<error, handling> X<exception, handling> - -=item eval BLOCK - -=item eval - -=for Pod::Functions catch exceptions or compile and run code - -In the first form, the return value of EXPR is parsed and executed as if it -were a little Perl program. The value of the expression (which is itself -determined within scalar context) is first parsed, and if there were no -errors, executed as a block within the lexical context of the current Perl -program. This means, that in particular, any outer lexical variables are -visible to it, and any package variable settings or subroutine and format -definitions remain afterwards. - -Note that the value is parsed every time the C<eval> executes. -If EXPR is omitted, evaluates C<$_>. This form is typically used to -delay parsing and subsequent execution of the text of EXPR until run time. - -If the C<unicode_eval> feature is enabled (which is the default under a -C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of -characters, so C<use utf8> declarations have no effect, and source filters -are forbidden. In the absence of the C<unicode_eval> feature, the string -will sometimes be treated as characters and sometimes as bytes, depending -on the internal encoding, and source filters activated within the C<eval> -exhibit the erratic, but historical, behaviour of affecting some outer file -scope that is still compiling. See also the L</evalbytes> keyword, which -always treats its input as a byte stream and works properly with source -filters, and the L<feature> pragma. - -In the second form, the code within the BLOCK is parsed only once--at the -same time the code surrounding the C<eval> itself was parsed--and executed -within the context of the current Perl program. This form is typically -used to trap exceptions more efficiently than the first (see below), while -also providing the benefit of checking the code within BLOCK at compile -time. - -The final semicolon, if any, may be omitted from the value of EXPR or within -the BLOCK. - -In both forms, the value returned is the value of the last expression -evaluated inside the mini-program; a return statement may be also used, just -as with subroutines. The expression providing the return value is evaluated -in void, scalar, or list context, depending on the context of the C<eval> -itself. See L</wantarray> for more on how the evaluation context can be -determined. - -If there is a syntax error or runtime error, or a C<die> statement is -executed, C<eval> returns C<undef> in scalar context -or an empty list in list context, and C<$@> is set to the error -message. (Prior to 5.16, a bug caused C<undef> to be returned -in list context for syntax errors, but not for runtime errors.) -If there was no error, C<$@> is set to the empty string. A -control flow operator like C<last> or C<goto> can bypass the setting of -C<$@>. Beware that using C<eval> neither silences Perl from printing -warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. -To do either of those, you have to use the C<$SIG{__WARN__}> facility, or -turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>. -See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>. - -Note that, because C<eval> traps otherwise-fatal errors, it is useful for -determining whether a particular feature (such as C<socket> or C<symlink>) -is implemented. It is also Perl's exception-trapping mechanism, where -the die operator is used to raise exceptions. - -If you want to trap errors when loading an XS module, some problems with -the binary interface (such as Perl version skew) may be fatal even with -C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>. - -If the code to be executed doesn't vary, you may use the eval-BLOCK -form to trap run-time errors without incurring the penalty of -recompiling each time. The error, if any, is still returned in C<$@>. -Examples: - - # make divide-by-zero nonfatal - eval { $answer = $a / $b; }; warn $@ if $@; - - # same thing, but less efficient - eval '$answer = $a / $b'; warn $@ if $@; - - # a compile-time error - eval { $answer = }; # WRONG - - # a run-time error - eval '$answer ='; # sets $@ - -Using the C<eval{}> form as an exception trap in libraries does have some -issues. Due to the current arguably broken state of C<__DIE__> hooks, you -may wish not to trigger any C<__DIE__> hooks that user code may have installed. -You can use the C<local $SIG{__DIE__}> construct for this purpose, -as this example shows: - - # a private exception trap for divide-by-zero - eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; - warn $@ if $@; - -This is especially significant, given that C<__DIE__> hooks can call -C<die> again, which has the effect of changing their error messages: - - # __DIE__ hooks may modify error messages - { - local $SIG{'__DIE__'} = - sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; - eval { die "foo lives here" }; - print $@ if $@; # prints "bar lives here" - } - -Because this promotes action at a distance, this counterintuitive behavior -may be fixed in a future release. - -With an C<eval>, you should be especially careful to remember what's -being looked at when: - - eval $x; # CASE 1 - eval "$x"; # CASE 2 - - eval '$x'; # CASE 3 - eval { $x }; # CASE 4 - - eval "\$$x++"; # CASE 5 - $$x++; # CASE 6 - -Cases 1 and 2 above behave identically: they run the code contained in -the variable $x. (Although case 2 has misleading double quotes making -the reader wonder what else might be happening (nothing is).) Cases 3 -and 4 likewise behave in the same way: they run the code C<'$x'>, which -does nothing but return the value of $x. (Case 4 is preferred for -purely visual reasons, but it also has the advantage of compiling at -compile-time instead of at run-time.) Case 5 is a place where -normally you I<would> like to use double quotes, except that in this -particular situation, you can just use symbolic references instead, as -in case 6. - -Before Perl 5.14, the assignment to C<$@> occurred before restoration -of localized variables, which means that for your code to run on older -versions, a temporary is required if you want to mask some but not all -errors: - - # alter $@ on nefarious repugnancy only - { - my $e; - { - local $@; # protect existing $@ - eval { test_repugnancy() }; - # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only - $@ =~ /nefarious/ and $e = $@; - } - die $e if defined $e - } - -C<eval BLOCK> does I<not> count as a loop, so the loop control statements -C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. - -An C<eval ''> executed within the C<DB> package doesn't see the usual -surrounding lexical scope, but rather the scope of the first non-DB piece -of code that called it. You don't normally need to worry about this unless -you are writing a Perl debugger. - -=item evalbytes EXPR -X<evalbytes> - -=item evalbytes - -=for Pod::Functions +evalbytes similar to string eval, but intend to parse a bytestream - -This function is like L</eval> with a string argument, except it always -parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A -string containing characters whose ordinal value exceeds 255 results in an -error. Source filters activated within the evaluated code apply to the -code itself. - -This function is only available under the C<evalbytes> feature, a -C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See -L<feature> for more information. - -=item exec LIST -X<exec> X<execute> - -=item exec PROGRAM LIST - -=for Pod::Functions abandon this program to run another - -The C<exec> function executes a system command I<and never returns>; -use C<system> instead of C<exec> if you want it to return. It fails and -returns false only if the command does not exist I<and> it is executed -directly instead of via your system's command shell (see below). - -Since it's a common mistake to use C<exec> instead of C<system>, Perl -warns you if C<exec> is called in void context and if there is a following -statement that isn't C<die>, C<warn>, or C<exit> (if C<-w> is set--but -you always do that, right?). If you I<really> want to follow an C<exec> -with some other statement, you can use one of these styles to avoid the warning: - - exec ('foo') or print STDERR "couldn't exec foo: $!"; - { exec ('foo') }; print STDERR "couldn't exec foo: $!"; - -If there is more than one argument in LIST, or if LIST is an array -with more than one value, calls execvp(3) with the arguments in LIST. -If there is only one scalar argument or an array with one element in it, -the argument is checked for shell metacharacters, and if there are any, -the entire argument is passed to the system's command shell for parsing -(this is C</bin/sh -c> on Unix platforms, but varies on other platforms). -If there are no shell metacharacters in the argument, it is split into -words and passed directly to C<execvp>, which is more efficient. -Examples: - - exec '/bin/echo', 'Your arguments are: ', @ARGV; - exec "sort $outfile | uniq"; - -If you don't really want to execute the first argument, but want to lie -to the program you are executing about its own name, you can specify -the program you actually want to run as an "indirect object" (without a -comma) in front of the LIST. (This always forces interpretation of the -LIST as a multivalued list, even if there is only a single scalar in -the list.) Example: - - $shell = '/bin/csh'; - exec $shell '-sh'; # pretend it's a login shell - -or, more directly, - - exec {'/bin/csh'} '-sh'; # pretend it's a login shell - -When the arguments get executed via the system shell, results are -subject to its quirks and capabilities. See L<perlop/"`STRING`"> -for details. - -Using an indirect object with C<exec> or C<system> is also more -secure. This usage (which also works fine with system()) forces -interpretation of the arguments as a multivalued list, even if the -list had just one argument. That way you're safe from the shell -expanding wildcards or splitting up words with whitespace in them. - - @args = ( "echo surprise" ); - - exec @args; # subject to shell escapes - # if @args == 1 - exec { $args[0] } @args; # safe even with one-arg list - -The first version, the one without the indirect object, ran the I<echo> -program, passing it C<"surprise"> an argument. The second version didn't; -it tried to run a program named I<"echo surprise">, didn't find it, and set -C<$?> to a non-zero value indicating failure. - -Beginning with v5.6.0, Perl attempts to flush all files opened for -output before the exec, but this may not be supported on some platforms -(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH -in English) or call the C<autoflush()> method of C<IO::Handle> on any -open handles to avoid lost output. - -Note that C<exec> will not call your C<END> blocks, nor will it invoke -C<DESTROY> methods on your objects. - -Portability issues: L<perlport/exec>. - -=item exists EXPR -X<exists> X<autovivification> - -=for Pod::Functions test whether a hash key is present - -Given an expression that specifies an element of a hash, returns true if the -specified element in the hash has ever been initialized, even if the -corresponding value is undefined. - - print "Exists\n" if exists $hash{$key}; - print "Defined\n" if defined $hash{$key}; - print "True\n" if $hash{$key}; - -exists may also be called on array elements, but its behavior is much less -obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware> -that calling exists on array values is deprecated and likely to be removed in -a future version of Perl. - - print "Exists\n" if exists $array[$index]; - print "Defined\n" if defined $array[$index]; - print "True\n" if $array[$index]; - -A hash or array element can be true only if it's defined and defined only if -it exists, but the reverse doesn't necessarily hold true. - -Given an expression that specifies the name of a subroutine, -returns true if the specified subroutine has ever been declared, even -if it is undefined. Mentioning a subroutine name for exists or defined -does not count as declaring it. Note that a subroutine that does not -exist may still be callable: its package may have an C<AUTOLOAD> -method that makes it spring into existence the first time that it is -called; see L<perlsub>. - - print "Exists\n" if exists &subroutine; - print "Defined\n" if defined &subroutine; - -Note that the EXPR can be arbitrarily complicated as long as the final -operation is a hash or array key lookup or subroutine name: - - if (exists $ref->{A}->{B}->{$key}) { } - if (exists $hash{A}{B}{$key}) { } - - if (exists $ref->{A}->{B}->[$ix]) { } - if (exists $hash{A}{B}[$ix]) { } - - if (exists &{$ref->{A}{B}{$key}}) { } - -Although the most deeply nested array or hash element will not spring into -existence just because its existence was tested, any intervening ones will. -Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring -into existence due to the existence test for the $key element above. -This happens anywhere the arrow operator is used, including even here: - - undef $ref; - if (exists $ref->{"Some key"}) { } - print $ref; # prints HASH(0x80d3d5c) - -This surprising autovivification in what does not at first--or even -second--glance appear to be an lvalue context may be fixed in a future -release. - -Use of a subroutine call, rather than a subroutine name, as an argument -to exists() is an error. - - exists ⊂ # OK - exists &sub(); # Error - -=item exit EXPR -X<exit> X<terminate> X<abort> - -=item exit - -=for Pod::Functions terminate this program - -Evaluates EXPR and exits immediately with that value. Example: - - $ans = <STDIN>; - exit 0 if $ans =~ /^[Xx]/; - -See also C<die>. If EXPR is omitted, exits with C<0> status. The only -universally recognized values for EXPR are C<0> for success and C<1> -for error; other values are subject to interpretation depending on the -environment in which the Perl program is running. For example, exiting -69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause -the mailer to return the item undelivered, but that's not true everywhere. - -Don't use C<exit> to abort a subroutine if there's any chance that -someone might want to trap whatever error happened. Use C<die> instead, -which can be trapped by an C<eval>. - -The exit() function does not always exit immediately. It calls any -defined C<END> routines first, but these C<END> routines may not -themselves abort the exit. Likewise any object destructors that need to -be called are called before the real exit. C<END> routines and destructors -can change the exit status by modifying C<$?>. If this is a problem, you -can call C<POSIX::_exit($status)> to avoid END and destructor processing. -See L<perlmod> for details. - -Portability issues: L<perlport/exit>. - -=item exp EXPR -X<exp> X<exponential> X<antilog> X<antilogarithm> X<e> - -=item exp - -=for Pod::Functions raise I<e> to a power - -Returns I<e> (the natural logarithm base) to the power of EXPR. -If EXPR is omitted, gives C<exp($_)>. - -=item fc EXPR -X<fc> X<foldcase> X<casefold> X<fold-case> X<case-fold> - -=item fc - -=for Pod::Functions +fc return casefolded version of a string - -Returns the casefolded version of EXPR. This is the internal function -implementing the C<\F> escape in double-quoted strings. - -Casefolding is the process of mapping strings to a form where case -differences are erased; comparing two strings in their casefolded -form is effectively a way of asking if two strings are equal, -regardless of case. - -Roughly, if you ever found yourself writing this - - lc($this) eq lc($that) # Wrong! - # or - uc($this) eq uc($that) # Also wrong! - # or - $this =~ /\Q$that/i # Right! - -Now you can write - - fc($this) eq fc($that) - -And get the correct results. - -Perl only implements the full form of casefolding. -For further information on casefolding, refer to -the Unicode Standard, specifically sections 3.13 C<Default Case Operations>, -4.2 C<Case-Normative>, and 5.18 C<Case Mappings>, -available at L<http://www.unicode.org/versions/latest/>, as well as the -Case Charts available at L<http://www.unicode.org/charts/case/>. - -If EXPR is omitted, uses C<$_>. - -This function behaves the same way under various pragma, such as in a locale, -as L</lc> does. - -While the Unicode Standard defines two additional forms of casefolding, -one for Turkic languages and one that never maps one character into multiple -characters, these are not provided by the Perl core; However, the CPAN module -C<Unicode::Casing> may be used to provide an implementation. - -This keyword is available only when the C<"fc"> feature is enabled, -or when prefixed with C<CORE::>; See L<feature>. Alternately, -include a C<use v5.16> or later to the current scope. - -=item fcntl FILEHANDLE,FUNCTION,SCALAR -X<fcntl> - -=for Pod::Functions file control system call - -Implements the fcntl(2) function. You'll probably have to say - - use Fcntl; - -first to get the correct constant definitions. Argument processing and -value returned work just like C<ioctl> below. -For example: - - use Fcntl; - fcntl($filehandle, F_GETFL, $packed_return_buffer) - or die "can't fcntl F_GETFL: $!"; - -You don't have to check for C<defined> on the return from C<fcntl>. -Like C<ioctl>, it maps a C<0> return from the system call into -C<"0 but true"> in Perl. This string is true in boolean context and C<0> -in numeric context. It is also exempt from the normal B<-w> warnings -on improper numeric conversions. - -Note that C<fcntl> raises an exception if used on a machine that -doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) -manpage to learn what functions are available on your system. - -Here's an example of setting a filehandle named C<REMOTE> to be -non-blocking at the system level. You'll have to negotiate C<$|> -on your own, though. - - use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); - - $flags = fcntl(REMOTE, F_GETFL, 0) - or die "Can't get flags for the socket: $!\n"; - - $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) - or die "Can't set flags for the socket: $!\n"; - -Portability issues: L<perlport/fcntl>. - -=item __FILE__ -X<__FILE__> - -=for Pod::Functions the name of the current source file - -A special token that returns the name of the file in which it occurs. - -=item fileno FILEHANDLE -X<fileno> - -=for Pod::Functions return file descriptor from filehandle - -Returns the file descriptor for a filehandle, or undefined if the -filehandle is not open. If there is no real file descriptor at the OS -level, as can happen with filehandles connected to memory objects via -C<open> with a reference for the third argument, -1 is returned. - -This is mainly useful for constructing -bitmaps for C<select> and low-level POSIX tty-handling operations. -If FILEHANDLE is an expression, the value is taken as an indirect -filehandle, generally its name. - -You can use this to find out whether two handles refer to the -same underlying descriptor: - - if (fileno(THIS) == fileno(THAT)) { - print "THIS and THAT are dups\n"; - } - -=item flock FILEHANDLE,OPERATION -X<flock> X<lock> X<locking> - -=for Pod::Functions lock an entire file with an advisory lock - -Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true -for success, false on failure. Produces a fatal error if used on a -machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). -C<flock> is Perl's portable file-locking interface, although it locks -entire files only, not records. - -Two potentially non-obvious but traditional C<flock> semantics are -that it waits indefinitely until the lock is granted, and that its locks -are B<merely advisory>. Such discretionary locks are more flexible, but -offer fewer guarantees. This means that programs that do not also use -C<flock> may modify files locked with C<flock>. See L<perlport>, -your port's specific documentation, and your system-specific local manpages -for details. It's best to assume traditional behavior if you're writing -portable programs. (But if you're not, you should as always feel perfectly -free to write for your own system's idiosyncrasies (sometimes called -"features"). Slavish adherence to portability concerns shouldn't get -in the way of your getting your job done.) - -OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with -LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but -you can use the symbolic names if you import them from the L<Fcntl> module, -either individually, or as a group using the C<:flock> tag. LOCK_SH -requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN -releases a previously requested lock. If LOCK_NB is bitwise-or'ed with -LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking -waiting for the lock; check the return status to see if you got it. - -To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE -before locking or unlocking it. - -Note that the emulation built with lockf(3) doesn't provide shared -locks, and it requires that FILEHANDLE be open with write intent. These -are the semantics that lockf(3) implements. Most if not all systems -implement lockf(3) in terms of fcntl(2) locking, though, so the -differing semantics shouldn't bite too many people. - -Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE -be open with read intent to use LOCK_SH and requires that it be open -with write intent to use LOCK_EX. - -Note also that some versions of C<flock> cannot lock things over the -network; you would need to use the more system-specific C<fcntl> for -that. If you like you can force Perl to ignore your system's flock(2) -function, and so provide its own fcntl(2)-based emulation, by passing -the switch C<-Ud_flock> to the F<Configure> program when you configure -and build a new Perl. - -Here's a mailbox appender for BSD systems. - - use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants - - sub lock { - my ($fh) = @_; - flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n"; - - # and, in case someone appended while we were waiting... - seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n"; - } - - sub unlock { - my ($fh) = @_; - flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n"; - } - - open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}") - or die "Can't open mailbox: $!"; - - lock($mbox); - print $mbox $msg,"\n\n"; - unlock($mbox); - -On systems that support a real flock(2), locks are inherited across fork() -calls, whereas those that must resort to the more capricious fcntl(2) -function lose their locks, making it seriously harder to write servers. - -See also L<DB_File> for other flock() examples. - -Portability issues: L<perlport/flock>. - -=item fork -X<fork> X<child> X<parent> - -=for Pod::Functions create a new process just like this one - -Does a fork(2) system call to create a new process running the -same program at the same point. It returns the child pid to the -parent process, C<0> to the child process, or C<undef> if the fork is -unsuccessful. File descriptors (and sometimes locks on those descriptors) -are shared, while everything else is copied. On most systems supporting -fork(), great care has gone into making it extremely efficient (for -example, using copy-on-write technology on data pages), making it the -dominant paradigm for multitasking over the last few decades. - -Beginning with v5.6.0, Perl attempts to flush all files opened for -output before forking the child process, but this may not be supported -on some platforms (see L<perlport>). To be safe, you may need to set -C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of -C<IO::Handle> on any open handles to avoid duplicate output. - -If you C<fork> without ever waiting on your children, you will -accumulate zombies. On some systems, you can avoid this by setting -C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of -forking and reaping moribund children. - -Note that if your forked child inherits system file descriptors like -STDIN and STDOUT that are actually connected by a pipe or socket, even -if you exit, then the remote server (such as, say, a CGI script or a -backgrounded job launched from a remote shell) won't think you're done. -You should reopen those to F</dev/null> if it's any issue. - -On some platforms such as Windows, where the fork() system call is not available, -Perl can be built to emulate fork() in the Perl interpreter. -The emulation is designed, at the level of the Perl program, -to be as compatible as possible with the "Unix" fork(). -However it has limitations that have to be considered in code intended to be portable. -See L<perlfork> for more details. - -Portability issues: L<perlport/fork>. - -=item format -X<format> - -=for Pod::Functions declare a picture format with use by the write() function - -Declare a picture format for use by the C<write> function. For -example: - - format Something = - Test: @<<<<<<<< @||||| @>>>>> - $str, $%, '$' . int($num) - . - - $str = "widget"; - $num = $cost/$quantity; - $~ = 'Something'; - write; - -See L<perlform> for many details and examples. - -=item formline PICTURE,LIST -X<formline> - -=for Pod::Functions internal function used for formats - -This is an internal function used by C<format>s, though you may call it, -too. It formats (see L<perlform>) a list of values according to the -contents of PICTURE, placing the output into the format output -accumulator, C<$^A> (or C<$ACCUMULATOR> in English). -Eventually, when a C<write> is done, the contents of -C<$^A> are written to some filehandle. You could also read C<$^A> -and then set C<$^A> back to C<"">. Note that a format typically -does one C<formline> per line of form, but the C<formline> function itself -doesn't care how many newlines are embedded in the PICTURE. This means -that the C<~> and C<~~> tokens treat the entire PICTURE as a single line. -You may therefore need to use multiple formlines to implement a single -record format, just like the C<format> compiler. - -Be careful if you put double quotes around the picture, because an C<@> -character may be taken to mean the beginning of an array name. -C<formline> always returns true. See L<perlform> for other examples. - -If you are trying to use this instead of C<write> to capture the output, -you may find it easier to open a filehandle to a scalar -(C<< open $fh, ">", \$output >>) and write to that instead. - -=item getc FILEHANDLE -X<getc> X<getchar> X<character> X<file, read> - -=item getc - -=for Pod::Functions get the next character from the filehandle - -Returns the next character from the input file attached to FILEHANDLE, -or the undefined value at end of file or if there was an error (in -the latter case C<$!> is set). If FILEHANDLE is omitted, reads from -STDIN. This is not particularly efficient. However, it cannot be -used by itself to fetch single characters without waiting for the user -to hit enter. For that, try something more like: - - if ($BSD_STYLE) { - system "stty cbreak </dev/tty >/dev/tty 2>&1"; - } - else { - system "stty", '-icanon', 'eol', "\001"; - } - - $key = getc(STDIN); - - if ($BSD_STYLE) { - system "stty -cbreak </dev/tty >/dev/tty 2>&1"; - } - else { - system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL - } - print "\n"; - -Determination of whether $BSD_STYLE should be set -is left as an exercise to the reader. - -The C<POSIX::getattr> function can do this more portably on -systems purporting POSIX compliance. See also the C<Term::ReadKey> -module from your nearest CPAN site; details on CPAN can be found under -L<perlmodlib/CPAN>. - -=item getlogin -X<getlogin> X<login> - -=for Pod::Functions return who logged in at this tty - -This implements the C library function of the same name, which on most -systems returns the current login from F</etc/utmp>, if any. If it -returns the empty string, use C<getpwuid>. - - $login = getlogin || getpwuid($<) || "Kilroy"; - -Do not consider C<getlogin> for authentication: it is not as -secure as C<getpwuid>. - -Portability issues: L<perlport/getlogin>. - -=item getpeername SOCKET -X<getpeername> X<peer> - -=for Pod::Functions find the other end of a socket connection - -Returns the packed sockaddr address of the other end of the SOCKET -connection. - - use Socket; - $hersockaddr = getpeername(SOCK); - ($port, $iaddr) = sockaddr_in($hersockaddr); - $herhostname = gethostbyaddr($iaddr, AF_INET); - $herstraddr = inet_ntoa($iaddr); - -=item getpgrp PID -X<getpgrp> X<group> - -=for Pod::Functions get process group - -Returns the current process group for the specified PID. Use -a PID of C<0> to get the current process group for the -current process. Will raise an exception if used on a machine that -doesn't implement getpgrp(2). If PID is omitted, returns the process -group of the current process. Note that the POSIX version of C<getpgrp> -does not accept a PID argument, so only C<PID==0> is truly portable. - -Portability issues: L<perlport/getpgrp>. - -=item getppid -X<getppid> X<parent> X<pid> - -=for Pod::Functions get parent process ID - -Returns the process id of the parent process. - -Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work -around non-POSIX thread semantics the minority of Linux systems (and -Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation -has since been removed. See the documentation for L<$$|perlvar/$$> for -details. - -Portability issues: L<perlport/getppid>. - -=item getpriority WHICH,WHO -X<getpriority> X<priority> X<nice> - -=for Pod::Functions get current nice value - -Returns the current priority for a process, a process group, or a user. -(See L<getpriority(2)>.) Will raise a fatal exception if used on a -machine that doesn't implement getpriority(2). - -Portability issues: L<perlport/getpriority>. - -=item getpwnam NAME -X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname> -X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr> -X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent> -X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent> -X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent> -X<endnetent> X<endprotoent> X<endservent> - -=for Pod::Functions get passwd record given user login name - -=item getgrnam NAME - -=for Pod::Functions get group record given group name - -=item gethostbyname NAME - -=for Pod::Functions get host record given name - -=item getnetbyname NAME - -=for Pod::Functions get networks record given name - -=item getprotobyname NAME - -=for Pod::Functions get protocol record given name - -=item getpwuid UID - -=for Pod::Functions get passwd record given user ID - -=item getgrgid GID - -=for Pod::Functions get group record given group user ID - -=item getservbyname NAME,PROTO - -=for Pod::Functions get services record given its name - -=item gethostbyaddr ADDR,ADDRTYPE - -=for Pod::Functions get host record given its address - -=item getnetbyaddr ADDR,ADDRTYPE - -=for Pod::Functions get network record given its address - -=item getprotobynumber NUMBER - -=for Pod::Functions get protocol record numeric protocol - -=item getservbyport PORT,PROTO - -=for Pod::Functions get services record given numeric port - -=item getpwent - -=for Pod::Functions get next passwd record - -=item getgrent - -=for Pod::Functions get next group record - -=item gethostent - -=for Pod::Functions get next hosts record - -=item getnetent - -=for Pod::Functions get next networks record - -=item getprotoent - -=for Pod::Functions get next protocols record - -=item getservent - -=for Pod::Functions get next services record - -=item setpwent - -=for Pod::Functions prepare passwd file for use - -=item setgrent - -=for Pod::Functions prepare group file for use - -=item sethostent STAYOPEN - -=for Pod::Functions prepare hosts file for use - -=item setnetent STAYOPEN - -=for Pod::Functions prepare networks file for use - -=item setprotoent STAYOPEN - -=for Pod::Functions prepare protocols file for use - -=item setservent STAYOPEN - -=for Pod::Functions prepare services file for use - -=item endpwent - -=for Pod::Functions be done using passwd file - -=item endgrent - -=for Pod::Functions be done using group file - -=item endhostent - -=for Pod::Functions be done using hosts file - -=item endnetent - -=for Pod::Functions be done using networks file - -=item endprotoent - -=for Pod::Functions be done using protocols file - -=item endservent - -=for Pod::Functions be done using services file - -These routines are the same as their counterparts in the -system C library. In list context, the return values from the -various get routines are as follows: - - ($name,$passwd,$uid,$gid, - $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* - ($name,$passwd,$gid,$members) = getgr* - ($name,$aliases,$addrtype,$length,@addrs) = gethost* - ($name,$aliases,$addrtype,$net) = getnet* - ($name,$aliases,$proto) = getproto* - ($name,$aliases,$port,$proto) = getserv* - -(If the entry doesn't exist you get an empty list.) - -The exact meaning of the $gcos field varies but it usually contains -the real name of the user (as opposed to the login name) and other -information pertaining to the user. Beware, however, that in many -system users are able to change this information and therefore it -cannot be trusted and therefore the $gcos is tainted (see -L<perlsec>). The $passwd and $shell, user's encrypted password and -login shell, are also tainted, for the same reason. - -In scalar context, you get the name, unless the function was a -lookup by name, in which case you get the other thing, whatever it is. -(If the entry doesn't exist you get the undefined value.) For example: - - $uid = getpwnam($name); - $name = getpwuid($num); - $name = getpwent(); - $gid = getgrnam($name); - $name = getgrgid($num); - $name = getgrent(); - #etc. - -In I<getpw*()> the fields $quota, $comment, and $expire are special -in that they are unsupported on many systems. If the -$quota is unsupported, it is an empty scalar. If it is supported, it -usually encodes the disk quota. If the $comment field is unsupported, -it is an empty scalar. If it is supported it usually encodes some -administrative comment about the user. In some systems the $quota -field may be $change or $age, fields that have to do with password -aging. In some systems the $comment field may be $class. The $expire -field, if present, encodes the expiration period of the account or the -password. For the availability and the exact meaning of these fields -in your system, please consult getpwnam(3) and your system's -F<pwd.h> file. You can also find out from within Perl what your -$quota and $comment fields mean and whether you have the $expire field -by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, -C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password -files are supported only if your vendor has implemented them in the -intuitive fashion that calling the regular C library routines gets the -shadow versions if you're running under privilege or if there exists -the shadow(3) functions as found in System V (this includes Solaris -and Linux). Those systems that implement a proprietary shadow password -facility are unlikely to be supported. - -The $members value returned by I<getgr*()> is a space-separated list of -the login names of the members of the group. - -For the I<gethost*()> functions, if the C<h_errno> variable is supported in -C, it will be returned to you via C<$?> if the function call fails. The -C<@addrs> value returned by a successful call is a list of raw -addresses returned by the corresponding library call. In the -Internet domain, each address is four bytes long; you can unpack it -by saying something like: - - ($a,$b,$c,$d) = unpack('W4',$addr[0]); - -The Socket library makes this slightly easier: - - use Socket; - $iaddr = inet_aton("127.1"); # or whatever address - $name = gethostbyaddr($iaddr, AF_INET); - - # or going the other way - $straddr = inet_ntoa($iaddr); - -In the opposite way, to resolve a hostname to the IP address -you can write this: - - use Socket; - $packed_ip = gethostbyname("www.perl.org"); - if (defined $packed_ip) { - $ip_address = inet_ntoa($packed_ip); - } - -Make sure C<gethostbyname()> is called in SCALAR context and that -its return value is checked for definedness. - -The C<getprotobynumber> function, even though it only takes one argument, -has the precedence of a list operator, so beware: - - getprotobynumber $number eq 'icmp' # WRONG - getprotobynumber($number eq 'icmp') # actually means this - getprotobynumber($number) eq 'icmp' # better this way - -If you get tired of remembering which element of the return list -contains which return value, by-name interfaces are provided -in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, -C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, -and C<User::grent>. These override the normal built-ins, supplying -versions that return objects with the appropriate names -for each field. For example: - - use File::stat; - use User::pwent; - $is_his = (stat($filename)->uid == pwent($whoever)->uid); - -Even though it looks as though they're the same method calls (uid), -they aren't, because a C<File::stat> object is different from -a C<User::pwent> object. - -Portability issues: L<perlport/getpwnam> to L<perlport/endservent>. - -=item getsockname SOCKET -X<getsockname> - -=for Pod::Functions retrieve the sockaddr for a given socket - -Returns the packed sockaddr address of this end of the SOCKET connection, -in case you don't know the address because you have several different -IPs that the connection might have come in on. - - use Socket; - $mysockaddr = getsockname(SOCK); - ($port, $myaddr) = sockaddr_in($mysockaddr); - printf "Connect to %s [%s]\n", - scalar gethostbyaddr($myaddr, AF_INET), - inet_ntoa($myaddr); - -=item getsockopt SOCKET,LEVEL,OPTNAME -X<getsockopt> - -=for Pod::Functions get socket options on a given socket - -Queries the option named OPTNAME associated with SOCKET at a given LEVEL. -Options may exist at multiple protocol levels depending on the socket -type, but at least the uppermost socket level SOL_SOCKET (defined in the -C<Socket> module) will exist. To query options at another level the -protocol number of the appropriate protocol controlling the option -should be supplied. For example, to indicate that an option is to be -interpreted by the TCP protocol, LEVEL should be set to the protocol -number of TCP, which you can get using C<getprotobyname>. - -The function returns a packed string representing the requested socket -option, or C<undef> on error, with the reason for the error placed in -C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME; -consult getsockopt(2) for details. A common case is that the option is an -integer, in which case the result is a packed integer, which you can decode -using C<unpack> with the C<i> (or C<I>) format. - -Here's an example to test whether Nagle's algorithm is enabled on a socket: - - use Socket qw(:all); - - defined(my $tcp = getprotobyname("tcp")) - or die "Could not determine the protocol number for tcp"; - # my $tcp = IPPROTO_TCP; # Alternative - my $packed = getsockopt($socket, $tcp, TCP_NODELAY) - or die "getsockopt TCP_NODELAY: $!"; - my $nodelay = unpack("I", $packed); - print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n"; - -Portability issues: L<perlport/getsockopt>. - -=item glob EXPR -X<glob> X<wildcard> X<filename, expansion> X<expand> - -=item glob - -=for Pod::Functions expand filenames using wildcards - -In list context, returns a (possibly empty) list of filename expansions on -the value of EXPR such as the standard Unix shell F</bin/csh> would do. In -scalar context, glob iterates through such filename expansions, returning -undef when the list is exhausted. This is the internal function -implementing the C<< <*.c> >> operator, but you can use it directly. If -EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in -more detail in L<perlop/"I/O Operators">. - -Note that C<glob> splits its arguments on whitespace and treats -each segment as separate pattern. As such, C<glob("*.c *.h")> -matches all files with a F<.c> or F<.h> extension. The expression -C<glob(".* *")> matches all files in the current working directory. -If you want to glob filenames that might contain whitespace, you'll -have to use extra quotes around the spacey filename to protect it. -For example, to glob filenames that have an C<e> followed by a space -followed by an C<f>, use either of: - - @spacies = <"*e f*">; - @spacies = glob '"*e f*"'; - @spacies = glob q("*e f*"); - -If you had to get a variable through, you could do this: - - @spacies = glob "'*${var}e f*'"; - @spacies = glob qq("*${var}e f*"); - -If non-empty braces are the only wildcard characters used in the -C<glob>, no filenames are matched, but potentially many strings -are returned. For example, this produces nine strings, one for -each pairing of fruits and colors: - - @many = glob "{apple,tomato,cherry}={green,yellow,red}"; - -Beginning with v5.6.0, this operator is implemented using the standard -C<File::Glob> extension. See L<File::Glob> for details, including -C<bsd_glob> which does not treat whitespace as a pattern separator. - -Portability issues: L<perlport/glob>. - -=item gmtime EXPR -X<gmtime> X<UTC> X<Greenwich> - -=item gmtime - -=for Pod::Functions convert UNIX time into record or string using Greenwich time - -Works just like L</localtime> but the returned values are -localized for the standard Greenwich time zone. - -Note: When called in list context, $isdst, the last value -returned by gmtime, is always C<0>. There is no -Daylight Saving Time in GMT. - -Portability issues: L<perlport/gmtime>. - -=item goto LABEL -X<goto> X<jump> X<jmp> - -=item goto EXPR - -=item goto &NAME - -=for Pod::Functions create spaghetti code - -The C<goto-LABEL> form finds the statement labeled with LABEL and -resumes execution there. It can't be used to get out of a block or -subroutine given to C<sort>. It can be used to go almost anywhere -else within the dynamic scope, including out of subroutines, but it's -usually better to use some other construct such as C<last> or C<die>. -The author of Perl has never felt the need to use this form of C<goto> -(in Perl, that is; C is another matter). (The difference is that C -does not offer named loops combined with loop control. Perl does, and -this replaces most structured uses of C<goto> in other languages.) - -The C<goto-EXPR> form expects a label name, whose scope will be resolved -dynamically. This allows for computed C<goto>s per FORTRAN, but isn't -necessarily recommended if you're optimizing for maintainability: - - goto ("FOO", "BAR", "GLARCH")[$i]; - -As shown in this example, C<goto-EXPR> is exempt from the "looks like a -function" rule. A pair of parentheses following it does not (necessarily) -delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>. - -Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is -deprecated and will issue a warning. Even then, it may not be used to -go into any construct that requires initialization, such as a -subroutine or a C<foreach> loop. It also can't be used to go into a -construct that is optimized away. - -The C<goto-&NAME> form is quite different from the other forms of -C<goto>. In fact, it isn't a goto in the normal sense at all, and -doesn't have the stigma associated with other gotos. Instead, it -exits the current subroutine (losing any changes set by local()) and -immediately calls in its place the named subroutine using the current -value of @_. This is used by C<AUTOLOAD> subroutines that wish to -load another subroutine and then pretend that the other subroutine had -been called in the first place (except that any modifications to C<@_> -in the current subroutine are propagated to the other subroutine.) -After the C<goto>, not even C<caller> will be able to tell that this -routine was called first. - -NAME needn't be the name of a subroutine; it can be a scalar variable -containing a code reference or a block that evaluates to a code -reference. - -=item grep BLOCK LIST -X<grep> - -=item grep EXPR,LIST - -=for Pod::Functions locate elements in a list test true against a given criterion - -This is similar in spirit to, but not the same as, grep(1) and its -relatives. In particular, it is not limited to using regular expressions. - -Evaluates the BLOCK or EXPR for each element of LIST (locally setting -C<$_> to each element) and returns the list value consisting of those -elements for which the expression evaluated to true. In scalar -context, returns the number of times the expression was true. - - @foo = grep(!/^#/, @bar); # weed out comments - -or equivalently, - - @foo = grep {!/^#/} @bar; # weed out comments - -Note that C<$_> is an alias to the list value, so it can be used to -modify the elements of the LIST. While this is useful and supported, -it can cause bizarre results if the elements of LIST are not variables. -Similarly, grep returns aliases into the original list, much as a for -loop's index variable aliases the list elements. That is, modifying an -element of a list returned by grep (for example, in a C<foreach>, C<map> -or another C<grep>) actually modifies the element in the original list. -This is usually something to be avoided when writing clear code. - -If C<$_> is lexical in the scope where the C<grep> appears (because it has -been declared with C<my $_>) then, in addition to being locally aliased to -the list elements, C<$_> keeps being lexical inside the block; i.e., it -can't be seen from the outside, avoiding any potential side-effects. - -See also L</map> for a list composed of the results of the BLOCK or EXPR. - -=item hex EXPR -X<hex> X<hexadecimal> - -=item hex - -=for Pod::Functions convert a string to a hexadecimal number - -Interprets EXPR as a hex string and returns the corresponding value. -(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see -L</oct>.) If EXPR is omitted, uses C<$_>. - - print hex '0xAf'; # prints '175' - print hex 'aF'; # same - -Hex strings may only represent integers. Strings that would cause -integer overflow trigger a warning. Leading whitespace is not stripped, -unlike oct(). To present something as hex, look into L</printf>, -L</sprintf>, and L</unpack>. - -=item import LIST -X<import> - -=for Pod::Functions patch a module's namespace into your own - -There is no builtin C<import> function. It is just an ordinary -method (subroutine) defined (or inherited) by modules that wish to export -names to another module. The C<use> function calls the C<import> method -for the package used. See also L</use>, L<perlmod>, and L<Exporter>. - -=item index STR,SUBSTR,POSITION -X<index> X<indexOf> X<InStr> - -=item index STR,SUBSTR - -=for Pod::Functions find a substring within a string - -The index function searches for one string within another, but without -the wildcard-like behavior of a full regular-expression pattern match. -It returns the position of the first occurrence of SUBSTR in STR at -or after POSITION. If POSITION is omitted, starts searching from the -beginning of the string. POSITION before the beginning of the string -or after its end is treated as if it were the beginning or the end, -respectively. POSITION and the return value are based at zero. -If the substring is not found, C<index> returns -1. - -=item int EXPR -X<int> X<integer> X<truncate> X<trunc> X<floor> - -=item int - -=for Pod::Functions get the integer portion of a number - -Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. -You should not use this function for rounding: one because it truncates -towards C<0>, and two because machine representations of floating-point -numbers can sometimes produce counterintuitive results. For example, -C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's -because it's really more like -268.99999999999994315658 instead. Usually, -the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> -functions will serve you better than will int(). - -=item ioctl FILEHANDLE,FUNCTION,SCALAR -X<ioctl> - -=for Pod::Functions system-dependent device control system call - -Implements the ioctl(2) function. You'll probably first have to say - - require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph - -to get the correct function definitions. If F<sys/ioctl.ph> doesn't -exist or doesn't have the correct definitions you'll have to roll your -own, based on your C header files such as F<< <sys/ioctl.h> >>. -(There is a Perl script called B<h2ph> that comes with the Perl kit that -may help you in this, but it's nontrivial.) SCALAR will be read and/or -written depending on the FUNCTION; a C pointer to the string value of SCALAR -will be passed as the third argument of the actual C<ioctl> call. (If SCALAR -has no string value but does have a numeric value, that value will be -passed rather than a pointer to the string value. To guarantee this to be -true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> -functions may be needed to manipulate the values of structures used by -C<ioctl>. - -The return value of C<ioctl> (and C<fcntl>) is as follows: - - if OS returns: then Perl returns: - -1 undefined value - 0 string "0 but true" - anything else that number - -Thus Perl returns true on success and false on failure, yet you can -still easily determine the actual value returned by the operating -system: - - $retval = ioctl(...) || -1; - printf "System returned %d\n", $retval; - -The special string C<"0 but true"> is exempt from B<-w> complaints -about improper numeric conversions. - -Portability issues: L<perlport/ioctl>. - -=item join EXPR,LIST -X<join> - -=for Pod::Functions join a list into a string using a separator - -Joins the separate strings of LIST into a single string with fields -separated by the value of EXPR, and returns that new string. Example: - - $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); - -Beware that unlike C<split>, C<join> doesn't take a pattern as its -first argument. Compare L</split>. - -=item keys HASH -X<keys> X<key> - -=item keys ARRAY - -=item keys EXPR - -=for Pod::Functions retrieve list of indices from a hash - -Called in list context, returns a list consisting of all the keys of the -named hash, or in Perl 5.12 or later only, the indices of an array. Perl -releases prior to 5.12 will produce a syntax error if you try to use an -array argument. In scalar context, returns the number of keys or indices. - -The keys of a hash are returned in an apparently random order. The actual -random order is subject to change in future versions of Perl, but it -is guaranteed to be the same order as either the C<values> or C<each> -function produces (given that the hash has not been modified). Since -Perl 5.8.1 the ordering can be different even between different runs of -Perl for security reasons (see L<perlsec/"Algorithmic Complexity -Attacks">). - -As a side effect, calling keys() resets the internal interator of the HASH or ARRAY -(see L</each>). In particular, calling keys() in void context resets -the iterator with no other overhead. - -Here is yet another way to print your environment: - - @keys = keys %ENV; - @values = values %ENV; - while (@keys) { - print pop(@keys), '=', pop(@values), "\n"; - } - -or how about sorted by key: - - foreach $key (sort(keys %ENV)) { - print $key, '=', $ENV{$key}, "\n"; - } - -The returned values are copies of the original keys in the hash, so -modifying them will not affect the original hash. Compare L</values>. - -To sort a hash by value, you'll need to use a C<sort> function. -Here's a descending numeric sort of a hash by its values: - - foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { - printf "%4d %s\n", $hash{$key}, $key; - } - -Used as an lvalue, C<keys> allows you to increase the number of hash buckets -allocated for the given hash. This can gain you a measure of efficiency if -you know the hash is going to get big. (This is similar to pre-extending -an array by assigning a larger number to $#array.) If you say - - keys %hash = 200; - -then C<%hash> will have at least 200 buckets allocated for it--256 of them, -in fact, since it rounds up to the next power of two. These -buckets will be retained even if you do C<%hash = ()>, use C<undef -%hash> if you want to free the storage while C<%hash> is still in scope. -You can't shrink the number of buckets allocated for the hash using -C<keys> in this way (but you needn't worry about doing this by accident, -as trying has no effect). C<keys @array> in an lvalue context is a syntax -error. - -Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain -a reference to an unblessed hash or array. The argument will be -dereferenced automatically. This aspect of C<keys> is considered highly -experimental. The exact behaviour may change in a future version of Perl. - - for (keys $hashref) { ... } - for (keys $obj->get_arrayref) { ... } - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.012; # so keys/values/each work on arrays - use 5.014; # so keys/values/each work on scalars (experimental) - -See also C<each>, C<values>, and C<sort>. - -=item kill SIGNAL, LIST - -=item kill SIGNAL -X<kill> X<signal> - -=for Pod::Functions send a signal to a process or process group - -Sends a signal to a list of processes. Returns the number of -processes successfully signaled (which is not necessarily the -same as the number actually killed). - - $cnt = kill 1, $child1, $child2; - kill 9, @goners; - -If SIGNAL is zero, no signal is sent to the process, but C<kill> -checks whether it's I<possible> to send a signal to it (that -means, to be brief, that the process is owned by the same user, or we are -the super-user). This is useful to check that a child process is still -alive (even if only as a zombie) and hasn't changed its UID. See -L<perlport> for notes on the portability of this construct. - -Unlike in the shell, if SIGNAL is negative, it kills process groups instead -of processes. That means you usually -want to use positive not negative signals. -You may also use a signal name in quotes. - -The behavior of kill when a I<PROCESS> number is zero or negative depends on -the operating system. For example, on POSIX-conforming systems, zero will -signal the current process group and -1 will signal all processes. - -See L<perlipc/"Signals"> for more details. - -On some platforms such as Windows where the fork() system call is not available. -Perl can be built to emulate fork() at the interpreter level. -This emulation has limitations related to kill that have to be considered, -for code running on Windows and in code intended to be portable. - -See L<perlfork> for more details. - -If there is no I<LIST> of processes, no signal is sent, and the return -value is 0. This form is sometimes used, however, because it causes -tainting checks to be run. But see -L<perlsec/Laundering and Detecting Tainted Data>. - -Portability issues: L<perlport/kill>. - -=item last LABEL -X<last> X<break> - -=item last - -=for Pod::Functions exit a block prematurely - -The C<last> command is like the C<break> statement in C (as used in -loops); it immediately exits the loop in question. If the LABEL is -omitted, the command refers to the innermost enclosing loop. The -C<continue> block, if any, is not executed: - - LINE: while (<STDIN>) { - last LINE if /^$/; # exit when done with header - #... - } - -C<last> cannot be used to exit a block that returns a value such as -C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<last> can be used to effect an early -exit out of such a block. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item lc EXPR -X<lc> X<lowercase> - -=item lc - -=for Pod::Functions return lower-case version of a string - -Returns a lowercased version of EXPR. This is the internal function -implementing the C<\L> escape in double-quoted strings. - -If EXPR is omitted, uses C<$_>. - -What gets returned depends on several factors: - -=over - -=item If C<use bytes> is in effect: - -=over - -=item On EBCDIC platforms - -The results are what the C language system call C<tolower()> returns. - -=item On ASCII platforms - -The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z> -respectively. - -=back - -=item Otherwise, if C<use locale> (but not C<use locale ':not_characters'>) is in effect: - -Respects current LC_CTYPE locale for code points < 256; and uses Unicode -semantics for the remaining code points (this last can only happen if -the UTF8 flag is also set). See L<perllocale>. - -A deficiency in this is that case changes that cross the 255/256 -boundary are not well-defined. For example, the lower case of LATIN CAPITAL -LETTER SHARP S (U+1E9E) in Unicode semantics is U+00DF (on ASCII -platforms). But under C<use locale>, the lower case of U+1E9E is -itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the -current locale, and Perl has no way of knowing if that character even -exists in the locale, much less what code point it is. Perl returns -the input character unchanged, for all instances (and there aren't -many) where the 255/256 boundary would otherwise be crossed. - -=item Otherwise, If EXPR has the UTF8 flag set: - -Unicode semantics are used for the case change. - -=item Otherwise, if C<use feature 'unicode_strings'> or C<use locale ':not_characters'>) is in effect: - -Unicode semantics are used for the case change. - -=item Otherwise: - -=over - -=item On EBCDIC platforms - -The results are what the C language system call C<tolower()> returns. - -=item On ASCII platforms - -ASCII semantics are used for the case change. The lowercase of any character -outside the ASCII range is the character itself. - -=back - -=back - -=item lcfirst EXPR -X<lcfirst> X<lowercase> - -=item lcfirst - -=for Pod::Functions return a string with just the next letter in lower case - -Returns the value of EXPR with the first character lowercased. This -is the internal function implementing the C<\l> escape in -double-quoted strings. - -If EXPR is omitted, uses C<$_>. - -This function behaves the same way under various pragmata, such as in a locale, -as L</lc> does. - -=item length EXPR -X<length> X<size> - -=item length - -=for Pod::Functions return the number of bytes in a string - -Returns the length in I<characters> of the value of EXPR. If EXPR is -omitted, returns the length of C<$_>. If EXPR is undefined, returns -C<undef>. - -This function cannot be used on an entire array or hash to find out how -many elements these have. For that, use C<scalar @array> and C<scalar keys -%hash>, respectively. - -Like all Perl character operations, length() normally deals in logical -characters, not physical bytes. For how many bytes a string encoded as -UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have -to C<use Encode> first). See L<Encode> and L<perlunicode>. - -=item __LINE__ -X<__LINE__> - -=for Pod::Functions the current source line number - -A special token that compiles to the current line number. - -=item link OLDFILE,NEWFILE -X<link> - -=for Pod::Functions create a hard link in the filesystem - -Creates a new filename linked to the old filename. Returns true for -success, false otherwise. - -Portability issues: L<perlport/link>. - -=item listen SOCKET,QUEUESIZE -X<listen> - -=for Pod::Functions register your socket as a server - -Does the same thing that the listen(2) system call does. Returns true if -it succeeded, false otherwise. See the example in -L<perlipc/"Sockets: Client/Server Communication">. - -=item local EXPR -X<local> - -=for Pod::Functions create a temporary value for a global variable (dynamic scoping) - -You really probably want to be using C<my> instead, because C<local> isn't -what most people think of as "local". See -L<perlsub/"Private Variables via my()"> for details. - -A local modifies the listed variables to be local to the enclosing -block, file, or eval. If more than one value is listed, the list must -be placed in parentheses. See L<perlsub/"Temporary Values via local()"> -for details, including issues with tied arrays and hashes. - -The C<delete local EXPR> construct can also be used to localize the deletion -of array/hash elements to the current block. -See L<perlsub/"Localized deletion of elements of composite types">. - -=item localtime EXPR -X<localtime> X<ctime> - -=item localtime - -=for Pod::Functions convert UNIX time into record or string using local time - -Converts a time as returned by the time function to a 9-element list -with the time analyzed for the local time zone. Typically used as -follows: - - # 0 1 2 3 4 5 6 7 8 - ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = - localtime(time); - -All list elements are numeric and come straight out of the C `struct -tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours -of the specified time. - -C<$mday> is the day of the month and C<$mon> the month in -the range C<0..11>, with 0 indicating January and 11 indicating December. -This makes it easy to get a month name from a list: - - my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ); - print "$abbr[$mon] $mday"; - # $mon=9, $mday=18 gives "Oct 18" - -C<$year> contains the number of years since 1900. To get a 4-digit -year write: - - $year += 1900; - -To get the last two digits of the year (e.g., "01" in 2001) do: - - $year = sprintf("%02d", $year % 100); - -C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating -Wednesday. C<$yday> is the day of the year, in the range C<0..364> -(or C<0..365> in leap years.) - -C<$isdst> is true if the specified time occurs during Daylight Saving -Time, false otherwise. - -If EXPR is omitted, C<localtime()> uses the current time (as returned -by time(3)). - -In scalar context, C<localtime()> returns the ctime(3) value: - - $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" - -The format of this scalar value is B<not> locale-dependent -but built into Perl. For GMT instead of local -time use the L</gmtime> builtin. See also the -C<Time::Local> module (for converting seconds, minutes, hours, and such back to -the integer value returned by time()), and the L<POSIX> module's strftime(3) -and mktime(3) functions. - -To get somewhat similar but locale-dependent date strings, set up your -locale environment variables appropriately (please see L<perllocale>) and -try for example: - - use POSIX qw(strftime); - $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; - # or for GMT formatted appropriately for your locale: - $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; - -Note that the C<%a> and C<%b>, the short forms of the day of the week -and the month of the year, may not necessarily be three characters wide. - -The L<Time::gmtime> and L<Time::localtime> modules provide a convenient, -by-name access mechanism to the gmtime() and localtime() functions, -respectively. - -For a comprehensive date and time representation look at the -L<DateTime> module on CPAN. - -Portability issues: L<perlport/localtime>. - -=item lock THING -X<lock> - -=for Pod::Functions +5.005 get a thread lock on a variable, subroutine, or method - -This function places an advisory lock on a shared variable or referenced -object contained in I<THING> until the lock goes out of scope. - -The value returned is the scalar itself, if the argument is a scalar, or a -reference, if the argument is a hash, array or subroutine. - -lock() is a "weak keyword" : this means that if you've defined a function -by this name (before any calls to it), that function will be called -instead. If you are not under C<use threads::shared> this does nothing. -See L<threads::shared>. - -=item log EXPR -X<log> X<logarithm> X<e> X<ln> X<base> - -=item log - -=for Pod::Functions retrieve the natural logarithm for a number - -Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, -returns the log of C<$_>. To get the -log of another base, use basic algebra: -The base-N log of a number is equal to the natural log of that number -divided by the natural log of N. For example: - - sub log10 { - my $n = shift; - return log($n)/log(10); - } - -See also L</exp> for the inverse operation. - -=item lstat FILEHANDLE -X<lstat> - -=item lstat EXPR - -=item lstat DIRHANDLE - -=item lstat - -=for Pod::Functions stat a symbolic link - -Does the same thing as the C<stat> function (including setting the -special C<_> filehandle) but stats a symbolic link instead of the file -the symbolic link points to. If symbolic links are unimplemented on -your system, a normal C<stat> is done. For much more detailed -information, please see the documentation for C<stat>. - -If EXPR is omitted, stats C<$_>. - -Portability issues: L<perlport/lstat>. - -=item m// - -=for Pod::Functions match a string with a regular expression pattern - -The match operator. See L<perlop/"Regexp Quote-Like Operators">. - -=item map BLOCK LIST -X<map> - -=item map EXPR,LIST - -=for Pod::Functions apply a change to a list to get back a new list with the changes - -Evaluates the BLOCK or EXPR for each element of LIST (locally setting -C<$_> to each element) and returns the list value composed of the -results of each such evaluation. In scalar context, returns the -total number of elements so generated. Evaluates BLOCK or EXPR in -list context, so each element of LIST may produce zero, one, or -more elements in the returned value. - - @chars = map(chr, @numbers); - -translates a list of numbers to the corresponding characters. - - my @squares = map { $_ * $_ } @numbers; - -translates a list of numbers to their squared values. - - my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers; - -shows that number of returned elements can differ from the number of -input elements. To omit an element, return an empty list (). -This could also be achieved by writing - - my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers; - -which makes the intention more clear. - -Map always returns a list, which can be -assigned to a hash such that the elements -become key/value pairs. See L<perldata> for more details. - - %hash = map { get_a_key_for($_) => $_ } @array; - -is just a funny way to write - - %hash = (); - foreach (@array) { - $hash{get_a_key_for($_)} = $_; - } - -Note that C<$_> is an alias to the list value, so it can be used to -modify the elements of the LIST. While this is useful and supported, -it can cause bizarre results if the elements of LIST are not variables. -Using a regular C<foreach> loop for this purpose would be clearer in -most cases. See also L</grep> for an array composed of those items of -the original list for which the BLOCK or EXPR evaluates to true. - -If C<$_> is lexical in the scope where the C<map> appears (because it has -been declared with C<my $_>), then, in addition to being locally aliased to -the list elements, C<$_> keeps being lexical inside the block; that is, it -can't be seen from the outside, avoiding any potential side-effects. - -C<{> starts both hash references and blocks, so C<map { ...> could be either -the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look -ahead for the closing C<}> it has to take a guess at which it's dealing with -based on what it finds just after the -C<{>. Usually it gets it right, but if it -doesn't it won't realize something is wrong until it gets to the C<}> and -encounters the missing (or unexpected) comma. The syntax error will be -reported close to the C<}>, but you'll need to change something near the C<{> -such as using a unary C<+> to give Perl some help: - - %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong - %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right - %hash = map { ("\L$_" => 1) } @array # this also works - %hash = map { lc($_) => 1 } @array # as does this. - %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! - - %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) - -or to force an anon hash constructor use C<+{>: - - @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end - -to get a list of anonymous hashes each with only one entry apiece. - -=item mkdir FILENAME,MASK -X<mkdir> X<md> X<directory, create> - -=item mkdir FILENAME - -=item mkdir - -=for Pod::Functions create a directory - -Creates the directory specified by FILENAME, with permissions -specified by MASK (as modified by C<umask>). If it succeeds it -returns true; otherwise it returns false and sets C<$!> (errno). -MASK defaults to 0777 if omitted, and FILENAME defaults -to C<$_> if omitted. - -In general, it is better to create directories with a permissive MASK -and let the user modify that with their C<umask> than it is to supply -a restrictive MASK and give the user no way to be more permissive. -The exceptions to this rule are when the file or directory should be -kept private (mail files, for instance). The perlfunc(1) entry on -C<umask> discusses the choice of MASK in more detail. - -Note that according to the POSIX 1003.1-1996 the FILENAME may have any -number of trailing slashes. Some operating and filesystems do not get -this right, so Perl automatically removes all trailing slashes to keep -everyone happy. - -To recursively create a directory structure, look at -the C<mkpath> function of the L<File::Path> module. - -=item msgctl ID,CMD,ARG -X<msgctl> - -=for Pod::Functions SysV IPC message control operations - -Calls the System V IPC function msgctl(2). You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is C<IPC_STAT>, -then ARG must be a variable that will hold the returned C<msqid_ds> -structure. Returns like C<ioctl>: the undefined value for error, -C<"0 but true"> for zero, or the actual return value otherwise. See also -L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and -C<IPC::Semaphore>. - -Portability issues: L<perlport/msgctl>. - -=item msgget KEY,FLAGS -X<msgget> - -=for Pod::Functions get SysV IPC message queue - -Calls the System V IPC function msgget(2). Returns the message queue -id, or C<undef> on error. See also -L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and -C<IPC::Msg>. - -Portability issues: L<perlport/msgget>. - -=item msgrcv ID,VAR,SIZE,TYPE,FLAGS -X<msgrcv> - -=for Pod::Functions receive a SysV IPC message from a message queue - -Calls the System V IPC function msgrcv to receive a message from -message queue ID into variable VAR with a maximum message size of -SIZE. Note that when a message is received, the message type as a -native long integer will be the first thing in VAR, followed by the -actual message. This packing may be opened with C<unpack("l! a*")>. -Taints the variable. Returns true if successful, false -on error. See also L<perlipc/"SysV IPC"> and the documentation for -C<IPC::SysV> and C<IPC::SysV::Msg>. - -Portability issues: L<perlport/msgrcv>. - -=item msgsnd ID,MSG,FLAGS -X<msgsnd> - -=for Pod::Functions send a SysV IPC message to a message queue - -Calls the System V IPC function msgsnd to send the message MSG to the -message queue ID. MSG must begin with the native long integer message -type, be followed by the length of the actual message, and then finally -the message itself. This kind of packing can be achieved with -C<pack("l! a*", $type, $message)>. Returns true if successful, -false on error. See also the C<IPC::SysV> -and C<IPC::SysV::Msg> documentation. - -Portability issues: L<perlport/msgsnd>. - -=item my EXPR -X<my> - -=item my TYPE EXPR - -=item my EXPR : ATTRS - -=item my TYPE EXPR : ATTRS - -=for Pod::Functions declare and assign a local variable (lexical scoping) - -A C<my> declares the listed variables to be local (lexically) to the -enclosing block, file, or C<eval>. If more than one value is listed, -the list must be placed in parentheses. - -The exact semantics and interface of TYPE and ATTRS are still -evolving. TYPE is currently bound to the use of the C<fields> pragma, -and attributes are handled using the C<attributes> pragma, or starting -from Perl 5.8.0 also via the C<Attribute::Handlers> module. See -L<perlsub/"Private Variables via my()"> for details, and L<fields>, -L<attributes>, and L<Attribute::Handlers>. - -=item next LABEL -X<next> X<continue> - -=item next - -=for Pod::Functions iterate a block prematurely - -The C<next> command is like the C<continue> statement in C; it starts -the next iteration of the loop: - - LINE: while (<STDIN>) { - next LINE if /^#/; # discard comments - #... - } - -Note that if there were a C<continue> block on the above, it would get -executed even on discarded lines. If LABEL is omitted, the command -refers to the innermost enclosing loop. - -C<next> cannot be used to exit a block which returns a value such as -C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<next> will exit such a block early. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item no MODULE VERSION LIST -X<no declarations> -X<unimporting> - -=item no MODULE VERSION - -=item no MODULE LIST - -=item no MODULE - -=item no VERSION - -=for Pod::Functions unimport some module symbols or semantics at compile time - -See the C<use> function, of which C<no> is the opposite. - -=item oct EXPR -X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin> - -=item oct - -=for Pod::Functions convert a string to an octal number - -Interprets EXPR as an octal string and returns the corresponding -value. (If EXPR happens to start off with C<0x>, interprets it as a -hex string. If EXPR starts off with C<0b>, it is interpreted as a -binary string. Leading whitespace is ignored in all three cases.) -The following will handle decimal, binary, octal, and hex in standard -Perl notation: - - $val = oct($val) if $val =~ /^0/; - -If EXPR is omitted, uses C<$_>. To go the other way (produce a number -in octal), use sprintf() or printf(): - - $dec_perms = (stat("filename"))[2] & 07777; - $oct_perm_str = sprintf "%o", $perms; - -The oct() function is commonly used when a string such as C<644> needs -to be converted into a file mode, for example. Although Perl -automatically converts strings into numbers as needed, this automatic -conversion assumes base 10. - -Leading white space is ignored without warning, as too are any trailing -non-digits, such as a decimal point (C<oct> only handles non-negative -integers, not negative integers or floating point). - -=item open FILEHANDLE,EXPR -X<open> X<pipe> X<file, open> X<fopen> - -=item open FILEHANDLE,MODE,EXPR - -=item open FILEHANDLE,MODE,EXPR,LIST - -=item open FILEHANDLE,MODE,REFERENCE - -=item open FILEHANDLE - -=for Pod::Functions open a file, pipe, or descriptor - -Opens the file whose filename is given by EXPR, and associates it with -FILEHANDLE. - -Simple examples to open a file for reading: - - open(my $fh, "<", "input.txt") - or die "cannot open < input.txt: $!"; - -and for writing: - - open(my $fh, ">", "output.txt") - or die "cannot open > output.txt: $!"; - -(The following is a comprehensive reference to open(): for a gentler -introduction you may consider L<perlopentut>.) - -If FILEHANDLE is an undefined scalar variable (or array or hash element), a -new filehandle is autovivified, meaning that the variable is assigned a -reference to a newly allocated anonymous filehandle. Otherwise if -FILEHANDLE is an expression, its value is the real filehandle. (This is -considered a symbolic reference, so C<use strict "refs"> should I<not> be -in effect.) - -If EXPR is omitted, the global (package) scalar variable of the same -name as the FILEHANDLE contains the filename. (Note that lexical -variables--those declared with C<my> or C<state>--will not work for this -purpose; so if you're using C<my> or C<state>, specify EXPR in your -call to open.) - -If three (or more) arguments are specified, the open mode (including -optional encoding) in the second argument are distinct from the filename in -the third. If MODE is C<< < >> or nothing, the file is opened for input. -If MODE is C<< > >>, the file is opened for output, with existing files -first being truncated ("clobbered") and nonexisting files newly created. -If MODE is C<<< >> >>>, the file is opened for appending, again being -created if necessary. - -You can put a C<+> in front of the C<< > >> or C<< < >> to -indicate that you want both read and write access to the file; thus -C<< +< >> is almost always preferred for read/write updates--the -C<< +> >> mode would clobber the file first. You can't usually use -either read-write mode for updating textfiles, since they have -variable-length records. See the B<-i> switch in L<perlrun> for a -better approach. The file is created with permissions of C<0666> -modified by the process's C<umask> value. - -These various prefixes correspond to the fopen(3) modes of C<r>, -C<r+>, C<w>, C<w+>, C<a>, and C<a+>. - -In the one- and two-argument forms of the call, the mode and filename -should be concatenated (in that order), preferably separated by white -space. You can--but shouldn't--omit the mode in these forms when that mode -is C<< < >>. It is always safe to use the two-argument form of C<open> if -the filename argument is a known literal. - -For three or more arguments if MODE is C<|->, the filename is -interpreted as a command to which output is to be piped, and if MODE -is C<-|>, the filename is interpreted as a command that pipes -output to us. In the two-argument (and one-argument) form, one should -replace dash (C<->) with the command. -See L<perlipc/"Using open() for IPC"> for more examples of this. -(You are not allowed to C<open> to a command that pipes both in I<and> -out, but see L<IPC::Open2>, L<IPC::Open3>, and -L<perlipc/"Bidirectional Communication with Another Process"> for -alternatives.) - -In the form of pipe opens taking three or more arguments, if LIST is specified -(extra arguments after the command name) then LIST becomes arguments -to the command invoked if the platform supports it. The meaning of -C<open> with more than three arguments for non-pipe modes is not yet -defined, but experimental "layers" may give extra LIST arguments -meaning. - -In the two-argument (and one-argument) form, opening C<< <- >> -or C<-> opens STDIN and opening C<< >- >> opens STDOUT. - -You may (and usually should) use the three-argument form of open to specify -I/O layers (sometimes referred to as "disciplines") to apply to the handle -that affect how the input and output are processed (see L<open> and -L<PerlIO> for more details). For example: - - open(my $fh, "<:encoding(UTF-8)", "filename") - || die "can't open UTF-8 encoded filename: $!"; - -opens the UTF8-encoded file containing Unicode characters; -see L<perluniintro>. Note that if layers are specified in the -three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>; -usually set by the B<open> pragma or the switch B<-CioD>) are ignored. -Those layers will also be ignored if you specifying a colon with no name -following it. In that case the default layer for the operating system -(:raw on Unix, :crlf on Windows) is used. - -Open returns nonzero on success, the undefined value otherwise. If -the C<open> involved a pipe, the return value happens to be the pid of -the subprocess. - -If you're running Perl on a system that distinguishes between text -files and binary files, then you should check out L</binmode> for tips -for dealing with this. The key distinction between systems that need -C<binmode> and those that don't is their text file formats. Systems -like Unix, Mac OS, and Plan 9, that end lines with a single -character and encode that character in C as C<"\n"> do not -need C<binmode>. The rest need it. - -When opening a file, it's seldom a good idea to continue -if the request failed, so C<open> is frequently used with -C<die>. Even if C<die> won't do what you want (say, in a CGI script, -where you want to format a suitable error message (but there are -modules that can help with that problem)) always check -the return value from opening a file. - -As a special case the three-argument form with a read/write mode and the third -argument being C<undef>: - - open(my $tmp, "+>", undef) or die ... - -opens a filehandle to an anonymous temporary file. Also using C<< +< >> -works for symmetry, but you really should consider writing something -to the temporary file first. You will need to seek() to do the -reading. - -Since v5.8.0, Perl has built using PerlIO by default. Unless you've -changed this (such as building Perl with C<Configure -Uuseperlio>), you can -open filehandles directly to Perl scalars via: - - open($fh, ">", \$variable) || .. - -To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first: - - close STDOUT; - open(STDOUT, ">", \$variable) - or die "Can't open STDOUT: $!"; - -General examples: - - $ARTICLE = 100; - open(ARTICLE) or die "Can't find article $ARTICLE: $!\n"; - while (<ARTICLE>) {... - - open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved) - # if the open fails, output is discarded - - open(my $dbase, "+<", "dbase.mine") # open for update - or die "Can't open 'dbase.mine' for update: $!"; - - open(my $dbase, "+<dbase.mine") # ditto - or die "Can't open 'dbase.mine' for update: $!"; - - open(ARTICLE, "-|", "caesar <$article") # decrypt article - or die "Can't start caesar: $!"; - - open(ARTICLE, "caesar <$article |") # ditto - or die "Can't start caesar: $!"; - - open(EXTRACT, "|sort >Tmp$$") # $$ is our process id - or die "Can't start sort: $!"; - - # in-memory files - open(MEMORY, ">", \$var) - or die "Can't open memory file: $!"; - print MEMORY "foo!\n"; # output will appear in $var - - # process argument list of files along with any includes - - foreach $file (@ARGV) { - process($file, "fh00"); - } - - sub process { - my($filename, $input) = @_; - $input++; # this is a string increment - unless (open($input, "<", $filename)) { - print STDERR "Can't open $filename: $!\n"; - return; - } - - local $_; - while (<$input>) { # note use of indirection - if (/^#include "(.*)"/) { - process($1, $input); - next; - } - #... # whatever - } - } - -See L<perliol> for detailed info on PerlIO. - -You may also, in the Bourne shell tradition, specify an EXPR beginning -with C<< >& >>, in which case the rest of the string is interpreted -as the name of a filehandle (or file descriptor, if numeric) to be -duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>, -C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. -The mode you specify should match the mode of the original filehandle. -(Duping a filehandle does not take into account any existing contents -of IO buffers.) If you use the three-argument -form, then you can pass either a -number, the name of a filehandle, or the normal "reference to a glob". - -Here is a script that saves, redirects, and restores C<STDOUT> and -C<STDERR> using various methods: - - #!/usr/bin/perl - open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!"; - open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!"; - - open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!"; - open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!"; - - select STDERR; $| = 1; # make unbuffered - select STDOUT; $| = 1; # make unbuffered - - print STDOUT "stdout 1\n"; # this works for - print STDERR "stderr 1\n"; # subprocesses too - - open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!"; - open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!"; - - print STDOUT "stdout 2\n"; - print STDERR "stderr 2\n"; - -If you specify C<< '<&=X' >>, where C<X> is a file descriptor number -or a filehandle, then Perl will do an equivalent of C's C<fdopen> of -that file descriptor (and not call C<dup(2)>); this is more -parsimonious of file descriptors. For example: - - # open for input, reusing the fileno of $fd - open(FILEHANDLE, "<&=$fd") - -or - - open(FILEHANDLE, "<&=", $fd) - -or - - # open for append, using the fileno of OLDFH - open(FH, ">>&=", OLDFH) - -or - - open(FH, ">>&=OLDFH") - -Being parsimonious on filehandles is also useful (besides being -parsimonious) for example when something is dependent on file -descriptors, like for example locking using flock(). If you do just -C<< open(A, ">>&B") >>, the filehandle A will not have the same file -descriptor as B, and therefore flock(A) will not flock(B) nor vice -versa. But with C<< open(A, ">>&=B") >>, the filehandles will share -the same underlying system file descriptor. - -Note that under Perls older than 5.8.0, Perl uses the standard C library's' -fdopen() to implement the C<=> functionality. On many Unix systems, -fdopen() fails when file descriptors exceed a certain value, typically 255. -For Perls 5.8.0 and later, PerlIO is (most often) the default. - -You can see whether your Perl was built with PerlIO by running C<perl -V> -and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you -have PerlIO; otherwise you don't. - -If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|> -with the one- or two-argument forms of C<open>), -an implicit C<fork> is done, so C<open> returns twice: in the parent -process it returns the pid -of the child process, and in the child process it returns (a defined) C<0>. -Use C<defined($pid)> or C<//> to determine whether the open was successful. - -For example, use either - - $child_pid = open(FROM_KID, "-|") // die "can't fork: $!"; - -or - $child_pid = open(TO_KID, "|-") // die "can't fork: $!"; - -followed by - - if ($child_pid) { - # am the parent: - # either write TO_KID or else read FROM_KID - ... - wait $child_pid; - } else { - # am the child; use STDIN/STDOUT normally - ... - exit; - } - -The filehandle behaves normally for the parent, but I/O to that -filehandle is piped from/to the STDOUT/STDIN of the child process. -In the child process, the filehandle isn't opened--I/O happens from/to -the new STDOUT/STDIN. Typically this is used like the normal -piped open when you want to exercise more control over just how the -pipe command gets executed, such as when running setuid and -you don't want to have to scan shell commands for metacharacters. - -The following blocks are more or less equivalent: - - open(FOO, "|tr '[a-z]' '[A-Z]'"); - open(FOO, "|-", "tr '[a-z]' '[A-Z]'"); - open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; - open(FOO, "|-", "tr", '[a-z]', '[A-Z]'); - - open(FOO, "cat -n '$file'|"); - open(FOO, "-|", "cat -n '$file'"); - open(FOO, "-|") || exec "cat", "-n", $file; - open(FOO, "-|", "cat", "-n", $file); - -The last two examples in each block show the pipe as "list form", which is -not yet supported on all platforms. A good rule of thumb is that if -your platform has a real C<fork()> (in other words, if your platform is -Unix, including Linux and MacOS X), you can use the list form. You would -want to use the list form of the pipe so you can pass literal arguments -to the command without risk of the shell interpreting any shell metacharacters -in them. However, this also bars you from opening pipes to commands -that intentionally contain shell metacharacters, such as: - - open(FOO, "|cat -n | expand -4 | lpr") - // die "Can't open pipeline to lpr: $!"; - -See L<perlipc/"Safe Pipe Opens"> for more examples of this. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before any operation that may do a fork, but this may not be -supported on some platforms (see L<perlport>). To be safe, you may need -to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method -of C<IO::Handle> on any open handles. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptor as determined by the value -of C<$^F>. See L<perlvar/$^F>. - -Closing any piped filehandle causes the parent process to wait for the -child to finish, then returns the status value in C<$?> and -C<${^CHILD_ERROR_NATIVE}>. - -The filename passed to the one- and two-argument forms of open() will -have leading and trailing whitespace deleted and normal -redirection characters honored. This property, known as "magic open", -can often be used to good effect. A user could specify a filename of -F<"rsh cat file |">, or you could change certain filenames as needed: - - $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; - open(FH, $filename) or die "Can't open $filename: $!"; - -Use the three-argument form to open a file with arbitrary weird characters in it, - - open(FOO, "<", $file) - || die "can't open < $file: $!"; - -otherwise it's necessary to protect any leading and trailing whitespace: - - $file =~ s#^(\s)#./$1#; - open(FOO, "< $file\0") - || die "open failed: $!"; - -(this may not work on some bizarre filesystems). One should -conscientiously choose between the I<magic> and I<three-argument> form -of open(): - - open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!"; - -will allow the user to specify an argument of the form C<"rsh cat file |">, -but will not work on a filename that happens to have a trailing space, while - - open(IN, "<", $ARGV[0]) - || die "can't open < $ARGV[0]: $!"; - -will have exactly the opposite restrictions. - -If you want a "real" C C<open> (see L<open(2)> on your system), then you -should use the C<sysopen> function, which involves no such magic (but may -use subtly different filemodes than Perl open(), which is mapped to C -fopen()). This is another way to protect your filenames from -interpretation. For example: - - use IO::Handle; - sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) - or die "sysopen $path: $!"; - $oldfh = select(HANDLE); $| = 1; select($oldfh); - print HANDLE "stuff $$\n"; - seek(HANDLE, 0, 0); - print "File contains: ", <HANDLE>; - -Using the constructor from the C<IO::Handle> package (or one of its -subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous -filehandles that have the scope of the variables used to hold them, then -automatically (but silently) close once their reference counts become -zero, typically at scope exit: - - use IO::File; - #... - sub read_myfile_munged { - my $ALL = shift; - # or just leave it undef to autoviv - my $handle = IO::File->new; - open($handle, "<", "myfile") or die "myfile: $!"; - $first = <$handle> - or return (); # Automatically closed here. - mung($first) or die "mung failed"; # Or here. - return (first, <$handle>) if $ALL; # Or here. - return $first; # Or here. - } - -B<WARNING:> The previous example has a bug because the automatic -close that happens when the refcount on C<handle> does not -properly detect and report failures. I<Always> close the handle -yourself and inspect the return value. - - close($handle) - || warn "close failed: $!"; - -See L</seek> for some details about mixing reading and writing. - -Portability issues: L<perlport/open>. - -=item opendir DIRHANDLE,EXPR -X<opendir> - -=for Pod::Functions open a directory - -Opens a directory named EXPR for processing by C<readdir>, C<telldir>, -C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. -DIRHANDLE may be an expression whose value can be used as an indirect -dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined -scalar variable (or array or hash element), the variable is assigned a -reference to a new anonymous dirhandle; that is, it's autovivified. -DIRHANDLEs have their own namespace separate from FILEHANDLEs. - -See the example at C<readdir>. - -=item ord EXPR -X<ord> X<encoding> - -=item ord - -=for Pod::Functions find a character's numeric representation - -Returns the numeric value of the first character of EXPR. -If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>. -(Note I<character>, not byte.) - -For the reverse, see L</chr>. -See L<perlunicode> for more about Unicode. - -=item our EXPR -X<our> X<global> - -=item our TYPE EXPR - -=item our EXPR : ATTRS - -=item our TYPE EXPR : ATTRS - -=for Pod::Functions +5.6.0 declare and assign a package variable (lexical scoping) - -C<our> associates a simple name with a package variable in the current -package for use within the current scope. When C<use strict 'vars'> is in -effect, C<our> lets you use declared global variables without qualifying -them with package names, within the lexical scope of the C<our> declaration. -In this way C<our> differs from C<use vars>, which is package-scoped. - -Unlike C<my> or C<state>, which allocates storage for a variable and -associates a simple name with that storage for use within the current -scope, C<our> associates a simple name with a package (read: global) -variable in the current package, for use within the current lexical scope. -In other words, C<our> has the same scoping rules as C<my> or C<state>, but -does not necessarily create a variable. - -If more than one value is listed, the list must be placed -in parentheses. - - our $foo; - our($bar, $baz); - -An C<our> declaration declares a global variable that will be visible -across its entire lexical scope, even across package boundaries. The -package in which the variable is entered is determined at the point -of the declaration, not at the point of use. This means the following -behavior holds: - - package Foo; - our $bar; # declares $Foo::bar for rest of lexical scope - $bar = 20; - - package Bar; - print $bar; # prints 20, as it refers to $Foo::bar - -Multiple C<our> declarations with the same name in the same lexical -scope are allowed if they are in different packages. If they happen -to be in the same package, Perl will emit warnings if you have asked -for them, just like multiple C<my> declarations. Unlike a second -C<my> declaration, which will bind the name to a fresh variable, a -second C<our> declaration in the same package, in the same scope, is -merely redundant. - - use warnings; - package Foo; - our $bar; # declares $Foo::bar for rest of lexical scope - $bar = 20; - - package Bar; - our $bar = 30; # declares $Bar::bar for rest of lexical scope - print $bar; # prints 30 - - our $bar; # emits warning but has no other effect - print $bar; # still prints 30 - -An C<our> declaration may also have a list of attributes associated -with it. - -The exact semantics and interface of TYPE and ATTRS are still -evolving. TYPE is currently bound to the use of the C<fields> pragma, -and attributes are handled using the C<attributes> pragma, or, starting -from Perl 5.8.0, also via the C<Attribute::Handlers> module. See -L<perlsub/"Private Variables via my()"> for details, and L<fields>, -L<attributes>, and L<Attribute::Handlers>. - -=item pack TEMPLATE,LIST -X<pack> - -=for Pod::Functions convert a list into a binary representation - -Takes a LIST of values and converts it into a string using the rules -given by the TEMPLATE. The resulting string is the concatenation of -the converted values. Typically, each converted value looks -like its machine-level representation. For example, on 32-bit machines -an integer may be represented by a sequence of 4 bytes, which will in -Perl be presented as a string that's 4 characters long. - -See L<perlpacktut> for an introduction to this function. - -The TEMPLATE is a sequence of characters that give the order and type -of values, as follows: - - a A string with arbitrary binary data, will be null padded. - A A text (ASCII) string, will be space padded. - Z A null-terminated (ASCIZ) string, will be null padded. - - b A bit string (ascending bit order inside each byte, - like vec()). - B A bit string (descending bit order inside each byte). - h A hex string (low nybble first). - H A hex string (high nybble first). - - c A signed char (8-bit) value. - C An unsigned char (octet) value. - W An unsigned char value (can be greater than 255). - - s A signed short (16-bit) value. - S An unsigned short value. - - l A signed long (32-bit) value. - L An unsigned long value. - - q A signed quad (64-bit) value. - Q An unsigned quad value. - (Quads are available only if your system supports 64-bit - integer values _and_ if Perl has been compiled to support - those. Raises an exception otherwise.) - - i A signed integer value. - I A unsigned integer value. - (This 'integer' is _at_least_ 32 bits wide. Its exact - size depends on what a local C compiler calls 'int'.) - - n An unsigned short (16-bit) in "network" (big-endian) order. - N An unsigned long (32-bit) in "network" (big-endian) order. - v An unsigned short (16-bit) in "VAX" (little-endian) order. - V An unsigned long (32-bit) in "VAX" (little-endian) order. - - j A Perl internal signed integer value (IV). - J A Perl internal unsigned integer value (UV). - - f A single-precision float in native format. - d A double-precision float in native format. - - F A Perl internal floating-point value (NV) in native format - D A float of long-double precision in native format. - (Long doubles are available only if your system supports - long double values _and_ if Perl has been compiled to - support those. Raises an exception otherwise.) - - p A pointer to a null-terminated string. - P A pointer to a structure (fixed-length string). - - u A uuencoded string. - U A Unicode character number. Encodes to a character in char- - acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in - byte mode. - - w A BER compressed integer (not an ASN.1 BER, see perlpacktut - for details). Its bytes represent an unsigned integer in - base 128, most significant digit first, with as few digits - as possible. Bit eight (the high bit) is set on each byte - except the last. - - x A null byte (a.k.a ASCII NUL, "\000", chr(0)) - X Back up a byte. - @ Null-fill or truncate to absolute position, counted from the - start of the innermost ()-group. - . Null-fill or truncate to absolute position specified by - the value. - ( Start of a ()-group. - -One or more modifiers below may optionally follow certain letters in the -TEMPLATE (the second column lists letters for which the modifier is valid): - - ! sSlLiI Forces native (short, long, int) sizes instead - of fixed (16-/32-bit) sizes. - - xX Make x and X act as alignment commands. - - nNvV Treat integers as signed instead of unsigned. - - @. Specify position as byte offset in the internal - representation of the packed string. Efficient - but dangerous. - - > sSiIlLqQ Force big-endian byte-order on the type. - jJfFdDpP (The "big end" touches the construct.) - - < sSiIlLqQ Force little-endian byte-order on the type. - jJfFdDpP (The "little end" touches the construct.) - -The C<< > >> and C<< < >> modifiers can also be used on C<()> groups -to force a particular byte-order on all components in that group, -including all its subgroups. - -The following rules apply: - -=over - -=item * - -Each letter may optionally be followed by a number indicating the repeat -count. A numeric repeat count may optionally be enclosed in brackets, as -in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from -the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>, -C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means -something else, described below. Supplying a C<*> for the repeat count -instead of a number means to use however many items are left, except for: - -=over - -=item * - -C<@>, C<x>, and C<X>, where it is equivalent to C<0>. - -=item * - -<.>, where it means relative to the start of the string. - -=item * - -C<u>, where it is equivalent to 1 (or 45, which here is equivalent). - -=back - -One can replace a numeric repeat count with a template letter enclosed in -brackets to use the packed byte length of the bracketed template for the -repeat count. - -For example, the template C<x[L]> skips as many bytes as in a packed long, -and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when -variable-expanded) unpacks. If the template in brackets contains alignment -commands (such as C<x![d]>), its packed length is calculated as if the -start of the template had the maximal possible alignment. - -When used with C<Z>, a C<*> as the repeat count is guaranteed to add a -trailing null byte, so the resulting string is always one byte longer than -the byte length of the item itself. - -When used with C<@>, the repeat count represents an offset from the start -of the innermost C<()> group. - -When used with C<.>, the repeat count determines the starting position to -calculate the value offset as follows: - -=over - -=item * - -If the repeat count is C<0>, it's relative to the current position. - -=item * - -If the repeat count is C<*>, the offset is relative to the start of the -packed string. - -=item * - -And if it's an integer I<n>, the offset is relative to the start of the -I<n>th innermost C<( )> group, or to the start of the string if I<n> is -bigger then the group level. - -=back - -The repeat count for C<u> is interpreted as the maximal number of bytes -to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat -count should not be more than 65. - -=item * - -The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a -string of length count, padding with nulls or spaces as needed. When -unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything -after the first null, and C<a> returns data with no stripping at all. - -If the value to pack is too long, the result is truncated. If it's too -long and an explicit count is provided, C<Z> packs only C<$count-1> bytes, -followed by a null byte. Thus C<Z> always packs a trailing null, except -when the count is 0. - -=item * - -Likewise, the C<b> and C<B> formats pack a string that's that many bits long. -Each such format generates 1 bit of the result. These are typically followed -by a repeat count like C<B8> or C<B64>. - -Each result bit is based on the least-significant bit of the corresponding -input character, i.e., on C<ord($char)%2>. In particular, characters C<"0"> -and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">. - -Starting from the beginning of the input string, each 8-tuple -of characters is converted to 1 character of output. With format C<b>, -the first character of the 8-tuple determines the least-significant bit of a -character; with format C<B>, it determines the most-significant bit of -a character. - -If the length of the input string is not evenly divisible by 8, the -remainder is packed as if the input string were padded by null characters -at the end. Similarly during unpacking, "extra" bits are ignored. - -If the input string is longer than needed, remaining characters are ignored. - -A C<*> for the repeat count uses all characters of the input field. -On unpacking, bits are converted to a string of C<0>s and C<1>s. - -=item * - -The C<h> and C<H> formats pack a string that many nybbles (4-bit groups, -representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long. - -For each such format, pack() generates 4 bits of result. -With non-alphabetical characters, the result is based on the 4 least-significant -bits of the input character, i.e., on C<ord($char)%16>. In particular, -characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes -C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result -is compatible with the usual hexadecimal digits, so that C<"a"> and -C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex -characters with this format. - -Starting from the beginning of the template to pack(), each pair -of characters is converted to 1 character of output. With format C<h>, the -first character of the pair determines the least-significant nybble of the -output character; with format C<H>, it determines the most-significant -nybble. - -If the length of the input string is not even, it behaves as if padded by -a null character at the end. Similarly, "extra" nybbles are ignored during -unpacking. - -If the input string is longer than needed, extra characters are ignored. - -A C<*> for the repeat count uses all characters of the input field. For -unpack(), nybbles are converted to a string of hexadecimal digits. - -=item * - -The C<p> format packs a pointer to a null-terminated string. You are -responsible for ensuring that the string is not a temporary value, as that -could potentially get deallocated before you got around to using the packed -result. The C<P> format packs a pointer to a structure of the size indicated -by the length. A null pointer is created if the corresponding value for -C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer -unpacks into C<undef>. - -If your system has a strange pointer size--meaning a pointer is neither as -big as an int nor as big as a long--it may not be possible to pack or -unpack pointers in big- or little-endian byte order. Attempting to do -so raises an exception. - -=item * - -The C</> template character allows packing and unpacking of a sequence of -items where the packed structure contains a packed item count followed by -the packed items themselves. This is useful when the structure you're -unpacking has encoded the sizes or repeat counts for some of its fields -within the structure itself as separate fields. - -For C<pack>, you write I<length-item>C</>I<sequence-item>, and the -I<length-item> describes how the length value is packed. Formats likely -to be of most use are integer-packing ones like C<n> for Java strings, -C<w> for ASN.1 or SNMP, and C<N> for Sun XDR. - -For C<pack>, I<sequence-item> may have a repeat count, in which case -the minimum of that and the number of available items is used as the argument -for I<length-item>. If it has no repeat count or uses a '*', the number -of available items is used. - -For C<unpack>, an internal stack of integer arguments unpacked so far is -used. You write C</>I<sequence-item> and the repeat count is obtained by -popping off the last element from the stack. The I<sequence-item> must not -have a repeat count. - -If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">), -the I<length-item> is the string length, not the number of strings. With -an explicit repeat count for pack, the packed string is adjusted to that -length. For example: - - This code: gives this result: - - unpack("W/a", "\004Gurusamy") ("Guru") - unpack("a3/A A*", "007 Bond J ") (" Bond", "J") - unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".") - - pack("n/a* w/a","hello,","world") "\000\006hello,\005world" - pack("a/W2", ord("a") .. ord("z")) "2ab" - -The I<length-item> is not returned explicitly from C<unpack>. - -Supplying a count to the I<length-item> format letter is only useful with -C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may -introduce C<"\000"> characters, which Perl does not regard as legal in -numeric strings. - -=item * - -The integer types C<s>, C<S>, C<l>, and C<L> may be -followed by a C<!> modifier to specify native shorts or -longs. As shown in the example above, a bare C<l> means -exactly 32 bits, although the native C<long> as seen by the local C compiler -may be larger. This is mainly an issue on 64-bit platforms. You can -see whether using C<!> makes any difference this way: - - printf "format s is %d, s! is %d\n", - length pack("s"), length pack("s!"); - - printf "format l is %d, l! is %d\n", - length pack("l"), length pack("l!"); - - -C<i!> and C<I!> are also allowed, but only for completeness' sake: -they are identical to C<i> and C<I>. - -The actual sizes (in bytes) of native shorts, ints, longs, and long -longs on the platform where Perl was built are also available from -the command line: - - $ perl -V:{short,int,long{,long}}size - shortsize='2'; - intsize='4'; - longsize='4'; - longlongsize='8'; - -or programmatically via the C<Config> module: - - use Config; - print $Config{shortsize}, "\n"; - print $Config{intsize}, "\n"; - print $Config{longsize}, "\n"; - print $Config{longlongsize}, "\n"; - -C<$Config{longlongsize}> is undefined on systems without -long long support. - -=item * - -The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are -inherently non-portable between processors and operating systems because -they obey native byteorder and endianness. For example, a 4-byte integer -0x12345678 (305419896 decimal) would be ordered natively (arranged in and -handled by the CPU registers) into bytes as - - 0x12 0x34 0x56 0x78 # big-endian - 0x78 0x56 0x34 0x12 # little-endian - -Basically, Intel and VAX CPUs are little-endian, while everybody else, -including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are -big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) -them in little-endian mode, but SGI/Cray uses them in big-endian mode. - -The names I<big-endian> and I<little-endian> are comic references to the -egg-eating habits of the little-endian Lilliputians and the big-endian -Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>. -This entered computer lingo via the paper "On Holy Wars and a Plea for -Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. - -Some systems may have even weirder byte orders such as - - 0x56 0x78 0x12 0x34 - 0x34 0x12 0x78 0x56 - -You can determine your system endianness with this incantation: - - printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678); - -The byteorder on the platform where Perl was built is also available -via L<Config>: - - use Config; - print "$Config{byteorder}\n"; - -or from the command line: - - $ perl -V:byteorder - -Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321"> -and C<"87654321"> are big-endian. - -For portably packed integers, either use the formats C<n>, C<N>, C<v>, -and C<V> or else use the C<< > >> and C<< < >> modifiers described -immediately below. See also L<perlport>. - -=item * - -Starting with Perl 5.9.2, integer and floating-point formats, along with -the C<p> and C<P> formats and C<()> groups, may all be followed by the -C<< > >> or C<< < >> endianness modifiers to respectively enforce big- -or little-endian byte-order. These modifiers are especially useful -given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers, -64-bit integers, or floating-point values. - -Here are some concerns to keep in mind when using an endianness modifier: - -=over - -=item * - -Exchanging signed integers between different platforms works only -when all platforms store them in the same format. Most platforms store -signed integers in two's-complement notation, so usually this is not an issue. - -=item * - -The C<< > >> or C<< < >> modifiers can only be used on floating-point -formats on big- or little-endian machines. Otherwise, attempting to -use them raises an exception. - -=item * - -Forcing big- or little-endian byte-order on floating-point values for -data exchange can work only if all platforms use the same -binary representation such as IEEE floating-point. Even if all -platforms are using IEEE, there may still be subtle differences. Being able -to use C<< > >> or C<< < >> on floating-point values can be useful, -but also dangerous if you don't know exactly what you're doing. -It is not a general way to portably store floating-point values. - -=item * - -When using C<< > >> or C<< < >> on a C<()> group, this affects -all types inside the group that accept byte-order modifiers, -including all subgroups. It is silently ignored for all other -types. You are not allowed to override the byte-order within a group -that already has a byte-order modifier suffix. - -=back - -=item * - -Real numbers (floats and doubles) are in native machine format only. -Due to the multiplicity of floating-point formats and the lack of a -standard "network" representation for them, no facility for interchange has been -made. This means that packed floating-point data written on one machine -may not be readable on another, even if both use IEEE floating-point -arithmetic (because the endianness of the memory representation is not part -of the IEEE spec). See also L<perlport>. - -If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >> -modifiers to force big- or little-endian byte-order on floating-point values. - -Because Perl uses doubles (or long doubles, if configured) internally for -all numeric calculation, converting from double into float and thence -to double again loses precision, so C<unpack("f", pack("f", $foo)>) -will not in general equal $foo. - -=item * - -Pack and unpack can operate in two modes: character mode (C<C0> mode) where -the packed string is processed per character, and UTF-8 mode (C<U0> mode) -where the packed string is processed in its UTF-8-encoded Unicode form on -a byte-by-byte basis. Character mode is the default -unless the format string starts with C<U>. You -can always switch mode mid-format with an explicit -C<C0> or C<U0> in the format. This mode remains in effect until the next -mode change, or until the end of the C<()> group it (directly) applies to. - -Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode -bytes is not necessarily obvious. Probably only the first of these -is what you want: - - $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | - perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)' - 03B1.03C9 - $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | - perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' - CE.B1.CF.89 - $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | - perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)' - CE.B1.CF.89 - $ perl -CS -E 'say "\x{3B1}\x{3C9}"' | - perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)' - C3.8E.C2.B1.C3.8F.C2.89 - -Those examples also illustrate that you should not try to use -C<pack>/C<unpack> as a substitute for the L<Encode> module. - -=item * - -You must yourself do any alignment or padding by inserting, for example, -enough C<"x">es while packing. There is no way for pack() and unpack() -to know where characters are going to or coming from, so they -handle their output and input as flat sequences of characters. - -=item * - -A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may -take a repeat count either as postfix, or for unpack(), also via the C</> -template character. Within each repetition of a group, positioning with -C<@> starts over at 0. Therefore, the result of - - pack("@1A((@2A)@3A)", qw[X Y Z]) - -is the string C<"\0X\0\0YZ">. - -=item * - -C<x> and C<X> accept the C<!> modifier to act as alignment commands: they -jump forward or back to the closest position aligned at a multiple of C<count> -characters. For example, to pack() or unpack() a C structure like - - struct { - char c; /* one signed, 8-bit character */ - double d; - char cc[2]; - } - -one may need to use the template C<c x![d] d c[2]>. This assumes that -doubles must be aligned to the size of double. - -For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1; -both are no-ops. - -=item * - -C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to -represent signed 16-/32-bit integers in big-/little-endian order. -This is portable only when all platforms sharing packed data use the -same binary representation for signed integers; for example, when all -platforms use two's-complement representation. - -=item * - -Comments can be embedded in a TEMPLATE using C<#> through the end of line. -White space can separate pack codes from each other, but modifiers and -repeat counts must follow immediately. Breaking complex templates into -individual line-by-line components, suitably annotated, can do as much to -improve legibility and maintainability of pack/unpack formats as C</x> can -for complicated pattern matches. - -=item * - -If TEMPLATE requires more arguments than pack() is given, pack() -assumes additional C<""> arguments. If TEMPLATE requires fewer arguments -than given, extra arguments are ignored. - -=back - -Examples: - - $foo = pack("WWWW",65,66,67,68); - # foo eq "ABCD" - $foo = pack("W4",65,66,67,68); - # same thing - $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9); - # same thing with Unicode circled letters. - $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); - # same thing with Unicode circled letters. You don't get the - # UTF-8 bytes because the U at the start of the format caused - # a switch to U0-mode, so the UTF-8 bytes get joined into - # characters - $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9); - # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9" - # This is the UTF-8 encoding of the string in the - # previous example - - $foo = pack("ccxxcc",65,66,67,68); - # foo eq "AB\0\0CD" - - # NOTE: The examples above featuring "W" and "c" are true - # only on ASCII and ASCII-derived systems such as ISO Latin 1 - # and UTF-8. On EBCDIC systems, the first example would be - # $foo = pack("WWWW",193,194,195,196); - - $foo = pack("s2",1,2); - # "\001\000\002\000" on little-endian - # "\000\001\000\002" on big-endian - - $foo = pack("a4","abcd","x","y","z"); - # "abcd" - - $foo = pack("aaaa","abcd","x","y","z"); - # "axyz" - - $foo = pack("a14","abcdefg"); - # "abcdefg\0\0\0\0\0\0\0" - - $foo = pack("i9pl", gmtime); - # a real struct tm (on my system anyway) - - $utmp_template = "Z8 Z8 Z16 L"; - $utmp = pack($utmp_template, @utmp1); - # a struct utmp (BSDish) - - @utmp2 = unpack($utmp_template, $utmp); - # "@utmp1" eq "@utmp2" - - sub bintodec { - unpack("N", pack("B32", substr("0" x 32 . shift, -32))); - } - - $foo = pack('sx2l', 12, 34); - # short 12, two zero bytes padding, long 34 - $bar = pack('s@4l', 12, 34); - # short 12, zero fill to position 4, long 34 - # $foo eq $bar - $baz = pack('s.l', 12, 4, 34); - # short 12, zero fill to position 4, long 34 - - $foo = pack('nN', 42, 4711); - # pack big-endian 16- and 32-bit unsigned integers - $foo = pack('S>L>', 42, 4711); - # exactly the same - $foo = pack('s<l<', -42, 4711); - # pack little-endian 16- and 32-bit signed integers - $foo = pack('(sl)<', -42, 4711); - # exactly the same - -The same template may generally also be used in unpack(). - -=item package NAMESPACE - -=item package NAMESPACE VERSION -X<package> X<module> X<namespace> X<version> - -=item package NAMESPACE BLOCK - -=item package NAMESPACE VERSION BLOCK -X<package> X<module> X<namespace> X<version> - -=for Pod::Functions declare a separate global namespace - -Declares the BLOCK or the rest of the compilation unit as being in the -given namespace. The scope of the package declaration is either the -supplied code BLOCK or, in the absence of a BLOCK, from the declaration -itself through the end of current scope (the enclosing block, file, or -C<eval>). That is, the forms without a BLOCK are operative through the end -of the current scope, just like the C<my>, C<state>, and C<our> operators. -All unqualified dynamic identifiers in this scope will be in the given -namespace, except where overridden by another C<package> declaration or -when they're one of the special identifiers that qualify into C<main::>, -like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables. - -A package statement affects dynamic variables only, including those -you've used C<local> on, but I<not> lexical variables, which are created -with C<my>, C<state>, or C<our>. Typically it would be the first -declaration in a file included by C<require> or C<use>. You can switch into a -package in more than one place, since this only determines which default -symbol table the compiler uses for the rest of that block. You can refer to -identifiers in other packages than the current one by prefixing the identifier -with the package name and a double colon, as in C<$SomePack::var> -or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main> -package as assumed. That is, C<$::sail> is equivalent to -C<$main::sail> (as well as to C<$main'sail>, still seen in ancient -code, mostly from Perl 4). - -If VERSION is provided, C<package> sets the C<$VERSION> variable in the given -namespace to a L<version> object with the VERSION provided. VERSION must be a -"strict" style version number as defined by the L<version> module: a positive -decimal number (integer or decimal-fraction) without exponentiation or else a -dotted-decimal v-string with a leading 'v' character and at least three -components. You should set C<$VERSION> only once per package. - -See L<perlmod/"Packages"> for more information about packages, modules, -and classes. See L<perlsub> for other scoping issues. - -=item __PACKAGE__ -X<__PACKAGE__> - -=for Pod::Functions +5.004 the current package - -A special token that returns the name of the package in which it occurs. - -=item pipe READHANDLE,WRITEHANDLE -X<pipe> - -=for Pod::Functions open a pair of connected filehandles - -Opens a pair of connected pipes like the corresponding system call. -Note that if you set up a loop of piped processes, deadlock can occur -unless you are very careful. In addition, note that Perl's pipes use -IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE -after each command, depending on the application. - -See L<IPC::Open2>, L<IPC::Open3>, and -L<perlipc/"Bidirectional Communication with Another Process"> -for examples of such things. - -On systems that support a close-on-exec flag on files, that flag is set -on all newly opened file descriptors whose C<fileno>s are I<higher> than -the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>. - -=item pop ARRAY -X<pop> X<stack> - -=item pop EXPR - -=item pop - -=for Pod::Functions remove the last element from an array and return it - -Pops and returns the last value of the array, shortening the array by -one element. - -Returns the undefined value if the array is empty, although this may also -happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the -main program, but the C<@_> array in subroutines, just like C<shift>. - -Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a -reference to an unblessed array. The argument will be dereferenced -automatically. This aspect of C<pop> is considered highly experimental. -The exact behaviour may change in a future version of Perl. - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.014; # so push/pop/etc work on scalars (experimental) - -=item pos SCALAR -X<pos> X<match, position> - -=item pos - -=for Pod::Functions find or set the offset for the last/next m//g search - -Returns the offset of where the last C<m//g> search left off for the -variable in question (C<$_> is used when the variable is not -specified). Note that 0 is a valid match offset. C<undef> indicates -that the search position is reset (usually due to match failure, but -can also be because no match has yet been run on the scalar). - -C<pos> directly accesses the location used by the regexp engine to -store the offset, so assigning to C<pos> will change that offset, and -so will also influence the C<\G> zero-width assertion in regular -expressions. Both of these effects take place for the next match, so -you can't affect the position with C<pos> during the current match, -such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>. - -Setting C<pos> also resets the I<matched with zero-length> flag, described -under L<perlre/"Repeated Patterns Matching a Zero-length Substring">. - -Because a failed C<m//gc> match doesn't reset the offset, the return -from C<pos> won't change either in this case. See L<perlre> and -L<perlop>. - -=item print FILEHANDLE LIST -X<print> - -=item print FILEHANDLE - -=item print LIST - -=item print - -=for Pod::Functions output a list to a filehandle - -Prints a string or a list of strings. Returns true if successful. -FILEHANDLE may be a scalar variable containing the name of or a reference -to the filehandle, thus introducing one level of indirection. (NOTE: If -FILEHANDLE is a variable and the next token is a term, it may be -misinterpreted as an operator unless you interpose a C<+> or put -parentheses around the arguments.) If FILEHANDLE is omitted, prints to the -last selected (see L</select>) output handle. If LIST is omitted, prints -C<$_> to the currently selected output handle. To use FILEHANDLE alone to -print the content of C<$_> to it, you must use a real filehandle like -C<FH>, not an indirect one like C<$fh>. To set the default output handle -to something other than STDOUT, use the select operation. - -The current value of C<$,> (if any) is printed between each LIST item. The -current value of C<$\> (if any) is printed after the entire LIST has been -printed. Because print takes a LIST, anything in the LIST is evaluated in -list context, including any subroutines whose return lists you pass to -C<print>. Be careful not to follow the print keyword with a left -parenthesis unless you want the corresponding right parenthesis to -terminate the arguments to the print; put parentheses around all arguments -(or interpose a C<+>, but that doesn't look as good). - -If you're storing handles in an array or hash, or in general whenever -you're using any expression more complex than a bareword handle or a plain, -unsubscripted scalar variable to retrieve it, you will have to use a block -returning the filehandle value instead, in which case the LIST may not be -omitted: - - print { $files[$i] } "stuff\n"; - print { $OK ? STDOUT : STDERR } "stuff\n"; - -Printing to a closed pipe or socket will generate a SIGPIPE signal. See -L<perlipc> for more on signal handling. - -=item printf FILEHANDLE FORMAT, LIST -X<printf> - -=item printf FILEHANDLE - -=item printf FORMAT, LIST - -=item printf - -=for Pod::Functions output a formatted list to a filehandle - -Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> -(the output record separator) is not appended. The first argument of the -list will be interpreted as the C<printf> format. See -L<sprintf|/sprintf FORMAT, LIST> for an -explanation of the format argument. If you omit the LIST, C<$_> is used; -to use FILEHANDLE without a LIST, you must use a real filehandle like -C<FH>, not an indirect one like C<$fh>. If C<use locale> (including -C<use locale ':not_characters'>) is in effect and -POSIX::setlocale() has been called, the character used for the decimal -separator in formatted floating-point numbers is affected by the LC_NUMERIC -locale setting. See L<perllocale> and L<POSIX>. - -Don't fall into the trap of using a C<printf> when a simple -C<print> would do. The C<print> is more efficient and less -error prone. - -=item prototype FUNCTION -X<prototype> - -=for Pod::Functions +5.002 get the prototype (if any) of a subroutine - -Returns the prototype of a function as a string (or C<undef> if the -function has no prototype). FUNCTION is a reference to, or the name of, -the function whose prototype you want to retrieve. - -If FUNCTION is a string starting with C<CORE::>, the rest is taken as a -name for a Perl builtin. If the builtin is not I<overridable> (such as -C<qw//>) or if its arguments cannot be adequately expressed by a prototype -(such as C<system>), prototype() returns C<undef>, because the builtin -does not really behave like a Perl function. Otherwise, the string -describing the equivalent prototype is returned. - -=item push ARRAY,LIST -X<push> X<stack> - -=item push EXPR,LIST - -=for Pod::Functions append one or more elements to an array - -Treats ARRAY as a stack by appending the values of LIST to the end of -ARRAY. The length of ARRAY increases by the length of LIST. Has the same -effect as - - for $value (LIST) { - $ARRAY[++$#ARRAY] = $value; - } - -but is more efficient. Returns the number of elements in the array following -the completed C<push>. - -Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a -reference to an unblessed array. The argument will be dereferenced -automatically. This aspect of C<push> is considered highly experimental. -The exact behaviour may change in a future version of Perl. - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.014; # so push/pop/etc work on scalars (experimental) - -=item q/STRING/ - -=for Pod::Functions singly quote a string - -=item qq/STRING/ - -=for Pod::Functions doubly quote a string - -=item qw/STRING/ - -=for Pod::Functions quote a list of words - -=item qx/STRING/ - -=for Pod::Functions backquote quote a string - -Generalized quotes. See L<perlop/"Quote-Like Operators">. - -=item qr/STRING/ - -=for Pod::Functions +5.005 compile pattern - -Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">. - -=item quotemeta EXPR -X<quotemeta> X<metacharacter> - -=item quotemeta - -=for Pod::Functions quote regular expression magic characters - -Returns the value of EXPR with all the ASCII non-"word" -characters backslashed. (That is, all ASCII characters not matching -C</[A-Za-z_0-9]/> will be preceded by a backslash in the -returned string, regardless of any locale settings.) -This is the internal function implementing -the C<\Q> escape in double-quoted strings. -(See below for the behavior on non-ASCII code points.) - -If EXPR is omitted, uses C<$_>. - -quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into -regular expressions, because by default an interpolated variable will be -considered a mini-regular expression. For example: - - my $sentence = 'The quick brown fox jumped over the lazy dog'; - my $substring = 'quick.*?fox'; - $sentence =~ s{$substring}{big bad wolf}; - -Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>. - -On the other hand: - - my $sentence = 'The quick brown fox jumped over the lazy dog'; - my $substring = 'quick.*?fox'; - $sentence =~ s{\Q$substring\E}{big bad wolf}; - -Or: - - my $sentence = 'The quick brown fox jumped over the lazy dog'; - my $substring = 'quick.*?fox'; - my $quoted_substring = quotemeta($substring); - $sentence =~ s{$quoted_substring}{big bad wolf}; - -Will both leave the sentence as is. -Normally, when accepting literal string -input from the user, quotemeta() or C<\Q> must be used. - -In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded -strings, but not quoted in UTF-8 strings. - -Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for -quoting non-ASCII characters; the quoting of ASCII characters is -unchanged. - -Also unchanged is the quoting of non-UTF-8 strings when outside the -scope of a C<use feature 'unicode_strings'>, which is to quote all -characters in the upper Latin1 range. This provides complete backwards -compatibility for old programs which do not use Unicode. (Note that -C<unicode_strings> is automatically enabled within the scope of a -S<C<use v5.12>> or greater.) - -Within the scope of C<use locale>, all non-ASCII Latin1 code points -are quoted whether the string is encoded as UTF-8 or not. As mentioned -above, locale does not affect the quoting of ASCII-range characters. -This protects against those locales where characters such as C<"|"> are -considered to be word characters. - -Otherwise, Perl quotes non-ASCII characters using an adaptation from -Unicode (see L<http://www.unicode.org/reports/tr31/>.) -The only code points that are quoted are those that have any of the -Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, -Default_Ignorable_Code_Point, or General_Category=Control. - -Of these properties, the two important ones are Pattern_Syntax and -Pattern_White_Space. They have been set up by Unicode for exactly this -purpose of deciding which characters in a regular expression pattern -should be quoted. No character that can be in an identifier has these -properties. - -Perl promises, that if we ever add regular expression pattern -metacharacters to the dozen already defined -(C<\ E<verbar> ( ) [ { ^ $ * + ? .>), that we will only use ones that have the -Pattern_Syntax property. Perl also promises, that if we ever add -characters that are considered to be white space in regular expressions -(currently mostly affected by C</x>), they will all have the -Pattern_White_Space property. - -Unicode promises that the set of code points that have these two -properties will never change, so something that is not quoted in v5.16 -will never need to be quoted in any future Perl release. (Not all the -code points that match Pattern_Syntax have actually had characters -assigned to them; so there is room to grow, but they are quoted -whether assigned or not. Perl, of course, would never use an -unassigned code point as an actual metacharacter.) - -Quoting characters that have the other 3 properties is done to enhance -the readability of the regular expression and not because they actually -need to be quoted for regular expression purposes (characters with the -White_Space property are likely to be indistinguishable on the page or -screen from those with the Pattern_White_Space property; and the other -two properties contain non-printing characters). - -=item rand EXPR -X<rand> X<random> - -=item rand - -=for Pod::Functions retrieve the next pseudorandom number - -Returns a random fractional number greater than or equal to C<0> and less -than the value of EXPR. (EXPR should be positive.) If EXPR is -omitted, the value C<1> is used. Currently EXPR with the value C<0> is -also special-cased as C<1> (this was undocumented before Perl 5.8.0 -and is subject to change in future versions of Perl). Automatically calls -C<srand> unless C<srand> has already been called. See also C<srand>. - -Apply C<int()> to the value returned by C<rand()> if you want random -integers instead of random fractional numbers. For example, - - int(rand(10)) - -returns a random integer between C<0> and C<9>, inclusive. - -(Note: If your rand function consistently returns numbers that are too -large or too small, then your version of Perl was probably compiled -with the wrong number of RANDBITS.) - -B<C<rand()> is not cryptographically secure. You should not rely -on it in security-sensitive situations.> As of this writing, a -number of third-party CPAN modules offer random number generators -intended by their authors to be cryptographically secure, -including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, -and L<Math::TrulyRandom>. - -=item read FILEHANDLE,SCALAR,LENGTH,OFFSET -X<read> X<file, read> - -=item read FILEHANDLE,SCALAR,LENGTH - -=for Pod::Functions fixed-length buffered input from a filehandle - -Attempts to read LENGTH I<characters> of data into variable SCALAR -from the specified FILEHANDLE. Returns the number of characters -actually read, C<0> at end of file, or undef if there was an error (in -the latter case C<$!> is also set). SCALAR will be grown or shrunk -so that the last character actually read is the last character of the -scalar after the read. - -An OFFSET may be specified to place the read data at some place in the -string other than the beginning. A negative OFFSET specifies -placement at that many characters counting backwards from the end of -the string. A positive OFFSET greater than the length of SCALAR -results in the string being padded to the required size with C<"\0"> -bytes before the result of the read is appended. - -The call is implemented in terms of either Perl's or your system's native -fread(3) library function. To get a true read(2) system call, see -L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>. - -Note the I<characters>: depending on the status of the filehandle, -either (8-bit) bytes or characters are read. By default, all -filehandles operate on bytes, but for example if the filehandle has -been opened with the C<:utf8> I/O layer (see L</open>, and the C<open> -pragma, L<open>), the I/O will operate on UTF8-encoded Unicode -characters, not bytes. Similarly for the C<:encoding> pragma: -in that case pretty much any characters can be read. - -=item readdir DIRHANDLE -X<readdir> - -=for Pod::Functions get a directory from a directory handle - -Returns the next directory entry for a directory opened by C<opendir>. -If used in list context, returns all the rest of the entries in the -directory. If there are no more entries, returns the undefined value in -scalar context and the empty list in list context. - -If you're planning to filetest the return values out of a C<readdir>, you'd -better prepend the directory in question. Otherwise, because we didn't -C<chdir> there, it would have been testing the wrong file. - - opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!"; - @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh); - closedir $dh; - -As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop, -which will set C<$_> on every iteration. - - opendir(my $dh, $some_dir) || die; - while(readdir $dh) { - print "$some_dir/$_\n"; - } - closedir $dh; - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious failures, put this sort of thing at the -top of your file to signal that your code will work I<only> on Perls of a -recent vintage: - - use 5.012; # so readdir assigns to $_ in a lone while test - -=item readline EXPR - -=item readline -X<readline> X<gets> X<fgets> - -=for Pod::Functions fetch a record from a file - -Reads from the filehandle whose typeglob is contained in EXPR (or from -C<*ARGV> if EXPR is not provided). In scalar context, each call reads and -returns the next line until end-of-file is reached, whereupon the -subsequent call returns C<undef>. In list context, reads until end-of-file -is reached and returns a list of lines. Note that the notion of "line" -used here is whatever you may have defined with C<$/> or -C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. - -When C<$/> is set to C<undef>, when C<readline> is in scalar -context (i.e., file slurp mode), and when an empty file is read, it -returns C<''> the first time, followed by C<undef> subsequently. - -This is the internal function implementing the C<< <EXPR> >> -operator, but you can use it directly. The C<< <EXPR> >> -operator is discussed in more detail in L<perlop/"I/O Operators">. - - $line = <STDIN>; - $line = readline(*STDIN); # same thing - -If C<readline> encounters an operating system error, C<$!> will be set -with the corresponding error message. It can be helpful to check -C<$!> when you are reading from filehandles you don't trust, such as a -tty or a socket. The following example uses the operator form of -C<readline> and dies if the result is not defined. - - while ( ! eof($fh) ) { - defined( $_ = <$fh> ) or die "readline failed: $!"; - ... - } - -Note that you have can't handle C<readline> errors that way with the -C<ARGV> filehandle. In that case, you have to open each element of -C<@ARGV> yourself since C<eof> handles C<ARGV> differently. - - foreach my $arg (@ARGV) { - open(my $fh, $arg) or warn "Can't open $arg: $!"; - - while ( ! eof($fh) ) { - defined( $_ = <$fh> ) - or die "readline failed for $arg: $!"; - ... - } - } - -=item readlink EXPR -X<readlink> - -=item readlink - -=for Pod::Functions determine where a symbolic link is pointing - -Returns the value of a symbolic link, if symbolic links are -implemented. If not, raises an exception. If there is a system -error, returns the undefined value and sets C<$!> (errno). If EXPR is -omitted, uses C<$_>. - -Portability issues: L<perlport/readlink>. - -=item readpipe EXPR - -=item readpipe -X<readpipe> - -=for Pod::Functions execute a system command and collect standard output - -EXPR is executed as a system command. -The collected standard output of the command is returned. -In scalar context, it comes back as a single (potentially -multi-line) string. In list context, returns a list of lines -(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). -This is the internal function implementing the C<qx/EXPR/> -operator, but you can use it directly. The C<qx/EXPR/> -operator is discussed in more detail in L<perlop/"I/O Operators">. -If EXPR is omitted, uses C<$_>. - -=item recv SOCKET,SCALAR,LENGTH,FLAGS -X<recv> - -=for Pod::Functions receive a message over a Socket - -Receives a message on a socket. Attempts to receive LENGTH characters -of data into variable SCALAR from the specified SOCKET filehandle. -SCALAR will be grown or shrunk to the length actually read. Takes the -same flags as the system call of the same name. Returns the address -of the sender if SOCKET's protocol supports this; returns an empty -string otherwise. If there's an error, returns the undefined value. -This call is actually implemented in terms of recvfrom(2) system call. -See L<perlipc/"UDP: Message Passing"> for examples. - -Note the I<characters>: depending on the status of the socket, either -(8-bit) bytes or characters are received. By default all sockets -operate on bytes, but for example if the socket has been changed using -binmode() to operate with the C<:encoding(utf8)> I/O layer (see the -C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode -characters, not bytes. Similarly for the C<:encoding> pragma: in that -case pretty much any characters can be read. - -=item redo LABEL -X<redo> - -=item redo - -=for Pod::Functions start this loop iteration over again - -The C<redo> command restarts the loop block without evaluating the -conditional again. The C<continue> block, if any, is not executed. If -the LABEL is omitted, the command refers to the innermost enclosing -loop. Programs that want to lie to themselves about what was just input -normally use this command: - - # a simpleminded Pascal comment stripper - # (warning: assumes no { or } in strings) - LINE: while (<STDIN>) { - while (s|({.*}.*){.*}|$1 |) {} - s|{.*}| |; - if (s|{.*| |) { - $front = $_; - while (<STDIN>) { - if (/}/) { # end of comment? - s|^|$front\{|; - redo LINE; - } - } - } - print; - } - -C<redo> cannot be used to retry a block that returns a value such as -C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit -a grep() or map() operation. - -Note that a block by itself is semantically identical to a loop -that executes once. Thus C<redo> inside such a block will effectively -turn it into a looping construct. - -See also L</continue> for an illustration of how C<last>, C<next>, and -C<redo> work. - -=item ref EXPR -X<ref> X<reference> - -=item ref - -=for Pod::Functions find out the type of thing being referenced - -Returns a non-empty string if EXPR is a reference, the empty -string otherwise. If EXPR -is not specified, C<$_> will be used. The value returned depends on the -type of thing the reference is a reference to. -Builtin types include: - - SCALAR - ARRAY - HASH - CODE - REF - GLOB - LVALUE - FORMAT - IO - VSTRING - Regexp - -If the referenced object has been blessed into a package, then that package -name is returned instead. You can think of C<ref> as a C<typeof> operator. - - if (ref($r) eq "HASH") { - print "r is a reference to a hash.\n"; - } - unless (ref($r)) { - print "r is not a reference at all.\n"; - } - -The return value C<LVALUE> indicates a reference to an lvalue that is not -a variable. You get this from taking the reference of function calls like -C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points -to a L<version string|perldata/"Version Strings">. - -The result C<Regexp> indicates that the argument is a regular expression -resulting from C<qr//>. - -See also L<perlref>. - -=item rename OLDNAME,NEWNAME -X<rename> X<move> X<mv> X<ren> - -=for Pod::Functions change a filename - -Changes the name of a file; an existing file NEWNAME will be -clobbered. Returns true for success, false otherwise. - -Behavior of this function varies wildly depending on your system -implementation. For example, it will usually not work across file system -boundaries, even though the system I<mv> command sometimes compensates -for this. Other restrictions include whether it works on directories, -open files, or pre-existing files. Check L<perlport> and either the -rename(2) manpage or equivalent system documentation for details. - -For a platform independent C<move> function look at the L<File::Copy> -module. - -Portability issues: L<perlport/rename>. - -=item require VERSION -X<require> - -=item require EXPR - -=item require - -=for Pod::Functions load in external functions from a library at runtime - -Demands a version of Perl specified by VERSION, or demands some semantics -specified by EXPR or by C<$_> if EXPR is not supplied. - -VERSION may be either a numeric argument such as 5.006, which will be -compared to C<$]>, or a literal of the form v5.6.1, which will be compared -to C<$^V> (aka $PERL_VERSION). An exception is raised if -VERSION is greater than the version of the current Perl interpreter. -Compare with L</use>, which can do a similar check at compile time. - -Specifying VERSION as a literal of the form v5.6.1 should generally be -avoided, because it leads to misleading error messages under earlier -versions of Perl that do not support this syntax. The equivalent numeric -version should be used instead. - - require v5.6.1; # run time version check - require 5.6.1; # ditto - require 5.006_001; # ditto; preferred for backwards compatibility - -Otherwise, C<require> demands that a library file be included if it -hasn't already been included. The file is included via the do-FILE -mechanism, which is essentially just a variety of C<eval> with the -caveat that lexical variables in the invoking script will be invisible -to the included code. Has semantics similar to the following subroutine: - - sub require { - my ($filename) = @_; - if (exists $INC{$filename}) { - return 1 if $INC{$filename}; - die "Compilation failed in require"; - } - my ($realfilename,$result); - ITER: { - foreach $prefix (@INC) { - $realfilename = "$prefix/$filename"; - if (-f $realfilename) { - $INC{$filename} = $realfilename; - $result = do $realfilename; - last ITER; - } - } - die "Can't find $filename in \@INC"; - } - if ($@) { - $INC{$filename} = undef; - die $@; - } elsif (!$result) { - delete $INC{$filename}; - die "$filename did not return true value"; - } else { - return $result; - } - } - -Note that the file will not be included twice under the same specified -name. - -The file must return true as the last statement to indicate -successful execution of any initialization code, so it's customary to -end such a file with C<1;> unless you're sure it'll return true -otherwise. But it's better just to put the C<1;>, in case you add more -statements. - -If EXPR is a bareword, the require assumes a "F<.pm>" extension and -replaces "F<::>" with "F</>" in the filename for you, -to make it easy to load standard modules. This form of loading of -modules does not risk altering your namespace. - -In other words, if you try this: - - require Foo::Bar; # a splendid bareword - -The require function will actually look for the "F<Foo/Bar.pm>" file in the -directories specified in the C<@INC> array. - -But if you try this: - - $class = 'Foo::Bar'; - require $class; # $class is not a bareword - #or - require "Foo::Bar"; # not a bareword because of the "" - -The require function will look for the "F<Foo::Bar>" file in the @INC array and -will complain about not finding "F<Foo::Bar>" there. In this case you can do: - - eval "require $class"; - -Now that you understand how C<require> looks for files with a -bareword argument, there is a little extra functionality going on behind -the scenes. Before C<require> looks for a "F<.pm>" extension, it will -first look for a similar filename with a "F<.pmc>" extension. If this file -is found, it will be loaded in place of any file ending in a "F<.pm>" -extension. - -You can also insert hooks into the import facility by putting Perl code -directly into the @INC array. There are three forms of hooks: subroutine -references, array references, and blessed objects. - -Subroutine references are the simplest case. When the inclusion system -walks through @INC and encounters a subroutine, this subroutine gets -called with two parameters, the first a reference to itself, and the -second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The -subroutine should return either nothing or else a list of up to three -values in the following order: - -=over - -=item 1 - -A filehandle, from which the file will be read. - -=item 2 - -A reference to a subroutine. If there is no filehandle (previous item), -then this subroutine is expected to generate one line of source code per -call, writing the line into C<$_> and returning 1, then finally at end of -file returning 0. If there is a filehandle, then the subroutine will be -called to act as a simple source filter, with the line as read in C<$_>. -Again, return 1 for each valid line, and 0 after all lines have been -returned. - -=item 3 - -Optional state for the subroutine. The state is passed in as C<$_[1]>. A -reference to the subroutine itself is passed in as C<$_[0]>. - -=back - -If an empty list, C<undef>, or nothing that matches the first 3 values above -is returned, then C<require> looks at the remaining elements of @INC. -Note that this filehandle must be a real filehandle (strictly a typeglob -or reference to a typeglob, whether blessed or unblessed); tied filehandles -will be ignored and processing will stop there. - -If the hook is an array reference, its first element must be a subroutine -reference. This subroutine is called as above, but the first parameter is -the array reference. This lets you indirectly pass arguments to -the subroutine. - -In other words, you can write: - - push @INC, \&my_sub; - sub my_sub { - my ($coderef, $filename) = @_; # $coderef is \&my_sub - ... - } - -or: - - push @INC, [ \&my_sub, $x, $y, ... ]; - sub my_sub { - my ($arrayref, $filename) = @_; - # Retrieve $x, $y, ... - my @parameters = @$arrayref[1..$#$arrayref]; - ... - } - -If the hook is an object, it must provide an INC method that will be -called as above, the first parameter being the object itself. (Note that -you must fully qualify the sub's name, as unqualified C<INC> is always forced -into package C<main>.) Here is a typical code layout: - - # In Foo.pm - package Foo; - sub new { ... } - sub Foo::INC { - my ($self, $filename) = @_; - ... - } - - # In the main program - push @INC, Foo->new(...); - -These hooks are also permitted to set the %INC entry -corresponding to the files they have loaded. See L<perlvar/%INC>. - -For a yet-more-powerful import facility, see L</use> and L<perlmod>. - -=item reset EXPR -X<reset> - -=item reset - -=for Pod::Functions clear all variables of a given name - -Generally used in a C<continue> block at the end of a loop to clear -variables and reset C<??> searches so that they work again. The -expression is interpreted as a list of single characters (hyphens -allowed for ranges). All variables and arrays beginning with one of -those letters are reset to their pristine state. If the expression is -omitted, one-match searches (C<?pattern?>) are reset to match again. -Only resets variables or searches in the current package. Always returns -1. Examples: - - reset 'X'; # reset all X variables - reset 'a-z'; # reset lower case variables - reset; # just reset ?one-time? searches - -Resetting C<"A-Z"> is not recommended because you'll wipe out your -C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package -variables; lexical variables are unaffected, but they clean themselves -up on scope exit anyway, so you'll probably want to use them instead. -See L</my>. - -=item return EXPR -X<return> - -=item return - -=for Pod::Functions get out of a function early - -Returns from a subroutine, C<eval>, or C<do FILE> with the value -given in EXPR. Evaluation of EXPR may be in list, scalar, or void -context, depending on how the return value will be used, and the context -may vary from one execution to the next (see L</wantarray>). If no EXPR -is given, returns an empty list in list context, the undefined value in -scalar context, and (of course) nothing at all in void context. - -(In the absence of an explicit C<return>, a subroutine, eval, -or do FILE automatically returns the value of the last expression -evaluated.) - -=item reverse LIST -X<reverse> X<rev> X<invert> - -=for Pod::Functions flip a string or a list - -In list context, returns a list value consisting of the elements -of LIST in the opposite order. In scalar context, concatenates the -elements of LIST and returns a string value with all characters -in the opposite order. - - print join(", ", reverse "world", "Hello"); # Hello, world - - print scalar reverse "dlrow ,", "olleH"; # Hello, world - -Used without arguments in scalar context, reverse() reverses C<$_>. - - $_ = "dlrow ,olleH"; - print reverse; # No output, list context - print scalar reverse; # Hello, world - -Note that reversing an array to itself (as in C<@a = reverse @a>) will -preserve non-existent elements whenever possible, i.e., for non magical -arrays or tied arrays with C<EXISTS> and C<DELETE> methods. - -This operator is also handy for inverting a hash, although there are some -caveats. If a value is duplicated in the original hash, only one of those -can be represented as a key in the inverted hash. Also, this has to -unwind one hash and build a whole new one, which may take some time -on a large hash, such as from a DBM file. - - %by_name = reverse %by_address; # Invert the hash - -=item rewinddir DIRHANDLE -X<rewinddir> - -=for Pod::Functions reset directory handle - -Sets the current position to the beginning of the directory for the -C<readdir> routine on DIRHANDLE. - -Portability issues: L<perlport/rewinddir>. - -=item rindex STR,SUBSTR,POSITION -X<rindex> - -=item rindex STR,SUBSTR - -=for Pod::Functions right-to-left substring search - -Works just like index() except that it returns the position of the I<last> -occurrence of SUBSTR in STR. If POSITION is specified, returns the -last occurrence beginning at or before that position. - -=item rmdir FILENAME -X<rmdir> X<rd> X<directory, remove> - -=item rmdir - -=for Pod::Functions remove a directory - -Deletes the directory specified by FILENAME if that directory is -empty. If it succeeds it returns true; otherwise it returns false and -sets C<$!> (errno). If FILENAME is omitted, uses C<$_>. - -To remove a directory tree recursively (C<rm -rf> on Unix) look at -the C<rmtree> function of the L<File::Path> module. - -=item s/// - -=for Pod::Functions replace a pattern with a string - -The substitution operator. See L<perlop/"Regexp Quote-Like Operators">. - -=item say FILEHANDLE LIST -X<say> - -=item say FILEHANDLE - -=item say LIST - -=item say - -=for Pod::Functions +say output a list to a filehandle, appending a newline - -Just like C<print>, but implicitly appends a newline. C<say LIST> is -simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use -FILEHANDLE without a LIST to print the contents of C<$_> to it, you must -use a real filehandle like C<FH>, not an indirect one like C<$fh>. - -This keyword is available only when the C<"say"> feature -is enabled, or when prefixed with C<CORE::>; see -L<feature>. Alternately, include a C<use v5.10> or later to the current -scope. - -=item scalar EXPR -X<scalar> X<context> - -=for Pod::Functions force a scalar context - -Forces EXPR to be interpreted in scalar context and returns the value -of EXPR. - - @counts = ( scalar @a, scalar @b, scalar @c ); - -There is no equivalent operator to force an expression to -be interpolated in list context because in practice, this is never -needed. If you really wanted to do so, however, you could use -the construction C<@{[ (some expression) ]}>, but usually a simple -C<(some expression)> suffices. - -Because C<scalar> is a unary operator, if you accidentally use a -parenthesized list for the EXPR, this behaves as a scalar comma expression, -evaluating all but the last element in void context and returning the final -element evaluated in scalar context. This is seldom what you want. - -The following single statement: - - print uc(scalar(&foo,$bar)),$baz; - -is the moral equivalent of these two: - - &foo; - print(uc($bar),$baz); - -See L<perlop> for more details on unary operators and the comma operator. - -=item seek FILEHANDLE,POSITION,WHENCE -X<seek> X<fseek> X<filehandle, position> - -=for Pod::Functions reposition file pointer for random-access I/O - -Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. -FILEHANDLE may be an expression whose value gives the name of the -filehandle. The values for WHENCE are C<0> to set the new position -I<in bytes> to POSITION; C<1> to set it to the current position plus -POSITION; and C<2> to set it to EOF plus POSITION, typically -negative. For WHENCE you may use the constants C<SEEK_SET>, -C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end -of the file) from the L<Fcntl> module. Returns C<1> on success, false -otherwise. - -Note the I<in bytes>: even if the filehandle has been set to -operate on characters (for example by using the C<:encoding(utf8)> open -layer), tell() will return byte offsets, not character offsets -(because implementing that would render seek() and tell() rather slow). - -If you want to position the file for C<sysread> or C<syswrite>, don't use -C<seek>, because buffering makes its effect on the file's read-write position -unpredictable and non-portable. Use C<sysseek> instead. - -Due to the rules and rigors of ANSI C, on some systems you have to do a -seek whenever you switch between reading and writing. Amongst other -things, this may have the effect of calling stdio's clearerr(3). -A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: - - seek(TEST,0,1); - -This is also useful for applications emulating C<tail -f>. Once you hit -EOF on your read and then sleep for a while, you (probably) have to stick in a -dummy seek() to reset things. The C<seek> doesn't change the position, -but it I<does> clear the end-of-file condition on the handle, so that the -next C<< <FILE> >> makes Perl try again to read something. (We hope.) - -If that doesn't work (some I/O implementations are particularly -cantankerous), you might need something like this: - - for (;;) { - for ($curpos = tell(FILE); $_ = <FILE>; - $curpos = tell(FILE)) { - # search for some stuff and put it into files - } - sleep($for_a_while); - seek(FILE, $curpos, 0); - } - -=item seekdir DIRHANDLE,POS -X<seekdir> - -=for Pod::Functions reposition directory pointer - -Sets the current position for the C<readdir> routine on DIRHANDLE. POS -must be a value returned by C<telldir>. C<seekdir> also has the same caveats -about possible directory compaction as the corresponding system library -routine. - -=item select FILEHANDLE -X<select> X<filehandle, default> - -=item select - -=for Pod::Functions reset default output or do I/O multiplexing - -Returns the currently selected filehandle. If FILEHANDLE is supplied, -sets the new current default filehandle for output. This has two -effects: first, a C<write> or a C<print> without a filehandle -default to this FILEHANDLE. Second, references to variables related to -output will refer to this output channel. - -For example, to set the top-of-form format for more than one -output channel, you might do the following: - - select(REPORT1); - $^ = 'report1_top'; - select(REPORT2); - $^ = 'report2_top'; - -FILEHANDLE may be an expression whose value gives the name of the -actual filehandle. Thus: - - $oldfh = select(STDERR); $| = 1; select($oldfh); - -Some programmers may prefer to think of filehandles as objects with -methods, preferring to write the last example as: - - use IO::Handle; - STDERR->autoflush(1); - -Portability issues: L<perlport/select>. - -=item select RBITS,WBITS,EBITS,TIMEOUT -X<select> - -This calls the select(2) syscall with the bit masks specified, which -can be constructed using C<fileno> and C<vec>, along these lines: - - $rin = $win = $ein = ''; - vec($rin, fileno(STDIN), 1) = 1; - vec($win, fileno(STDOUT), 1) = 1; - $ein = $rin | $win; - -If you want to select on many filehandles, you may wish to write a -subroutine like this: - - sub fhbits { - my @fhlist = @_; - my $bits = ""; - for my $fh (@fhlist) { - vec($bits, fileno($fh), 1) = 1; - } - return $bits; - } - $rin = fhbits(*STDIN, *TTY, *MYSOCK); - -The usual idiom is: - - ($nfound,$timeleft) = - select($rout=$rin, $wout=$win, $eout=$ein, $timeout); - -or to block until something becomes ready just do this - - $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); - -Most systems do not bother to return anything useful in $timeleft, so -calling select() in scalar context just returns $nfound. - -Any of the bit masks can also be undef. The timeout, if specified, is -in seconds, which may be fractional. Note: not all implementations are -capable of returning the $timeleft. If not, they always return -$timeleft equal to the supplied $timeout. - -You can effect a sleep of 250 milliseconds this way: - - select(undef, undef, undef, 0.25); - -Note that whether C<select> gets restarted after signals (say, SIGALRM) -is implementation-dependent. See also L<perlport> for notes on the -portability of C<select>. - -On error, C<select> behaves just like select(2): it returns --1 and sets C<$!>. - -On some Unixes, select(2) may report a socket file descriptor as "ready for -reading" even when no data is available, and thus any subsequent C<read> -would block. This can be avoided if you always use O_NONBLOCK on the -socket. See select(2) and fcntl(2) for further details. - -The standard C<IO::Select> module provides a user-friendlier interface -to C<select>, mostly because it does all the bit-mask work for you. - -B<WARNING>: One should not attempt to mix buffered I/O (like C<read> -or <FH>) with C<select>, except as permitted by POSIX, and even -then only on POSIX systems. You have to use C<sysread> instead. - -Portability issues: L<perlport/select>. - -=item semctl ID,SEMNUM,CMD,ARG -X<semctl> - -=for Pod::Functions SysV semaphore control operations - -Calls the System V IPC function semctl(2). You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is IPC_STAT or -GETALL, then ARG must be a variable that will hold the returned -semid_ds structure or semaphore value array. Returns like C<ioctl>: -the undefined value for error, "C<0 but true>" for zero, or the actual -return value otherwise. The ARG must consist of a vector of native -short integers, which may be created with C<pack("s!",(0)x$nsem)>. -See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> -documentation. - -Portability issues: L<perlport/semctl>. - -=item semget KEY,NSEMS,FLAGS -X<semget> - -=for Pod::Functions get set of SysV semaphores - -Calls the System V IPC function semget(2). Returns the semaphore id, or -the undefined value on error. See also -L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> -documentation. - -Portability issues: L<perlport/semget>. - -=item semop KEY,OPSTRING -X<semop> - -=for Pod::Functions SysV semaphore operations - -Calls the System V IPC function semop(2) for semaphore operations -such as signalling and waiting. OPSTRING must be a packed array of -semop structures. Each semop structure can be generated with -C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING -implies the number of semaphore operations. Returns true if -successful, false on error. As an example, the -following code waits on semaphore $semnum of semaphore id $semid: - - $semop = pack("s!3", $semnum, -1, 0); - die "Semaphore trouble: $!\n" unless semop($semid, $semop); - -To signal the semaphore, replace C<-1> with C<1>. See also -L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> -documentation. - -Portability issues: L<perlport/semop>. - -=item send SOCKET,MSG,FLAGS,TO -X<send> - -=item send SOCKET,MSG,FLAGS - -=for Pod::Functions send a message over a socket - -Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET -filehandle. Takes the same flags as the system call of the same name. On -unconnected sockets, you must specify a destination to I<send to>, in which -case it does a sendto(2) syscall. Returns the number of characters sent, -or the undefined value on error. The sendmsg(2) syscall is currently -unimplemented. See L<perlipc/"UDP: Message Passing"> for examples. - -Note the I<characters>: depending on the status of the socket, either -(8-bit) bytes or characters are sent. By default all sockets operate -on bytes, but for example if the socket has been changed using -binmode() to operate with the C<:encoding(utf8)> I/O layer (see -L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8 -encoded Unicode characters, not bytes. Similarly for the C<:encoding> -pragma: in that case pretty much any characters can be sent. - -=item setpgrp PID,PGRP -X<setpgrp> X<group> - -=for Pod::Functions set the process group of a process - -Sets the current process group for the specified PID, C<0> for the current -process. Raises an exception when used on a machine that doesn't -implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, -it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not -accept any arguments, so only C<setpgrp(0,0)> is portable. See also -C<POSIX::setsid()>. - -Portability issues: L<perlport/setpgrp>. - -=item setpriority WHICH,WHO,PRIORITY -X<setpriority> X<priority> X<nice> X<renice> - -=for Pod::Functions set a process's nice value - -Sets the current priority for a process, a process group, or a user. -(See setpriority(2).) Raises an exception when used on a machine -that doesn't implement setpriority(2). - -Portability issues: L<perlport/setpriority>. - -=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL -X<setsockopt> - -=for Pod::Functions set some socket options - -Sets the socket option requested. Returns C<undef> on error. -Use integer constants provided by the C<Socket> module for -LEVEL and OPNAME. Values for LEVEL can also be obtained from -getprotobyname. OPTVAL might either be a packed string or an integer. -An integer OPTVAL is shorthand for pack("i", OPTVAL). - -An example disabling Nagle's algorithm on a socket: - - use Socket qw(IPPROTO_TCP TCP_NODELAY); - setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1); - -Portability issues: L<perlport/setsockopt>. - -=item shift ARRAY -X<shift> - -=item shift EXPR - -=item shift - -=for Pod::Functions remove the first element of an array, and return it - -Shifts the first value of the array off and returns it, shortening the -array by 1 and moving everything down. If there are no elements in the -array, returns the undefined value. If ARRAY is omitted, shifts the -C<@_> array within the lexical scope of subroutines and formats, and the -C<@ARGV> array outside a subroutine and also within the lexical scopes -established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, -C<UNITCHECK {}>, and C<END {}> constructs. - -Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a -reference to an unblessed array. The argument will be dereferenced -automatically. This aspect of C<shift> is considered highly experimental. -The exact behaviour may change in a future version of Perl. - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.014; # so push/pop/etc work on scalars (experimental) - -See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the -same thing to the left end of an array that C<pop> and C<push> do to the -right end. - -=item shmctl ID,CMD,ARG -X<shmctl> - -=for Pod::Functions SysV shared memory operations - -Calls the System V IPC function shmctl. You'll probably have to say - - use IPC::SysV; - -first to get the correct constant definitions. If CMD is C<IPC_STAT>, -then ARG must be a variable that will hold the returned C<shmid_ds> -structure. Returns like ioctl: C<undef> for error; "C<0> but -true" for zero; and the actual return value otherwise. -See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. - -Portability issues: L<perlport/shmctl>. - -=item shmget KEY,SIZE,FLAGS -X<shmget> - -=for Pod::Functions get SysV shared memory segment identifier - -Calls the System V IPC function shmget. Returns the shared memory -segment id, or C<undef> on error. -See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. - -Portability issues: L<perlport/shmget>. - -=item shmread ID,VAR,POS,SIZE -X<shmread> -X<shmwrite> - -=for Pod::Functions read SysV shared memory - -=item shmwrite ID,STRING,POS,SIZE - -=for Pod::Functions write SysV shared memory - -Reads or writes the System V shared memory segment ID starting at -position POS for size SIZE by attaching to it, copying in/out, and -detaching from it. When reading, VAR must be a variable that will -hold the data read. When writing, if STRING is too long, only SIZE -bytes are used; if STRING is too short, nulls are written to fill out -SIZE bytes. Return true if successful, false on error. -shmread() taints the variable. See also L<perlipc/"SysV IPC">, -C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN. - -Portability issues: L<perlport/shmread> and L<perlport/shmwrite>. - -=item shutdown SOCKET,HOW -X<shutdown> - -=for Pod::Functions close down just half of a socket connection - -Shuts down a socket connection in the manner indicated by HOW, which -has the same interpretation as in the syscall of the same name. - - shutdown(SOCKET, 0); # I/we have stopped reading data - shutdown(SOCKET, 1); # I/we have stopped writing data - shutdown(SOCKET, 2); # I/we have stopped using this socket - -This is useful with sockets when you want to tell the other -side you're done writing but not done reading, or vice versa. -It's also a more insistent form of close because it also -disables the file descriptor in any forked copies in other -processes. - -Returns C<1> for success; on error, returns C<undef> if -the first argument is not a valid filehandle, or returns C<0> and sets -C<$!> for any other failure. - -=item sin EXPR -X<sin> X<sine> X<asin> X<arcsine> - -=item sin - -=for Pod::Functions return the sine of a number - -Returns the sine of EXPR (expressed in radians). If EXPR is omitted, -returns sine of C<$_>. - -For the inverse sine operation, you may use the C<Math::Trig::asin> -function, or use this relation: - - sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } - -=item sleep EXPR -X<sleep> X<pause> - -=item sleep - -=for Pod::Functions block for some number of seconds - -Causes the script to sleep for (integer) EXPR seconds, or forever if no -argument is given. Returns the integer number of seconds actually slept. - -May be interrupted if the process receives a signal such as C<SIGALRM>. - - eval { - local $SIG{ALARM} = sub { die "Alarm!\n" }; - sleep; - }; - die $@ unless $@ eq "Alarm!\n"; - -You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep> -is often implemented using C<alarm>. - -On some older systems, it may sleep up to a full second less than what -you requested, depending on how it counts seconds. Most modern systems -always sleep the full amount. They may appear to sleep longer than that, -however, because your process might not be scheduled right away in a -busy multitasking system. - -For delays of finer granularity than one second, the Time::HiRes module -(from CPAN, and starting from Perl 5.8 part of the standard -distribution) provides usleep(). You may also use Perl's four-argument -version of select() leaving the first three arguments undefined, or you -might be able to use the C<syscall> interface to access setitimer(2) if -your system supports it. See L<perlfaq8> for details. - -See also the POSIX module's C<pause> function. - -=item socket SOCKET,DOMAIN,TYPE,PROTOCOL -X<socket> - -=for Pod::Functions create a socket - -Opens a socket of the specified kind and attaches it to filehandle -SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for -the syscall of the same name. You should C<use Socket> first -to get the proper definitions imported. See the examples in -L<perlipc/"Sockets: Client/Server Communication">. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptor, as determined by the -value of $^F. See L<perlvar/$^F>. - -=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL -X<socketpair> - -=for Pod::Functions create a pair of sockets - -Creates an unnamed pair of sockets in the specified domain, of the -specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as -for the syscall of the same name. If unimplemented, raises an exception. -Returns true if successful. - -On systems that support a close-on-exec flag on files, the flag will -be set for the newly opened file descriptors, as determined by the value -of $^F. See L<perlvar/$^F>. - -Some systems defined C<pipe> in terms of C<socketpair>, in which a call -to C<pipe(Rdr, Wtr)> is essentially: - - use Socket; - socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); - shutdown(Rdr, 1); # no more writing for reader - shutdown(Wtr, 0); # no more reading for writer - -See L<perlipc> for an example of socketpair use. Perl 5.8 and later will -emulate socketpair using IP sockets to localhost if your system implements -sockets but not socketpair. - -Portability issues: L<perlport/socketpair>. - -=item sort SUBNAME LIST -X<sort> X<qsort> X<quicksort> X<mergesort> - -=item sort BLOCK LIST - -=item sort LIST - -=for Pod::Functions sort a list of values - -In list context, this sorts the LIST and returns the sorted list value. -In scalar context, the behaviour of C<sort()> is undefined. - -If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison -order. If SUBNAME is specified, it gives the name of a subroutine -that returns an integer less than, equal to, or greater than C<0>, -depending on how the elements of the list are to be ordered. (The -C<< <=> >> and C<cmp> operators are extremely useful in such routines.) -SUBNAME may be a scalar variable name (unsubscripted), in which case -the value provides the name of (or a reference to) the actual -subroutine to use. In place of a SUBNAME, you can provide a BLOCK as -an anonymous, in-line sort subroutine. - -If the subroutine's prototype is C<($$)>, the elements to be compared are -passed by reference in C<@_>, as for a normal subroutine. This is slower -than unprototyped subroutines, where the elements to be compared are passed -into the subroutine as the package global variables $a and $b (see example -below). Note that in the latter case, it is usually highly counter-productive -to declare $a and $b as lexicals. - -If the subroutine is an XSUB, the elements to be compared are pushed on to -the stack, the way arguments are usually passed to XSUBs. $a and $b are -not set. - -The values to be compared are always passed by reference and should not -be modified. - -You also cannot exit out of the sort block or subroutine using any of the -loop control operators described in L<perlsyn> or with C<goto>. - -When C<use locale> (but not C<use locale 'not_characters'>) is in -effect, C<sort LIST> sorts LIST according to the -current collation locale. See L<perllocale>. - -sort() returns aliases into the original list, much as a for loop's index -variable aliases the list elements. That is, modifying an element of a -list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>) -actually modifies the element in the original list. This is usually -something to be avoided when writing clear code. - -Perl 5.6 and earlier used a quicksort algorithm to implement sort. -That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort -preserves the input order of elements that compare equal. Although -quicksort's run time is O(NlogN) when averaged over all arrays of -length N, the time can be O(N**2), I<quadratic> behavior, for some -inputs.) In 5.7, the quicksort implementation was replaced with -a stable mergesort algorithm whose worst-case behavior is O(NlogN). -But benchmarks indicated that for some inputs, on some platforms, -the original quicksort was faster. 5.8 has a sort pragma for -limited control of the sort. Its rather blunt control of the -underlying algorithm may not persist into future Perls, but the -ability to characterize the input or output in implementation -independent ways quite probably will. See L<the sort pragma|sort>. - -Examples: - - # sort lexically - @articles = sort @files; - - # same thing, but with explicit sort routine - @articles = sort {$a cmp $b} @files; - - # now case-insensitively - @articles = sort {fc($a) cmp fc($b)} @files; - - # same thing in reversed order - @articles = sort {$b cmp $a} @files; - - # sort numerically ascending - @articles = sort {$a <=> $b} @files; - - # sort numerically descending - @articles = sort {$b <=> $a} @files; - - # this sorts the %age hash by value instead of key - # using an in-line function - @eldest = sort { $age{$b} <=> $age{$a} } keys %age; - - # sort using explicit subroutine name - sub byage { - $age{$a} <=> $age{$b}; # presuming numeric - } - @sortedclass = sort byage @class; - - sub backwards { $b cmp $a } - @harry = qw(dog cat x Cain Abel); - @george = qw(gone chased yz Punished Axed); - print sort @harry; - # prints AbelCaincatdogx - print sort backwards @harry; - # prints xdogcatCainAbel - print sort @george, 'to', @harry; - # prints AbelAxedCainPunishedcatchaseddoggonetoxyz - - # inefficiently sort by descending numeric compare using - # the first integer after the first = sign, or the - # whole record case-insensitively otherwise - - my @new = sort { - ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] - || - fc($a) cmp fc($b) - } @old; - - # same thing, but much more efficiently; - # we'll build auxiliary indices instead - # for speed - my @nums = @caps = (); - for (@old) { - push @nums, ( /=(\d+)/ ? $1 : undef ); - push @caps, fc($_); - } - - my @new = @old[ sort { - $nums[$b] <=> $nums[$a] - || - $caps[$a] cmp $caps[$b] - } 0..$#old - ]; - - # same thing, but without any temps - @new = map { $_->[0] } - sort { $b->[1] <=> $a->[1] - || - $a->[2] cmp $b->[2] - } map { [$_, /=(\d+)/, fc($_)] } @old; - - # using a prototype allows you to use any comparison subroutine - # as a sort subroutine (including other package's subroutines) - package other; - sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here - - package main; - @new = sort other::backwards @old; - - # guarantee stability, regardless of algorithm - use sort 'stable'; - @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; - - # force use of mergesort (not portable outside Perl 5.8) - use sort '_mergesort'; # note discouraging _ - @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old; - -Warning: syntactical care is required when sorting the list returned from -a function. If you want to sort the list returned by the function call -C<find_records(@key)>, you can use: - - @contact = sort { $a cmp $b } find_records @key; - @contact = sort +find_records(@key); - @contact = sort &find_records(@key); - @contact = sort(find_records(@key)); - -If instead you want to sort the array @key with the comparison routine -C<find_records()> then you can use: - - @contact = sort { find_records() } @key; - @contact = sort find_records(@key); - @contact = sort(find_records @key); - @contact = sort(find_records (@key)); - -If you're using strict, you I<must not> declare $a -and $b as lexicals. They are package globals. That means -that if you're in the C<main> package and type - - @articles = sort {$b <=> $a} @files; - -then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), -but if you're in the C<FooPack> package, it's the same as typing - - @articles = sort {$FooPack::b <=> $FooPack::a} @files; - -The comparison function is required to behave. If it returns -inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and -sometimes saying the opposite, for example) the results are not -well-defined. - -Because C<< <=> >> returns C<undef> when either operand is C<NaN> -(not-a-number), be careful when sorting with a -comparison function like C<< $a <=> $b >> any lists that might contain a -C<NaN>. The following example takes advantage that C<NaN != NaN> to -eliminate any C<NaN>s from the input list. - - @result = sort { $a <=> $b } grep { $_ == $_ } @input; - -=item splice ARRAY or EXPR,OFFSET,LENGTH,LIST -X<splice> - -=item splice ARRAY or EXPR,OFFSET,LENGTH - -=item splice ARRAY or EXPR,OFFSET - -=item splice ARRAY or EXPR - -=for Pod::Functions add or remove elements anywhere in an array - -Removes the elements designated by OFFSET and LENGTH from an array, and -replaces them with the elements of LIST, if any. In list context, -returns the elements removed from the array. In scalar context, -returns the last element removed, or C<undef> if no elements are -removed. The array grows or shrinks as necessary. -If OFFSET is negative then it starts that far from the end of the array. -If LENGTH is omitted, removes everything from OFFSET onward. -If LENGTH is negative, removes the elements from OFFSET onward -except for -LENGTH elements at the end of the array. -If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is -past the end of the array, Perl issues a warning, and splices at the -end of the array. - -The following equivalences hold (assuming C<< $#a >= $i >> ) - - push(@a,$x,$y) splice(@a,@a,0,$x,$y) - pop(@a) splice(@a,-1) - shift(@a) splice(@a,0,1) - unshift(@a,$x,$y) splice(@a,0,0,$x,$y) - $a[$i] = $y splice(@a,$i,1,$y) - -Example, assuming array lengths are passed before arrays: - - sub aeq { # compare two list values - my(@a) = splice(@_,0,shift); - my(@b) = splice(@_,0,shift); - return 0 unless @a == @b; # same len? - while (@a) { - return 0 if pop(@a) ne pop(@b); - } - return 1; - } - if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } - -Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a -reference to an unblessed array. The argument will be dereferenced -automatically. This aspect of C<splice> is considered highly experimental. -The exact behaviour may change in a future version of Perl. - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.014; # so push/pop/etc work on scalars (experimental) - -=item split /PATTERN/,EXPR,LIMIT -X<split> - -=item split /PATTERN/,EXPR - -=item split /PATTERN/ - -=item split - -=for Pod::Functions split up a string using a regexp delimiter - -Splits the string EXPR into a list of strings and returns the -list in list context, or the size of the list in scalar context. - -If only PATTERN is given, EXPR defaults to C<$_>. - -Anything in EXPR that matches PATTERN is taken to be a separator -that separates the EXPR into substrings (called "I<fields>") that -do B<not> include the separator. Note that a separator may be -longer than one character or even have no characters at all (the -empty string, which is a zero-width match). - -The PATTERN need not be constant; an expression may be used -to specify a pattern that varies at runtime. - -If PATTERN matches the empty string, the EXPR is split at the match -position (between characters). As an example, the following: - - print join(':', split('b', 'abc')), "\n"; - -uses the 'b' in 'abc' as a separator to produce the output 'a:c'. -However, this: - - print join(':', split('', 'abc')), "\n"; - -uses empty string matches as separators to produce the output -'a:b:c'; thus, the empty string may be used to split EXPR into a -list of its component characters. - -As a special case for C<split>, the empty pattern given in -L<match operator|perlop/"m/PATTERN/msixpodualgc"> syntax (C<//>) specifically matches the empty string, which is contrary to its usual -interpretation as the last successful match. - -If PATTERN is C</^/>, then it is treated as if it used the -L<multiline modifier|perlreref/OPERATORS> (C</^/m>), since it -isn't much use otherwise. - -As another special case, C<split> emulates the default behavior of the -command line tool B<awk> when the PATTERN is either omitted or a I<literal -string> composed of a single space character (such as S<C<' '>> or -S<C<"\x20">>, but not e.g. S<C</ />>). In this case, any leading -whitespace in EXPR is removed before splitting occurs, and the PATTERN is -instead treated as if it were C</\s+/>; in particular, this means that -I<any> contiguous whitespace (not just a single space character) is used as -a separator. However, this special treatment can be avoided by specifying -the pattern S<C</ />> instead of the string S<C<" ">>, thereby allowing -only a single space character to be a separator. - -If omitted, PATTERN defaults to a single space, S<C<" ">>, triggering -the previously described I<awk> emulation. - -If LIMIT is specified and positive, it represents the maximum number -of fields into which the EXPR may be split; in other words, LIMIT is -one greater than the maximum number of times EXPR may be split. Thus, -the LIMIT value C<1> means that EXPR may be split a maximum of zero -times, producing a maximum of one field (namely, the entire value of -EXPR). For instance: - - print join(':', split(//, 'abc', 1)), "\n"; - -produces the output 'abc', and this: - - print join(':', split(//, 'abc', 2)), "\n"; - -produces the output 'a:bc', and each of these: - - print join(':', split(//, 'abc', 3)), "\n"; - print join(':', split(//, 'abc', 4)), "\n"; - -produces the output 'a:b:c'. - -If LIMIT is negative, it is treated as if it were instead arbitrarily -large; as many fields as possible are produced. - -If LIMIT is omitted (or, equivalently, zero), then it is usually -treated as if it were instead negative but with the exception that -trailing empty fields are stripped (empty leading fields are always -preserved); if all fields are empty, then all fields are considered to -be trailing (and are thus stripped in this case). Thus, the following: - - print join(':', split(',', 'a,b,c,,,')), "\n"; - -produces the output 'a:b:c', but the following: - - print join(':', split(',', 'a,b,c,,,', -1)), "\n"; - -produces the output 'a:b:c:::'. - -In time-critical applications, it is worthwhile to avoid splitting -into more fields than necessary. Thus, when assigning to a list, -if LIMIT is omitted (or zero), then LIMIT is treated as though it -were one larger than the number of variables in the list; for the -following, LIMIT is implicitly 4: - - ($login, $passwd, $remainder) = split(/:/); - -Note that splitting an EXPR that evaluates to the empty string always -produces zero fields, regardless of the LIMIT specified. - -An empty leading field is produced when there is a positive-width -match at the beginning of EXPR. For instance: - - print join(':', split(/ /, ' abc')), "\n"; - -produces the output ':abc'. However, a zero-width match at the -beginning of EXPR never produces an empty field, so that: - - print join(':', split(//, ' abc')); - -produces the output S<' :a:b:c'> (rather than S<': :a:b:c'>). - -An empty trailing field, on the other hand, is produced when there is a -match at the end of EXPR, regardless of the length of the match -(of course, unless a non-zero LIMIT is given explicitly, such fields are -removed, as in the last example). Thus: - - print join(':', split(//, ' abc', -1)), "\n"; - -produces the output S<' :a:b:c:'>. - -If the PATTERN contains -L<capturing groups|perlretut/Grouping things and hierarchical matching>, -then for each separator, an additional field is produced for each substring -captured by a group (in the order in which the groups are specified, -as per L<backreferences|perlretut/Backreferences>); if any group does not -match, then it captures the C<undef> value instead of a substring. Also, -note that any such additional field is produced whenever there is a -separator (that is, whenever a split occurs), and such an additional field -does B<not> count towards the LIMIT. Consider the following expressions -evaluated in list context (each returned list is provided in the associated -comment): - - split(/-|,/, "1-10,20", 3) - # ('1', '10', '20') - - split(/(-|,)/, "1-10,20", 3) - # ('1', '-', '10', ',', '20') - - split(/-|(,)/, "1-10,20", 3) - # ('1', undef, '10', ',', '20') - - split(/(-)|,/, "1-10,20", 3) - # ('1', '-', '10', undef, '20') - - split(/(-)|(,)/, "1-10,20", 3) - # ('1', '-', undef, '10', undef, ',', '20') - -=item sprintf FORMAT, LIST -X<sprintf> - -=for Pod::Functions formatted print into a string - -Returns a string formatted by the usual C<printf> conventions of the C -library function C<sprintf>. See below for more details -and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of -the general principles. - -For example: - - # Format number with up to 8 leading zeroes - $result = sprintf("%08d", $number); - - # Round number to 3 digits after decimal point - $rounded = sprintf("%.3f", $number); - -Perl does its own C<sprintf> formatting: it emulates the C -function sprintf(3), but doesn't use it except for floating-point -numbers, and even then only standard modifiers are allowed. -Non-standard extensions in your local sprintf(3) are -therefore unavailable from Perl. - -Unlike C<printf>, C<sprintf> does not do what you probably mean when you -pass it an array as your first argument. -The array is given scalar context, -and instead of using the 0th element of the array as the format, Perl will -use the count of elements in the array as the format, which is almost never -useful. - -Perl's C<sprintf> permits the following universally-known conversions: - - %% a percent sign - %c a character with the given number - %s a string - %d a signed integer, in decimal - %u an unsigned integer, in decimal - %o an unsigned integer, in octal - %x an unsigned integer, in hexadecimal - %e a floating-point number, in scientific notation - %f a floating-point number, in fixed decimal notation - %g a floating-point number, in %e or %f notation - -In addition, Perl permits the following widely-supported conversions: - - %X like %x, but using upper-case letters - %E like %e, but using an upper-case "E" - %G like %g, but with an upper-case "E" (if applicable) - %b an unsigned integer, in binary - %B like %b, but using an upper-case "B" with the # flag - %p a pointer (outputs the Perl value's address in hexadecimal) - %n special: *stores* the number of characters output so far - into the next argument in the parameter list - -Finally, for backward (and we do mean "backward") compatibility, Perl -permits these unnecessary but widely-supported conversions: - - %i a synonym for %d - %D a synonym for %ld - %U a synonym for %lu - %O a synonym for %lo - %F a synonym for %f - -Note that the number of exponent digits in the scientific notation produced -by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the -exponent less than 100 is system-dependent: it may be three or less -(zero-padded as necessary). In other words, 1.23 times ten to the -99th may be either "1.23e99" or "1.23e099". - -Between the C<%> and the format letter, you may specify several -additional attributes controlling the interpretation of the format. -In order, these are: - -=over 4 - -=item format parameter index - -An explicit format parameter index, such as C<2$>. By default sprintf -will format the next unused argument in the list, but this allows you -to take the arguments out of order: - - printf '%2$d %1$d', 12, 34; # prints "34 12" - printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1" - -=item flags - -one or more of: - - space prefix non-negative number with a space - + prefix non-negative number with a plus sign - - left-justify within the field - 0 use zeros, not spaces, to right-justify - # ensure the leading "0" for any octal, - prefix non-zero hexadecimal with "0x" or "0X", - prefix non-zero binary with "0b" or "0B" - -For example: - - printf '<% d>', 12; # prints "< 12>" - printf '<%+d>', 12; # prints "<+12>" - printf '<%6s>', 12; # prints "< 12>" - printf '<%-6s>', 12; # prints "<12 >" - printf '<%06s>', 12; # prints "<000012>" - printf '<%#o>', 12; # prints "<014>" - printf '<%#x>', 12; # prints "<0xc>" - printf '<%#X>', 12; # prints "<0XC>" - printf '<%#b>', 12; # prints "<0b1100>" - printf '<%#B>', 12; # prints "<0B1100>" - -When a space and a plus sign are given as the flags at once, -a plus sign is used to prefix a positive number. - - printf '<%+ d>', 12; # prints "<+12>" - printf '<% +d>', 12; # prints "<+12>" - -When the # flag and a precision are given in the %o conversion, -the precision is incremented if it's necessary for the leading "0". - - printf '<%#.5o>', 012; # prints "<00012>" - printf '<%#.5o>', 012345; # prints "<012345>" - printf '<%#.0o>', 0; # prints "<0>" - -=item vector flag - -This flag tells Perl to interpret the supplied string as a vector of -integers, one for each character in the string. Perl applies the format to -each integer in turn, then joins the resulting strings with a separator (a -dot C<.> by default). This can be useful for displaying ordinal values of -characters in arbitrary strings: - - printf "%vd", "AB\x{100}"; # prints "65.66.256" - printf "version is v%vd\n", $^V; # Perl's version - -Put an asterisk C<*> before the C<v> to override the string to -use to separate the numbers: - - printf "address is %*vX\n", ":", $addr; # IPv6 address - printf "bits are %0*v8b\n", " ", $bits; # random bitstring - -You can also explicitly specify the argument number to use for -the join string using something like C<*2$v>; for example: - - printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses - -=item (minimum) width - -Arguments are usually formatted to be only as wide as required to -display the given value. You can override the width by putting -a number here, or get the width from the next argument (with C<*>) -or from a specified argument (e.g., with C<*2$>): - - printf "<%s>", "a"; # prints "<a>" - printf "<%6s>", "a"; # prints "< a>" - printf "<%*s>", 6, "a"; # prints "< a>" - printf "<%*2$s>", "a", 6; # prints "< a>" - printf "<%2s>", "long"; # prints "<long>" (does not truncate) - -If a field width obtained through C<*> is negative, it has the same -effect as the C<-> flag: left-justification. - -=item precision, or maximum width -X<precision> - -You can specify a precision (for numeric conversions) or a maximum -width (for string conversions) by specifying a C<.> followed by a number. -For floating-point formats except C<g> and C<G>, this specifies -how many places right of the decimal point to show (the default being 6). -For example: - - # these examples are subject to system-specific variation - printf '<%f>', 1; # prints "<1.000000>" - printf '<%.1f>', 1; # prints "<1.0>" - printf '<%.0f>', 1; # prints "<1>" - printf '<%e>', 10; # prints "<1.000000e+01>" - printf '<%.1e>', 10; # prints "<1.0e+01>" - -For "g" and "G", this specifies the maximum number of digits to show, -including those prior to the decimal point and those after it; for -example: - - # These examples are subject to system-specific variation. - printf '<%g>', 1; # prints "<1>" - printf '<%.10g>', 1; # prints "<1>" - printf '<%g>', 100; # prints "<100>" - printf '<%.1g>', 100; # prints "<1e+02>" - printf '<%.2g>', 100.01; # prints "<1e+02>" - printf '<%.5g>', 100.01; # prints "<100.01>" - printf '<%.4g>', 100.01; # prints "<100>" - -For integer conversions, specifying a precision implies that the -output of the number itself should be zero-padded to this width, -where the 0 flag is ignored: - - printf '<%.6d>', 1; # prints "<000001>" - printf '<%+.6d>', 1; # prints "<+000001>" - printf '<%-10.6d>', 1; # prints "<000001 >" - printf '<%10.6d>', 1; # prints "< 000001>" - printf '<%010.6d>', 1; # prints "< 000001>" - printf '<%+10.6d>', 1; # prints "< +000001>" - - printf '<%.6x>', 1; # prints "<000001>" - printf '<%#.6x>', 1; # prints "<0x000001>" - printf '<%-10.6x>', 1; # prints "<000001 >" - printf '<%10.6x>', 1; # prints "< 000001>" - printf '<%010.6x>', 1; # prints "< 000001>" - printf '<%#10.6x>', 1; # prints "< 0x000001>" - -For string conversions, specifying a precision truncates the string -to fit the specified width: - - printf '<%.5s>', "truncated"; # prints "<trunc>" - printf '<%10.5s>', "truncated"; # prints "< trunc>" - -You can also get the precision from the next argument using C<.*>: - - printf '<%.6x>', 1; # prints "<000001>" - printf '<%.*x>', 6, 1; # prints "<000001>" - -If a precision obtained through C<*> is negative, it counts -as having no precision at all. - - printf '<%.*s>', 7, "string"; # prints "<string>" - printf '<%.*s>', 3, "string"; # prints "<str>" - printf '<%.*s>', 0, "string"; # prints "<>" - printf '<%.*s>', -1, "string"; # prints "<string>" - - printf '<%.*d>', 1, 0; # prints "<0>" - printf '<%.*d>', 0, 0; # prints "<>" - printf '<%.*d>', -1, 0; # prints "<0>" - -You cannot currently get the precision from a specified number, -but it is intended that this will be possible in the future, for -example using C<.*2$>: - - printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>" - -=item size - -For numeric conversions, you can specify the size to interpret the -number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer -conversions (C<d u o x X b i D U O>), numbers are usually assumed to be -whatever the default integer size is on your platform (usually 32 or 64 -bits), but you can override this to use instead one of the standard C types, -as supported by the compiler used to build Perl: - - hh interpret integer as C type "char" or "unsigned char" - on Perl 5.14 or later - h interpret integer as C type "short" or "unsigned short" - j interpret integer as C type "intmax_t" on Perl 5.14 - or later, and only with a C99 compiler (unportable) - l interpret integer as C type "long" or "unsigned long" - q, L, or ll interpret integer as C type "long long", "unsigned long long", - or "quad" (typically 64-bit integers) - t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later - z interpret integer as C type "size_t" on Perl 5.14 or later - -As of 5.14, none of these raises an exception if they are not supported on -your platform. However, if warnings are enabled, a warning of the -C<printf> warning class is issued on an unsupported conversion flag. -Should you instead prefer an exception, do this: - - use warnings FATAL => "printf"; - -If you would like to know about a version dependency before you -start running the program, put something like this at its top: - - use 5.014; # for hh/j/t/z/ printf modifiers - -You can find out whether your Perl supports quads via L<Config>: - - use Config; - if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) { - print "Nice quads!\n"; - } - -For floating-point conversions (C<e f g E F G>), numbers are usually assumed -to be the default floating-point size on your platform (double or long double), -but you can force "long double" with C<q>, C<L>, or C<ll> if your -platform supports them. You can find out whether your Perl supports long -doubles via L<Config>: - - use Config; - print "long doubles\n" if $Config{d_longdbl} eq "define"; - -You can find out whether Perl considers "long double" to be the default -floating-point size to use on your platform via L<Config>: - - use Config; - if ($Config{uselongdouble} eq "define") { - print "long doubles by default\n"; - } - -It can also be that long doubles and doubles are the same thing: - - use Config; - ($Config{doublesize} == $Config{longdblsize}) && - print "doubles are long doubles\n"; - -The size specifier C<V> has no effect for Perl code, but is supported for -compatibility with XS code. It means "use the standard size for a Perl -integer or floating-point number", which is the default. - -=item order of arguments - -Normally, sprintf() takes the next unused argument as the value to -format for each format specification. If the format specification -uses C<*> to require additional arguments, these are consumed from -the argument list in the order they appear in the format -specification I<before> the value to format. Where an argument is -specified by an explicit index, this does not affect the normal -order for the arguments, even when the explicitly specified index -would have been the next argument. - -So: - - printf "<%*.*s>", $a, $b, $c; - -uses C<$a> for the width, C<$b> for the precision, and C<$c> -as the value to format; while: - - printf "<%*1$.*s>", $a, $b; - -would use C<$a> for the width and precision, and C<$b> as the -value to format. - -Here are some more examples; be aware that when using an explicit -index, the C<$> may need escaping: - - printf "%2\$d %d\n", 12, 34; # will print "34 12\n" - printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n" - printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n" - printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n" - -=back - -If C<use locale> (including C<use locale 'not_characters'>) is in effect -and POSIX::setlocale() has been called, -the character used for the decimal separator in formatted floating-point -numbers is affected by the LC_NUMERIC locale. See L<perllocale> -and L<POSIX>. - -=item sqrt EXPR -X<sqrt> X<root> X<square root> - -=item sqrt - -=for Pod::Functions square root function - -Return the positive square root of EXPR. If EXPR is omitted, uses -C<$_>. Works only for non-negative operands unless you've -loaded the C<Math::Complex> module. - - use Math::Complex; - print sqrt(-4); # prints 2i - -=item srand EXPR -X<srand> X<seed> X<randseed> - -=item srand - -=for Pod::Functions seed the random number generator - -Sets and returns the random number seed for the C<rand> operator. - -The point of the function is to "seed" the C<rand> function so that C<rand> -can produce a different sequence each time you run your program. When -called with a parameter, C<srand> uses that for the seed; otherwise it -(semi-)randomly chooses a seed. In either case, starting with Perl 5.14, -it returns the seed. To signal that your code will work I<only> on Perls -of a recent vintage: - - use 5.014; # so srand returns the seed - -If C<srand()> is not called explicitly, it is called implicitly without a -parameter at the first use of the C<rand> operator. However, this was not true -of versions of Perl before 5.004, so if your script will run under older -Perl versions, it should call C<srand>; otherwise most programs won't call -C<srand()> at all. - -But there are a few situations in recent Perls where programs are likely to -want to call C<srand>. One is for generating predictable results generally for -testing or debugging. There, you use C<srand($seed)>, with the same C<$seed> -each time. Another case is that you may want to call C<srand()> -after a C<fork()> to avoid child processes sharing the same seed value as the -parent (and consequently each other). - -Do B<not> call C<srand()> (i.e., without an argument) more than once per -process. The internal state of the random number generator should -contain more entropy than can be provided by any seed, so calling -C<srand()> again actually I<loses> randomness. - -Most implementations of C<srand> take an integer and will silently -truncate decimal numbers. This means C<srand(42)> will usually -produce the same results as C<srand(42.1)>. To be safe, always pass -C<srand> an integer. - -In versions of Perl prior to 5.004 the default seed was just the -current C<time>. This isn't a particularly good seed, so many old -programs supply their own seed value (often C<time ^ $$> or C<time ^ -($$ + ($$ << 15))>), but that isn't necessary any more. - -Frequently called programs (like CGI scripts) that simply use - - time ^ $$ - -for a seed can fall prey to the mathematical property that - - a^b == (a+1)^(b+1) - -one-third of the time. So don't do that. - -A typical use of the returned seed is for a test program which has too many -combinations to test comprehensively in the time available to it each run. It -can test a random subset each time, and should there be a failure, log the seed -used for that run so that it can later be used to reproduce the same results. - -B<C<rand()> is not cryptographically secure. You should not rely -on it in security-sensitive situations.> As of this writing, a -number of third-party CPAN modules offer random number generators -intended by their authors to be cryptographically secure, -including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>, -and L<Math::TrulyRandom>. - -=item stat FILEHANDLE -X<stat> X<file, status> X<ctime> - -=item stat EXPR - -=item stat DIRHANDLE - -=item stat - -=for Pod::Functions get a file's status information - -Returns a 13-element list giving the status info for a file, either -the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is -omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically -used as follows: - - ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, - $atime,$mtime,$ctime,$blksize,$blocks) - = stat($filename); - -Not all fields are supported on all filesystem types. Here are the -meanings of the fields: - - 0 dev device number of filesystem - 1 ino inode number - 2 mode file mode (type and permissions) - 3 nlink number of (hard) links to the file - 4 uid numeric user ID of file's owner - 5 gid numeric group ID of file's owner - 6 rdev the device identifier (special files only) - 7 size total size of file, in bytes - 8 atime last access time in seconds since the epoch - 9 mtime last modify time in seconds since the epoch - 10 ctime inode change time in seconds since the epoch (*) - 11 blksize preferred block size for file system I/O - 12 blocks actual number of blocks allocated - -(The epoch was at 00:00 January 1, 1970 GMT.) - -(*) Not all fields are supported on all filesystem types. Notably, the -ctime field is non-portable. In particular, you cannot expect it to be a -"creation time"; see L<perlport/"Files and Filesystems"> for details. - -If C<stat> is passed the special filehandle consisting of an underline, no -stat is done, but the current contents of the stat structure from the -last C<stat>, C<lstat>, or filetest are returned. Example: - - if (-x $file && (($d) = stat(_)) && $d < 0) { - print "$file is executable NFS file\n"; - } - -(This works on machines only for which the device number is negative -under NFS.) - -Because the mode contains both the file type and its permissions, you -should mask off the file type portion and (s)printf using a C<"%o"> -if you want to see the real permissions. - - $mode = (stat($filename))[2]; - printf "Permissions are %04o\n", $mode & 07777; - -In scalar context, C<stat> returns a boolean value indicating success -or failure, and, if successful, sets the information associated with -the special filehandle C<_>. - -The L<File::stat> module provides a convenient, by-name access mechanism: - - use File::stat; - $sb = stat($filename); - printf "File is %s, size is %s, perm %04o, mtime %s\n", - $filename, $sb->size, $sb->mode & 07777, - scalar localtime $sb->mtime; - -You can import symbolic mode constants (C<S_IF*>) and functions -(C<S_IS*>) from the Fcntl module: - - use Fcntl ':mode'; - - $mode = (stat($filename))[2]; - - $user_rwx = ($mode & S_IRWXU) >> 6; - $group_read = ($mode & S_IRGRP) >> 3; - $other_execute = $mode & S_IXOTH; - - printf "Permissions are %04o\n", S_IMODE($mode), "\n"; - - $is_setuid = $mode & S_ISUID; - $is_directory = S_ISDIR($mode); - -You could write the last two using the C<-u> and C<-d> operators. -Commonly available C<S_IF*> constants are: - - # Permissions: read, write, execute, for user, group, others. - - S_IRWXU S_IRUSR S_IWUSR S_IXUSR - S_IRWXG S_IRGRP S_IWGRP S_IXGRP - S_IRWXO S_IROTH S_IWOTH S_IXOTH - - # Setuid/Setgid/Stickiness/SaveText. - # Note that the exact meaning of these is system-dependent. - - S_ISUID S_ISGID S_ISVTX S_ISTXT - - # File types. Not all are necessarily available on - # your system. - - S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR - S_IFIFO S_IFSOCK S_IFWHT S_ENFMT - - # The following are compatibility aliases for S_IRUSR, - # S_IWUSR, and S_IXUSR. - - S_IREAD S_IWRITE S_IEXEC - -and the C<S_IF*> functions are - - S_IMODE($mode) the part of $mode containing the permission - bits and the setuid/setgid/sticky bits - - S_IFMT($mode) the part of $mode containing the file type - which can be bit-anded with (for example) - S_IFREG or with the following functions - - # The operators -f, -d, -l, -b, -c, -p, and -S. - - S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) - S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) - - # No direct -X operator counterpart, but for the first one - # the -g operator is often equivalent. The ENFMT stands for - # record flocking enforcement, a platform-dependent feature. - - S_ISENFMT($mode) S_ISWHT($mode) - -See your native chmod(2) and stat(2) documentation for more details -about the C<S_*> constants. To get status info for a symbolic link -instead of the target file behind the link, use the C<lstat> function. - -Portability issues: L<perlport/stat>. - -=item state EXPR -X<state> - -=item state TYPE EXPR - -=item state EXPR : ATTRS - -=item state TYPE EXPR : ATTRS - -=for Pod::Functions +state declare and assign a persistent lexical variable - -C<state> declares a lexically scoped variable, just like C<my>. -However, those variables will never be reinitialized, contrary to -lexical variables that are reinitialized each time their enclosing block -is entered. -See L<perlsub/"Persistent Private Variables"> for details. - -C<state> variables are enabled only when the C<use feature "state"> pragma -is in effect, unless the keyword is written as C<CORE::state>. -See also L<feature>. - -=item study SCALAR -X<study> - -=item study - -=for Pod::Functions optimize input data for repeated searches - -Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of -doing many pattern matches on the string before it is next modified. -This may or may not save time, depending on the nature and number of -patterns you are searching and the distribution of character -frequencies in the string to be searched; you probably want to compare -run times with and without it to see which is faster. Those loops -that scan for many short constant strings (including the constant -parts of more complex patterns) will benefit most. -(The way C<study> works is this: a linked list of every -character in the string to be searched is made, so we know, for -example, where all the C<'k'> characters are. From each search string, -the rarest character is selected, based on some static frequency tables -constructed from some C programs and English text. Only those places -that contain this "rarest" character are examined.) - -For example, here is a loop that inserts index producing entries -before any line containing a certain pattern: - - while (<>) { - study; - print ".IX foo\n" if /\bfoo\b/; - print ".IX bar\n" if /\bbar\b/; - print ".IX blurfl\n" if /\bblurfl\b/; - # ... - print; - } - -In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f> -will be looked at, because C<f> is rarer than C<o>. In general, this is -a big win except in pathological cases. The only question is whether -it saves you more time than it took to build the linked list in the -first place. - -Note that if you have to look for strings that you don't know till -runtime, you can build an entire loop as a string and C<eval> that to -avoid recompiling all your patterns all the time. Together with -undefining C<$/> to input entire files as one record, this can be quite -fast, often faster than specialized programs like fgrep(1). The following -scans a list of files (C<@files>) for a list of words (C<@words>), and prints -out the names of those files that contain a match: - - $search = 'while (<>) { study;'; - foreach $word (@words) { - $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; - } - $search .= "}"; - @ARGV = @files; - undef $/; - eval $search; # this screams - $/ = "\n"; # put back to normal input delimiter - foreach $file (sort keys(%seen)) { - print $file, "\n"; - } - -=item sub NAME BLOCK -X<sub> - -=item sub NAME (PROTO) BLOCK - -=item sub NAME : ATTRS BLOCK - -=item sub NAME (PROTO) : ATTRS BLOCK - -=for Pod::Functions declare a subroutine, possibly anonymously - -This is subroutine definition, not a real function I<per se>. Without a -BLOCK it's just a forward declaration. Without a NAME, it's an anonymous -function declaration, so does return a value: the CODE ref of the closure -just created. - -See L<perlsub> and L<perlref> for details about subroutines and -references; see L<attributes> and L<Attribute::Handlers> for more -information about attributes. - -=item __SUB__ -X<__SUB__> - -=for Pod::Functions +current_sub the current subroutine, or C<undef> if not in a subroutine - -A special token that returns the a reference to the current subroutine, or -C<undef> outside of a subroutine. - -This token is only available under C<use v5.16> or the "current_sub" -feature. See L<feature>. - -=item substr EXPR,OFFSET,LENGTH,REPLACEMENT -X<substr> X<substring> X<mid> X<left> X<right> - -=item substr EXPR,OFFSET,LENGTH - -=item substr EXPR,OFFSET - -=for Pod::Functions get or alter a portion of a string - -Extracts a substring out of EXPR and returns it. First character is at -offset zero. If OFFSET is negative, starts -that far back from the end of the string. If LENGTH is omitted, returns -everything through the end of the string. If LENGTH is negative, leaves that -many characters off the end of the string. - - my $s = "The black cat climbed the green tree"; - my $color = substr $s, 4, 5; # black - my $middle = substr $s, 4, -11; # black cat climbed the - my $end = substr $s, 14; # climbed the green tree - my $tail = substr $s, -4; # tree - my $z = substr $s, -4, 2; # tr - -You can use the substr() function as an lvalue, in which case EXPR -must itself be an lvalue. If you assign something shorter than LENGTH, -the string will shrink, and if you assign something longer than LENGTH, -the string will grow to accommodate it. To keep the string the same -length, you may need to pad or chop your value using C<sprintf>. - -If OFFSET and LENGTH specify a substring that is partly outside the -string, only the part within the string is returned. If the substring -is beyond either end of the string, substr() returns the undefined -value and produces a warning. When used as an lvalue, specifying a -substring that is entirely outside the string raises an exception. -Here's an example showing the behavior for boundary cases: - - my $name = 'fred'; - substr($name, 4) = 'dy'; # $name is now 'freddy' - my $null = substr $name, 6, 2; # returns "" (no warning) - my $oops = substr $name, 7; # returns undef, with warning - substr($name, 7) = 'gap'; # raises an exception - -An alternative to using substr() as an lvalue is to specify the -replacement string as the 4th argument. This allows you to replace -parts of the EXPR and return what was there before in one operation, -just as you can with splice(). - - my $s = "The black cat climbed the green tree"; - my $z = substr $s, 14, 7, "jumped from"; # climbed - # $s is now "The black cat jumped from the green tree" - -Note that the lvalue returned by the three-argument version of substr() acts as -a 'magic bullet'; each time it is assigned to, it remembers which part -of the original string is being modified; for example: - - $x = '1234'; - for (substr($x,1,2)) { - $_ = 'a'; print $x,"\n"; # prints 1a4 - $_ = 'xyz'; print $x,"\n"; # prints 1xyz4 - $x = '56789'; - $_ = 'pq'; print $x,"\n"; # prints 5pq9 - } - -With negative offsets, it remembers its position from the end of the string -when the target string is modified: - - $x = '1234'; - for (substr($x, -3, 2)) { - $_ = 'a'; print $x,"\n"; # prints 1a4, as above - $x = 'abcdefg'; - print $_,"\n"; # prints f - } - -Prior to Perl version 5.10, the result of using an lvalue multiple times was -unspecified. Prior to 5.16, the result with negative offsets was -unspecified. - -=item symlink OLDFILE,NEWFILE -X<symlink> X<link> X<symbolic link> X<link, symbolic> - -=for Pod::Functions create a symbolic link to a file - -Creates a new filename symbolically linked to the old filename. -Returns C<1> for success, C<0> otherwise. On systems that don't support -symbolic links, raises an exception. To check for that, -use eval: - - $symlink_exists = eval { symlink("",""); 1 }; - -Portability issues: L<perlport/symlink>. - -=item syscall NUMBER, LIST -X<syscall> X<system call> - -=for Pod::Functions execute an arbitrary system call - -Calls the system call specified as the first element of the list, -passing the remaining elements as arguments to the system call. If -unimplemented, raises an exception. The arguments are interpreted -as follows: if a given argument is numeric, the argument is passed as -an int. If not, the pointer to the string value is passed. You are -responsible to make sure a string is pre-extended long enough to -receive any result that might be written into a string. You can't use a -string literal (or other read-only string) as an argument to C<syscall> -because Perl has to assume that any string pointer might be written -through. If your -integer arguments are not literals and have never been interpreted in a -numeric context, you may need to add C<0> to them to force them to look -like numbers. This emulates the C<syswrite> function (or vice versa): - - require 'syscall.ph'; # may need to run h2ph - $s = "hi there\n"; - syscall(&SYS_write, fileno(STDOUT), $s, length $s); - -Note that Perl supports passing of up to only 14 arguments to your syscall, -which in practice should (usually) suffice. - -Syscall returns whatever value returned by the system call it calls. -If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). -Note that some system calls I<can> legitimately return C<-1>. The proper -way to handle such calls is to assign C<$!=0> before the call, then -check the value of C<$!> if C<syscall> returns C<-1>. - -There's a problem with C<syscall(&SYS_pipe)>: it returns the file -number of the read end of the pipe it creates, but there is no way -to retrieve the file number of the other end. You can avoid this -problem by using C<pipe> instead. - -Portability issues: L<perlport/syscall>. - -=item sysopen FILEHANDLE,FILENAME,MODE -X<sysopen> - -=item sysopen FILEHANDLE,FILENAME,MODE,PERMS - -=for Pod::Functions +5.002 open a file, pipe, or descriptor - -Opens the file whose filename is given by FILENAME, and associates it with -FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real -filehandle wanted; an undefined scalar will be suitably autovivified. This -function calls the underlying operating system's I<open>(2) function with the -parameters FILENAME, MODE, and PERMS. - -The possible values and flag bits of the MODE parameter are -system-dependent; they are available via the standard module C<Fcntl>. See -the documentation of your operating system's I<open>(2) syscall to see -which values and flag bits are available. You may combine several flags -using the C<|>-operator. - -Some of the most common values are C<O_RDONLY> for opening the file in -read-only mode, C<O_WRONLY> for opening the file in write-only mode, -and C<O_RDWR> for opening the file in read-write mode. -X<O_RDONLY> X<O_RDWR> X<O_WRONLY> - -For historical reasons, some values work on almost every system -supported by Perl: 0 means read-only, 1 means write-only, and 2 -means read/write. We know that these values do I<not> work under -OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to -use them in new code. - -If the file named by FILENAME does not exist and the C<open> call creates -it (typically because MODE includes the C<O_CREAT> flag), then the value of -PERMS specifies the permissions of the newly created file. If you omit -the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. -These permission values need to be in octal, and are modified by your -process's current C<umask>. -X<O_CREAT> - -In many systems the C<O_EXCL> flag is available for opening files in -exclusive mode. This is B<not> locking: exclusiveness means here that -if the file already exists, sysopen() fails. C<O_EXCL> may not work -on network filesystems, and has no effect unless the C<O_CREAT> flag -is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from -being opened if it is a symbolic link. It does not protect against -symbolic links in the file's path. -X<O_EXCL> - -Sometimes you may want to truncate an already-existing file. This -can be done using the C<O_TRUNC> flag. The behavior of -C<O_TRUNC> with C<O_RDONLY> is undefined. -X<O_TRUNC> - -You should seldom if ever use C<0644> as argument to C<sysopen>, because -that takes away the user's option to have a more permissive umask. -Better to omit it. See the perlfunc(1) entry on C<umask> for more -on this. - -Note that C<sysopen> depends on the fdopen() C library function. -On many Unix systems, fdopen() is known to fail when file descriptors -exceed a certain value, typically 255. If you need more file -descriptors than that, consider rebuilding Perl to use the C<sfio> -library, or perhaps using the POSIX::open() function. - -See L<perlopentut> for a kinder, gentler explanation of opening files. - -Portability issues: L<perlport/sysopen>. - -=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET -X<sysread> - -=item sysread FILEHANDLE,SCALAR,LENGTH - -=for Pod::Functions fixed-length unbuffered input from a filehandle - -Attempts to read LENGTH bytes of data into variable SCALAR from the -specified FILEHANDLE, using the read(2). It bypasses -buffered IO, so mixing this with other kinds of reads, C<print>, -C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the -perlio or stdio layers usually buffers data. Returns the number of -bytes actually read, C<0> at end of file, or undef if there was an -error (in the latter case C<$!> is also set). SCALAR will be grown or -shrunk so that the last byte actually read is the last byte of the -scalar after the read. - -An OFFSET may be specified to place the read data at some place in the -string other than the beginning. A negative OFFSET specifies -placement at that many characters counting backwards from the end of -the string. A positive OFFSET greater than the length of SCALAR -results in the string being padded to the required size with C<"\0"> -bytes before the result of the read is appended. - -There is no syseof() function, which is ok, since eof() doesn't work -well on device files (like ttys) anyway. Use sysread() and check -for a return value for 0 to decide whether you're done. - -Note that if the filehandle has been marked as C<:utf8> Unicode -characters are read instead of bytes (the LENGTH, OFFSET, and the -return value of sysread() are in Unicode characters). -The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. -See L</binmode>, L</open>, and the C<open> pragma, L<open>. - -=item sysseek FILEHANDLE,POSITION,WHENCE -X<sysseek> X<lseek> - -=for Pod::Functions +5.004 position I/O pointer on handle used with sysread and syswrite - -Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may -be an expression whose value gives the name of the filehandle. The values -for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it -to the current position plus POSITION; and C<2> to set it to EOF plus -POSITION, typically negative. - -Note the I<in bytes>: even if the filehandle has been set to operate -on characters (for example by using the C<:encoding(utf8)> I/O layer), -tell() will return byte offsets, not character offsets (because -implementing that would render sysseek() unacceptably slow). - -sysseek() bypasses normal buffered IO, so mixing it with reads other -than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>, -C<seek>, C<tell>, or C<eof> may cause confusion. - -For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, -and C<SEEK_END> (start of the file, current position, end of the file) -from the Fcntl module. Use of the constants is also more portable -than relying on 0, 1, and 2. For example to define a "systell" function: - - use Fcntl 'SEEK_CUR'; - sub systell { sysseek($_[0], 0, SEEK_CUR) } - -Returns the new position, or the undefined value on failure. A position -of zero is returned as the string C<"0 but true">; thus C<sysseek> returns -true on success and false on failure, yet you can still easily determine -the new position. - -=item system LIST -X<system> X<shell> - -=item system PROGRAM LIST - -=for Pod::Functions run a separate program - -Does exactly the same thing as C<exec LIST>, except that a fork is -done first and the parent process waits for the child process to -exit. Note that argument processing varies depending on the -number of arguments. If there is more than one argument in LIST, -or if LIST is an array with more than one value, starts the program -given by the first element of the list with arguments given by the -rest of the list. If there is only one scalar argument, the argument -is checked for shell metacharacters, and if there are any, the -entire argument is passed to the system's command shell for parsing -(this is C</bin/sh -c> on Unix platforms, but varies on other -platforms). If there are no shell metacharacters in the argument, -it is split into words and passed directly to C<execvp>, which is -more efficient. - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before any operation that may do a fork, but this may not be -supported on some platforms (see L<perlport>). To be safe, you may need -to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method -of C<IO::Handle> on any open handles. - -The return value is the exit status of the program as returned by the -C<wait> call. To get the actual exit value, shift right by eight (see -below). See also L</exec>. This is I<not> what you want to use to capture -the output from a command; for that you should use merely backticks or -C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 -indicates a failure to start the program or an error of the wait(2) system -call (inspect $! for the reason). - -If you'd like to make C<system> (and many other bits of Perl) die on error, -have a look at the L<autodie> pragma. - -Like C<exec>, C<system> allows you to lie to a program about its name if -you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. - -Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of -C<system>, if you expect your program to terminate on receipt of these -signals you will need to arrange to do so yourself based on the return -value. - - @args = ("command", "arg1", "arg2"); - system(@args) == 0 - or die "system @args failed: $?" - -If you'd like to manually inspect C<system>'s failure, you can check all -possible failure modes by inspecting C<$?> like this: - - if ($? == -1) { - print "failed to execute: $!\n"; - } - elsif ($? & 127) { - printf "child died with signal %d, %s coredump\n", - ($? & 127), ($? & 128) ? 'with' : 'without'; - } - else { - printf "child exited with value %d\n", $? >> 8; - } - -Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}> -with the C<W*()> calls from the POSIX module. - -When C<system>'s arguments are executed indirectly by the shell, -results and return codes are subject to its quirks. -See L<perlop/"`STRING`"> and L</exec> for details. - -Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD> -handler. See L<perlipc> for details. - -Portability issues: L<perlport/system>. - -=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET -X<syswrite> - -=item syswrite FILEHANDLE,SCALAR,LENGTH - -=item syswrite FILEHANDLE,SCALAR - -=for Pod::Functions fixed-length unbuffered output to a filehandle - -Attempts to write LENGTH bytes of data from variable SCALAR to the -specified FILEHANDLE, using write(2). If LENGTH is -not specified, writes whole SCALAR. It bypasses buffered IO, so -mixing this with reads (other than C<sysread())>, C<print>, C<write>, -C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and -stdio layers usually buffer data. Returns the number of bytes -actually written, or C<undef> if there was an error (in this case the -errno variable C<$!> is also set). If the LENGTH is greater than the -data available in the SCALAR after the OFFSET, only as much data as is -available will be written. - -An OFFSET may be specified to write the data from some part of the -string other than the beginning. A negative OFFSET specifies writing -that many characters counting backwards from the end of the string. -If SCALAR is of length zero, you can only use an OFFSET of 0. - -B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters -encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and -return value of syswrite() are in (UTF8-encoded Unicode) characters. -The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer. -Alternately, if the handle is not marked with an encoding but you -attempt to write characters with code points over 255, raises an exception. -See L</binmode>, L</open>, and the C<open> pragma, L<open>. - -=item tell FILEHANDLE -X<tell> - -=item tell - -=for Pod::Functions get current seekpointer on a filehandle - -Returns the current position I<in bytes> for FILEHANDLE, or -1 on -error. FILEHANDLE may be an expression whose value gives the name of -the actual filehandle. If FILEHANDLE is omitted, assumes the file -last read. - -Note the I<in bytes>: even if the filehandle has been set to -operate on characters (for example by using the C<:encoding(utf8)> open -layer), tell() will return byte offsets, not character offsets (because -that would render seek() and tell() rather slow). - -The return value of tell() for the standard streams like the STDIN -depends on the operating system: it may return -1 or something else. -tell() on pipes, fifos, and sockets usually returns -1. - -There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. - -Do not use tell() (or other buffered I/O operations) on a filehandle -that has been manipulated by sysread(), syswrite(), or sysseek(). -Those functions ignore the buffering, while tell() does not. - -=item telldir DIRHANDLE -X<telldir> - -=for Pod::Functions get current seekpointer on a directory handle - -Returns the current position of the C<readdir> routines on DIRHANDLE. -Value may be given to C<seekdir> to access a particular location in a -directory. C<telldir> has the same caveats about possible directory -compaction as the corresponding system library routine. - -=item tie VARIABLE,CLASSNAME,LIST -X<tie> - -=for Pod::Functions +5.002 bind a variable to an object class - -This function binds a variable to a package class that will provide the -implementation for the variable. VARIABLE is the name of the variable -to be enchanted. CLASSNAME is the name of a class implementing objects -of correct type. Any additional arguments are passed to the C<new> -method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, -or C<TIEHASH>). Typically these are arguments such as might be passed -to the C<dbm_open()> function of C. The object returned by the C<new> -method is also returned by the C<tie> function, which would be useful -if you want to access other methods in CLASSNAME. - -Note that functions such as C<keys> and C<values> may return huge lists -when used on large objects, like DBM files. You may prefer to use the -C<each> function to iterate over such. Example: - - # print out history file offsets - use NDBM_File; - tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); - while (($key,$val) = each %HIST) { - print $key, ' = ', unpack('L',$val), "\n"; - } - untie(%HIST); - -A class implementing a hash should have the following methods: - - TIEHASH classname, LIST - FETCH this, key - STORE this, key, value - DELETE this, key - CLEAR this - EXISTS this, key - FIRSTKEY this - NEXTKEY this, lastkey - SCALAR this - DESTROY this - UNTIE this - -A class implementing an ordinary array should have the following methods: - - TIEARRAY classname, LIST - FETCH this, key - STORE this, key, value - FETCHSIZE this - STORESIZE this, count - CLEAR this - PUSH this, LIST - POP this - SHIFT this - UNSHIFT this, LIST - SPLICE this, offset, length, LIST - EXTEND this, count - DESTROY this - UNTIE this - -A class implementing a filehandle should have the following methods: - - TIEHANDLE classname, LIST - READ this, scalar, length, offset - READLINE this - GETC this - WRITE this, scalar, length, offset - PRINT this, LIST - PRINTF this, format, LIST - BINMODE this - EOF this - FILENO this - SEEK this, position, whence - TELL this - OPEN this, mode, LIST - CLOSE this - DESTROY this - UNTIE this - -A class implementing a scalar should have the following methods: - - TIESCALAR classname, LIST - FETCH this, - STORE this, value - DESTROY this - UNTIE this - -Not all methods indicated above need be implemented. See L<perltie>, -L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. - -Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module -for you; you need to do that explicitly yourself. See L<DB_File> -or the F<Config> module for interesting C<tie> implementations. - -For further details see L<perltie>, L<"tied VARIABLE">. - -=item tied VARIABLE -X<tied> - -=for Pod::Functions get a reference to the object underlying a tied variable - -Returns a reference to the object underlying VARIABLE (the same value -that was originally returned by the C<tie> call that bound the variable -to a package.) Returns the undefined value if VARIABLE isn't tied to a -package. - -=item time -X<time> X<epoch> - -=for Pod::Functions return number of seconds since 1970 - -Returns the number of non-leap seconds since whatever time the system -considers to be the epoch, suitable for feeding to C<gmtime> and -C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970; -a prominent exception being Mac OS Classic which uses 00:00:00, January 1, -1904 in the current local time zone for its epoch. - -For measuring time in better granularity than one second, use the -L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or, -if you have gettimeofday(2), you may be able to use the C<syscall> -interface of Perl. See L<perlfaq8> for details. - -For date and time processing look at the many related modules on CPAN. -For a comprehensive date and time representation look at the -L<DateTime> module. - -=item times -X<times> - -=for Pod::Functions return elapsed time for self and child processes - -Returns a four-element list giving the user and system times in -seconds for this process and any exited children of this process. - - ($user,$system,$cuser,$csystem) = times; - -In scalar context, C<times> returns C<$user>. - -Children's times are only included for terminated children. - -Portability issues: L<perlport/times>. - -=item tr/// - -=for Pod::Functions transliterate a string - -The transliteration operator. Same as C<y///>. See -L<perlop/"Quote and Quote-like Operators">. - -=item truncate FILEHANDLE,LENGTH -X<truncate> - -=item truncate EXPR,LENGTH - -=for Pod::Functions shorten a file - -Truncates the file opened on FILEHANDLE, or named by EXPR, to the -specified length. Raises an exception if truncate isn't implemented -on your system. Returns true if successful, C<undef> on error. - -The behavior is undefined if LENGTH is greater than the length of the -file. - -The position in the file of FILEHANDLE is left unchanged. You may want to -call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file. - -Portability issues: L<perlport/truncate>. - -=item uc EXPR -X<uc> X<uppercase> X<toupper> - -=item uc - -=for Pod::Functions return upper-case version of a string - -Returns an uppercased version of EXPR. This is the internal function -implementing the C<\U> escape in double-quoted strings. -It does not attempt to do titlecase mapping on initial letters. See -L</ucfirst> for that. - -If EXPR is omitted, uses C<$_>. - -This function behaves the same way under various pragma, such as in a locale, -as L</lc> does. - -=item ucfirst EXPR -X<ucfirst> X<uppercase> - -=item ucfirst - -=for Pod::Functions return a string with just the next letter in upper case - -Returns the value of EXPR with the first character in uppercase -(titlecase in Unicode). This is the internal function implementing -the C<\u> escape in double-quoted strings. - -If EXPR is omitted, uses C<$_>. - -This function behaves the same way under various pragma, such as in a locale, -as L</lc> does. - -=item umask EXPR -X<umask> - -=item umask - -=for Pod::Functions set file creation mode mask - -Sets the umask for the process to EXPR and returns the previous value. -If EXPR is omitted, merely returns the current umask. - -The Unix permission C<rwxr-x---> is represented as three sets of three -bits, or three octal digits: C<0750> (the leading 0 indicates octal -and isn't one of the digits). The C<umask> value is such a number -representing disabled permissions bits. The permission (or "mode") -values you pass C<mkdir> or C<sysopen> are modified by your umask, so -even if you tell C<sysopen> to create a file with permissions C<0777>, -if your umask is C<0022>, then the file will actually be created with -permissions C<0755>. If your C<umask> were C<0027> (group can't -write; others can't read, write, or execute), then passing -C<sysopen> C<0666> would create a file with mode C<0640> (because -C<0666 &~ 027> is C<0640>). - -Here's some advice: supply a creation mode of C<0666> for regular -files (in C<sysopen>) and one of C<0777> for directories (in -C<mkdir>) and executable files. This gives users the freedom of -choice: if they want protected files, they might choose process umasks -of C<022>, C<027>, or even the particularly antisocial mask of C<077>. -Programs should rarely if ever make policy decisions better left to -the user. The exception to this is when writing files that should be -kept private: mail files, web browser cookies, I<.rhosts> files, and -so on. - -If umask(2) is not implemented on your system and you are trying to -restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>), -raises an exception. If umask(2) is not implemented and you are -not trying to restrict access for yourself, returns C<undef>. - -Remember that a umask is a number, usually given in octal; it is I<not> a -string of octal digits. See also L</oct>, if all you have is a string. - -Portability issues: L<perlport/umask>. - -=item undef EXPR -X<undef> X<undefine> - -=item undef - -=for Pod::Functions remove a variable or function definition - -Undefines the value of EXPR, which must be an lvalue. Use only on a -scalar value, an array (using C<@>), a hash (using C<%>), a subroutine -(using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}> -will probably not do what you expect on most predefined variables or -DBM list values, so don't do that; see L</delete>. Always returns the -undefined value. You can omit the EXPR, in which case nothing is -undefined, but you still get an undefined value that you could, for -instance, return from a subroutine, assign to a variable, or pass as a -parameter. Examples: - - undef $foo; - undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; - undef @ary; - undef %hash; - undef &mysub; - undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. - return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; - select undef, undef, undef, 0.25; - ($a, $b, undef, $c) = &foo; # Ignore third value returned - -Note that this is a unary operator, not a list operator. - -=item unlink LIST -X<unlink> X<delete> X<remove> X<rm> X<del> - -=item unlink - -=for Pod::Functions remove one link to a file - -Deletes a list of files. On success, it returns the number of files -it successfully deleted. On failure, it returns false and sets C<$!> -(errno): - - my $unlinked = unlink 'a', 'b', 'c'; - unlink @goners; - unlink glob "*.bak"; - -On error, C<unlink> will not tell you which files it could not remove. -If you want to know which files you could not remove, try them one -at a time: - - foreach my $file ( @goners ) { - unlink $file or warn "Could not unlink $file: $!"; - } - -Note: C<unlink> will not attempt to delete directories unless you are -superuser and the B<-U> flag is supplied to Perl. Even if these -conditions are met, be warned that unlinking a directory can inflict -damage on your filesystem. Finally, using C<unlink> on directories is -not supported on many operating systems. Use C<rmdir> instead. - -If LIST is omitted, C<unlink> uses C<$_>. - -=item unpack TEMPLATE,EXPR -X<unpack> - -=item unpack TEMPLATE - -=for Pod::Functions convert binary structure into normal perl variables - -C<unpack> does the reverse of C<pack>: it takes a string -and expands it out into a list of values. -(In scalar context, it returns merely the first value produced.) - -If EXPR is omitted, unpacks the C<$_> string. -See L<perlpacktut> for an introduction to this function. - -The string is broken into chunks described by the TEMPLATE. Each chunk -is converted separately to a value. Typically, either the string is a result -of C<pack>, or the characters of the string represent a C structure of some -kind. - -The TEMPLATE has the same format as in the C<pack> function. -Here's a subroutine that does substring: - - sub substr { - my($what,$where,$howmuch) = @_; - unpack("x$where a$howmuch", $what); - } - -and then there's - - sub ordinal { unpack("W",$_[0]); } # same as ord() - -In addition to fields allowed in pack(), you may prefix a field with -a %<number> to indicate that -you want a <number>-bit checksum of the items instead of the items -themselves. Default is a 16-bit checksum. Checksum is calculated by -summing numeric values of expanded values (for string fields the sum of -C<ord($char)> is taken; for bit fields the sum of zeroes and ones). - -For example, the following -computes the same number as the System V sum program: - - $checksum = do { - local $/; # slurp! - unpack("%32W*",<>) % 65535; - }; - -The following efficiently counts the number of set bits in a bit vector: - - $setbits = unpack("%32b*", $selectmask); - -The C<p> and C<P> formats should be used with care. Since Perl -has no way of checking whether the value passed to C<unpack()> -corresponds to a valid memory location, passing a pointer value that's -not known to be valid is likely to have disastrous consequences. - -If there are more pack codes or if the repeat count of a field or a group -is larger than what the remainder of the input string allows, the result -is not well defined: the repeat count may be decreased, or -C<unpack()> may produce empty strings or zeros, or it may raise an exception. -If the input string is longer than one described by the TEMPLATE, -the remainder of that input string is ignored. - -See L</pack> for more examples and notes. - -=item unshift ARRAY,LIST -X<unshift> - -=item unshift EXPR,LIST - -=for Pod::Functions prepend more elements to the beginning of a list - -Does the opposite of a C<shift>. Or the opposite of a C<push>, -depending on how you look at it. Prepends list to the front of the -array and returns the new number of elements in the array. - - unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/; - -Note the LIST is prepended whole, not one element at a time, so the -prepended elements stay in the same order. Use C<reverse> to do the -reverse. - -Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold -a reference to an unblessed array. The argument will be dereferenced -automatically. This aspect of C<unshift> is considered highly -experimental. The exact behaviour may change in a future version of Perl. - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.014; # so push/pop/etc work on scalars (experimental) - -=item untie VARIABLE -X<untie> - -=for Pod::Functions break a tie binding to a variable - -Breaks the binding between a variable and a package. -(See L<tie|/tie VARIABLE,CLASSNAME,LIST>.) -Has no effect if the variable is not tied. - -=item use Module VERSION LIST -X<use> X<module> X<import> - -=item use Module VERSION - -=item use Module LIST - -=item use Module - -=item use VERSION - -=for Pod::Functions load in a module at compile time and import its namespace - -Imports some semantics into the current package from the named module, -generally by aliasing certain subroutine or variable names into your -package. It is exactly equivalent to - - BEGIN { require Module; Module->import( LIST ); } - -except that Module I<must> be a bareword. -The importation can be made conditional; see L<if>. - -In the peculiar C<use VERSION> form, VERSION may be either a positive -decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string -of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An -exception is raised if VERSION is greater than the version of the -current Perl interpreter; Perl will not attempt to parse the rest of the -file. Compare with L</require>, which can do a similar check at run time. -Symmetrically, C<no VERSION> allows you to specify that you want a version -of Perl older than the specified one. - -Specifying VERSION as a literal of the form v5.6.1 should generally be -avoided, because it leads to misleading error messages under earlier -versions of Perl (that is, prior to 5.6.0) that do not support this -syntax. The equivalent numeric version should be used instead. - - use v5.6.1; # compile time version check - use 5.6.1; # ditto - use 5.006_001; # ditto; preferred for backwards compatibility - -This is often useful if you need to check the current Perl version before -C<use>ing library modules that won't work with older versions of Perl. -(We try not to do this more than we have to.) - -C<use VERSION> also enables all features available in the requested -version as defined by the C<feature> pragma, disabling any features -not in the requested version's feature bundle. See L<feature>. -Similarly, if the specified Perl version is greater than or equal to -5.11.0, strictures are enabled lexically as -with C<use strict>. Any explicit use of -C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes -before it. In both cases, the F<feature.pm> and F<strict.pm> files are -not actually loaded. - -The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The -C<require> makes sure the module is loaded into memory if it hasn't been -yet. The C<import> is not a builtin; it's just an ordinary static method -call into the C<Module> package to tell the module to import the list of -features back into the current package. The module can implement its -C<import> method any way it likes, though most modules just choose to -derive their C<import> method via inheritance from the C<Exporter> class that -is defined in the C<Exporter> module. See L<Exporter>. If no C<import> -method can be found then the call is skipped, even if there is an AUTOLOAD -method. - -If you do not want to call the package's C<import> method (for instance, -to stop your namespace from being altered), explicitly supply the empty list: - - use Module (); - -That is exactly equivalent to - - BEGIN { require Module } - -If the VERSION argument is present between Module and LIST, then the -C<use> will call the VERSION method in class Module with the given -version as an argument. The default VERSION method, inherited from -the UNIVERSAL class, croaks if the given version is larger than the -value of the variable C<$Module::VERSION>. - -Again, there is a distinction between omitting LIST (C<import> called -with no arguments) and an explicit empty LIST C<()> (C<import> not -called). Note that there is no comma after VERSION! - -Because this is a wide-open interface, pragmas (compiler directives) -are also implemented this way. Currently implemented pragmas are: - - use constant; - use diagnostics; - use integer; - use sigtrap qw(SEGV BUS); - use strict qw(subs vars refs); - use subs qw(afunc blurfl); - use warnings qw(all); - use sort qw(stable _quicksort _mergesort); - -Some of these pseudo-modules import semantics into the current -block scope (like C<strict> or C<integer>, unlike ordinary modules, -which import symbols into the current package (which are effective -through the end of the file). - -Because C<use> takes effect at compile time, it doesn't respect the -ordinary flow control of the code being compiled. In particular, putting -a C<use> inside the false branch of a conditional doesn't prevent it -from being processed. If a module or pragma only needs to be loaded -conditionally, this can be done using the L<if> pragma: - - use if $] < 5.008, "utf8"; - use if WANT_WARNINGS, warnings => qw(all); - -There's a corresponding C<no> declaration that unimports meanings imported -by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. -It behaves just as C<import> does with VERSION, an omitted or empty LIST, -or no unimport method being found. - - no integer; - no strict 'refs'; - no warnings; - -Care should be taken when using the C<no VERSION> form of C<no>. It is -I<only> meant to be used to assert that the running Perl is of a earlier -version than its argument and I<not> to undo the feature-enabling side effects -of C<use VERSION>. - -See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> -for the C<-M> and C<-m> command-line options to Perl that give C<use> -functionality from the command-line. - -=item utime LIST -X<utime> - -=for Pod::Functions set a file's last access and modify times - -Changes the access and modification times on each file of a list of -files. The first two elements of the list must be the NUMERIC access -and modification times, in that order. Returns the number of files -successfully changed. The inode change time of each file is set -to the current time. For example, this code has the same effect as the -Unix touch(1) command when the files I<already exist> and belong to -the user running the program: - - #!/usr/bin/perl - $atime = $mtime = time; - utime $atime, $mtime, @ARGV; - -Since Perl 5.7.2, if the first two elements of the list are C<undef>, -the utime(2) syscall from your C library is called with a null second -argument. On most systems, this will set the file's access and -modification times to the current time (i.e., equivalent to the example -above) and will work even on files you don't own provided you have write -permission: - - for $file (@ARGV) { - utime(undef, undef, $file) - || warn "couldn't touch $file: $!"; - } - -Under NFS this will use the time of the NFS server, not the time of -the local machine. If there is a time synchronization problem, the -NFS server and local machine will have different times. The Unix -touch(1) command will in fact normally use this form instead of the -one shown in the first example. - -Passing only one of the first two elements as C<undef> is -equivalent to passing a 0 and will not have the effect -described when both are C<undef>. This also triggers an -uninitialized warning. - -On systems that support futimes(2), you may pass filehandles among the -files. On systems that don't support futimes(2), passing filehandles raises -an exception. Filehandles must be passed as globs or glob references to be -recognized; barewords are considered filenames. - -Portability issues: L<perlport/utime>. - -=item values HASH -X<values> - -=item values ARRAY - -=item values EXPR - -=for Pod::Functions return a list of the values in a hash - -In list context, returns a list consisting of all the values of the named -hash. In Perl 5.12 or later only, will also return a list of the values of -an array; prior to that release, attempting to use an array argument will -produce a syntax error. In scalar context, returns the number of values. - -When called on a hash, the values are returned in an apparently random -order. The actual random order is subject to change in future versions of -Perl, but it is guaranteed to be the same order as either the C<keys> or -C<each> function would produce on the same (unmodified) hash. Since Perl -5.8.1 the ordering is different even between different runs of Perl for -security reasons (see L<perlsec/"Algorithmic Complexity Attacks">). - -As a side effect, calling values() resets the HASH or ARRAY's internal -iterator, see L</each>. (In particular, calling values() in void context -resets the iterator with no other overhead. Apart from resetting the -iterator, C<values @array> in list context is the same as plain C<@array>. -(We recommend that you use void context C<keys @array> for this, but -reasoned that taking C<values @array> out would require more -documentation than leaving it in.) - -Note that the values are not copied, which means modifying them will -modify the contents of the hash: - - for (values %hash) { s/foo/bar/g } # modifies %hash values - for (@hash{keys %hash}) { s/foo/bar/g } # same - -Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold -a reference to an unblessed hash or array. The argument will be -dereferenced automatically. This aspect of C<values> is considered highly -experimental. The exact behaviour may change in a future version of Perl. - - for (values $hashref) { ... } - for (values $obj->get_arrayref) { ... } - -To avoid confusing would-be users of your code who are running earlier -versions of Perl with mysterious syntax errors, put this sort of thing at -the top of your file to signal that your code will work I<only> on Perls of -a recent vintage: - - use 5.012; # so keys/values/each work on arrays - use 5.014; # so keys/values/each work on scalars (experimental) - -See also C<keys>, C<each>, and C<sort>. - -=item vec EXPR,OFFSET,BITS -X<vec> X<bit> X<bit vector> - -=for Pod::Functions test or set particular bits in a string - -Treats the string in EXPR as a bit vector made up of elements of -width BITS and returns the value of the element specified by OFFSET -as an unsigned integer. BITS therefore specifies the number of bits -that are reserved for each element in the bit vector. This must -be a power of two from 1 to 32 (or 64, if your platform supports -that). - -If BITS is 8, "elements" coincide with bytes of the input string. - -If BITS is 16 or more, bytes of the input string are grouped into chunks -of size BITS/8, and each group is converted to a number as with -pack()/unpack() with big-endian formats C<n>/C<N> (and analogously -for BITS==64). See L<"pack"> for details. - -If bits is 4 or less, the string is broken into bytes, then the bits -of each byte are broken into 8/BITS groups. Bits of a byte are -numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, -C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, -breaking the single input byte C<chr(0x36)> into two groups gives a list -C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. - -C<vec> may also be assigned to, in which case parentheses are needed -to give the expression the correct precedence as in - - vec($image, $max_x * $x + $y, 8) = 3; - -If the selected element is outside the string, the value 0 is returned. -If an element off the end of the string is written to, Perl will first -extend the string with sufficiently many zero bytes. It is an error -to try to write off the beginning of the string (i.e., negative OFFSET). - -If the string happens to be encoded as UTF-8 internally (and thus has -the UTF8 flag set), this is ignored by C<vec>, and it operates on the -internal byte string, not the conceptual character string, even if you -only have characters with values less than 256. - -Strings created with C<vec> can also be manipulated with the logical -operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit -vector operation is desired when both operands are strings. -See L<perlop/"Bitwise String Operators">. - -The following code will build up an ASCII string saying C<'PerlPerlPerl'>. -The comments show the string after each step. Note that this code works -in the same way on big-endian or little-endian machines. - - my $foo = ''; - vec($foo, 0, 32) = 0x5065726C; # 'Perl' - - # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits - print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') - - vec($foo, 2, 16) = 0x5065; # 'PerlPe' - vec($foo, 3, 16) = 0x726C; # 'PerlPerl' - vec($foo, 8, 8) = 0x50; # 'PerlPerlP' - vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' - vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" - vec($foo, 21, 4) = 7; # 'PerlPerlPer' - # 'r' is "\x72" - vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" - vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" - vec($foo, 94, 1) = 1; # 'PerlPerlPerl' - # 'l' is "\x6c" - -To transform a bit vector into a string or list of 0's and 1's, use these: - - $bits = unpack("b*", $vector); - @bits = split(//, unpack("b*", $vector)); - -If you know the exact length in bits, it can be used in place of the C<*>. - -Here is an example to illustrate how the bits actually fall in place: - - #!/usr/bin/perl -wl - - print <<'EOT'; - 0 1 2 3 - unpack("V",$_) 01234567890123456789012345678901 - ------------------------------------------------------------------ - EOT - - for $w (0..3) { - $width = 2**$w; - for ($shift=0; $shift < $width; ++$shift) { - for ($off=0; $off < 32/$width; ++$off) { - $str = pack("B*", "0"x32); - $bits = (1<<$shift); - vec($str, $off, $width) = $bits; - $res = unpack("b*",$str); - $val = unpack("V", $str); - write; - } - } - } - - format STDOUT = - vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> - $off, $width, $bits, $val, $res - . - __END__ - -Regardless of the machine architecture on which it runs, the -example above should print the following table: - - 0 1 2 3 - unpack("V",$_) 01234567890123456789012345678901 - ------------------------------------------------------------------ - vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 - vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 - vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 - vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 - vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 - vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 - vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 - vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 - vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 - vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 - vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 - vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 - vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 - vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 - vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 - vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 - vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 - vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 - vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 - vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 - vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 - vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 - vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 - vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 - vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 - vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 - vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 - vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 - vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 - vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 - vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 - vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 - vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 - vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 - vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 - vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 - vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 - vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 - vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 - vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 - vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 - vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 - vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 - vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 - vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 - vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 - vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 - vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 - vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 - vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 - vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 - vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 - vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 - vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 - vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 - vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 - vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 - vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 - vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 - vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 - vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 - vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 - vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 - vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 - vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 - vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 - vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 - vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 - vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 - vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 - vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 - vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 - vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 - vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 - vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 - vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 - vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 - vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 - vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 - vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 - vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 - vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 - vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 - vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 - vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 - vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 - vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 - vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 - vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 - vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 - vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 - vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 - vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 - vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 - vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 - vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 - vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 - vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 - vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 - vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 - vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 - vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 - vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 - vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 - vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 - vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 - vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 - vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 - vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 - vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 - vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 - vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 - vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 - vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 - vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 - vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 - vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 - vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 - vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 - -=item wait -X<wait> - -=for Pod::Functions wait for any child process to die - -Behaves like wait(2) on your system: it waits for a child -process to terminate and returns the pid of the deceased process, or -C<-1> if there are no child processes. The status is returned in C<$?> -and C<${^CHILD_ERROR_NATIVE}>. -Note that a return value of C<-1> could mean that child processes are -being automatically reaped, as described in L<perlipc>. - -If you use wait in your handler for $SIG{CHLD} it may accidentally for the -child created by qx() or system(). See L<perlipc> for details. - -Portability issues: L<perlport/wait>. - -=item waitpid PID,FLAGS -X<waitpid> - -=for Pod::Functions wait for a particular child process to die - -Waits for a particular child process to terminate and returns the pid of -the deceased process, or C<-1> if there is no such child process. On some -systems, a value of 0 indicates that there are processes still running. -The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say - - use POSIX ":sys_wait_h"; - #... - do { - $kid = waitpid(-1, WNOHANG); - } while $kid > 0; - -then you can do a non-blocking wait for all pending zombie processes. -Non-blocking wait is available on machines supporting either the -waitpid(2) or wait4(2) syscalls. However, waiting for a particular -pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the -system call by remembering the status values of processes that have -exited but have not been harvested by the Perl script yet.) - -Note that on some systems, a return value of C<-1> could mean that child -processes are being automatically reaped. See L<perlipc> for details, -and for other examples. - -Portability issues: L<perlport/waitpid>. - -=item wantarray -X<wantarray> X<context> - -=for Pod::Functions get void vs scalar vs list context of current subroutine call - -Returns true if the context of the currently executing subroutine or -C<eval> is looking for a list value. Returns false if the context is -looking for a scalar. Returns the undefined value if the context is -looking for no value (void context). - - return unless defined wantarray; # don't bother doing more - my @a = complex_calculation(); - return wantarray ? @a : "@a"; - -C<wantarray()>'s result is unspecified in the top level of a file, -in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or -in a C<DESTROY> method. - -This function should have been named wantlist() instead. - -=item warn LIST -X<warn> X<warning> X<STDERR> - -=for Pod::Functions print debugging info - -Prints the value of LIST to STDERR. If the last element of LIST does -not end in a newline, it appends the same file/line number text as C<die> -does. - -If the output is empty and C<$@> already contains a value (typically from a -previous eval) that value is used after appending C<"\t...caught"> -to C<$@>. This is useful for staying almost, but not entirely similar to -C<die>. - -If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. - -No message is printed if there is a C<$SIG{__WARN__}> handler -installed. It is the handler's responsibility to deal with the message -as it sees fit (like, for instance, converting it into a C<die>). Most -handlers must therefore arrange to actually display the -warnings that they are not prepared to deal with, by calling C<warn> -again in the handler. Note that this is quite safe and will not -produce an endless loop, since C<__WARN__> hooks are not called from -inside one. - -You will find this behavior is slightly different from that of -C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can -instead call C<die> again to change it). - -Using a C<__WARN__> handler provides a powerful way to silence all -warnings (even the so-called mandatory ones). An example: - - # wipe out *all* compile-time warnings - BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } - my $foo = 10; - my $foo = 20; # no warning about duplicate my $foo, - # but hey, you asked for it! - # no compile-time or run-time warnings before here - $DOWARN = 1; - - # run-time warnings enabled after here - warn "\$foo is alive and $foo!"; # does show up - -See L<perlvar> for details on setting C<%SIG> entries and for more -examples. See the Carp module for other kinds of warnings using its -carp() and cluck() functions. - -=item write FILEHANDLE -X<write> - -=item write EXPR - -=item write - -=for Pod::Functions print a picture record - -Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, -using the format associated with that file. By default the format for -a file is the one having the same name as the filehandle, but the -format for the current output channel (see the C<select> function) may be set -explicitly by assigning the name of the format to the C<$~> variable. - -Top of form processing is handled automatically: if there is insufficient -room on the current page for the formatted record, the page is advanced by -writing a form feed, a special top-of-page format is used to format the new -page header before the record is written. By default, the top-of-page -format is the name of the filehandle with "_TOP" appended. This would be a -problem with autovivified filehandles, but it may be dynamically set to the -format of your choice by assigning the name to the C<$^> variable while -that filehandle is selected. The number of lines remaining on the current -page is in variable C<$->, which can be set to C<0> to force a new page. - -If FILEHANDLE is unspecified, output goes to the current default output -channel, which starts out as STDOUT but may be changed by the -C<select> operator. If the FILEHANDLE is an EXPR, then the expression -is evaluated and the resulting string is used to look up the name of -the FILEHANDLE at run time. For more on formats, see L<perlform>. - -Note that write is I<not> the opposite of C<read>. Unfortunately. - -=item y/// - -=for Pod::Functions transliterate a string - -The transliteration operator. Same as C<tr///>. See -L<perlop/"Quote and Quote-like Operators">. - -=back - -=head2 Non-function Keywords by Cross-reference - -=head3 perldata - -=over - -=item __DATA__ - -=item __END__ - -These keywords are documented in L<perldata/"Special Literals">. - -=back - -=head3 perlmod - -=over - -=item BEGIN - -=item CHECK - -=item END - -=item INIT - -=item UNITCHECK - -These compile phase keywords are documented in L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">. - -=back - -=head3 perlobj - -=over - -=item DESTROY - -This method keyword is documented in L<perlobj/"Destructors">. - -=back - -=head3 perlop - -=over - -=item and - -=item cmp - -=item eq - -=item ge - -=item gt - -=item if - -=item le - -=item lt - -=item ne - -=item not - -=item or - -=item x - -=item xor - -These operators are documented in L<perlop>. - -=back - -=head3 perlsub - -=over - -=item AUTOLOAD - -This keyword is documented in L<perlsub/"Autoloading">. - -=back - -=head3 perlsyn - -=over - -=item else - -=item elseif - -=item elsif - -=item for - -=item foreach - -=item unless - -=item until - -=item while - -These flow-control keywords are documented in L<perlsyn/"Compound Statements">. - -=back - -=over - -=item default - -=item given - -=item when - -These flow-control keywords related to the experimental switch feature are -documented in L<perlsyn/"Switch Statements"> . - -=back - -=cut |