diff options
Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perlfaq5.pod')
| -rw-r--r-- | Master/tlpkg/tlperl/lib/pods/perlfaq5.pod | 1545 |
1 files changed, 1545 insertions, 0 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perlfaq5.pod b/Master/tlpkg/tlperl/lib/pods/perlfaq5.pod new file mode 100644 index 00000000000..bd969f4393f --- /dev/null +++ b/Master/tlpkg/tlperl/lib/pods/perlfaq5.pod @@ -0,0 +1,1545 @@ +=head1 NAME + +perlfaq5 - Files and Formats + +=head1 DESCRIPTION + +This section deals with I/O and the "f" issues: filehandles, flushing, +formats, and footers. + +=head2 How do I flush/unbuffer an output filehandle? Why must I do this? +X<flush> X<buffer> X<unbuffer> X<autoflush> + +(contributed by brian d foy) + +You might like to read Mark Jason Dominus's "Suffering From Buffering" +at http://perl.plover.com/FAQs/Buffering.html . + +Perl normally buffers output so it doesn't make a system call for every +bit of output. By saving up output, it makes fewer expensive system calls. +For instance, in this little bit of code, you want to print a dot to the +screen for every line you process to watch the progress of your program. +Instead of seeing a dot for every line, Perl buffers the output and you +have a long wait before you see a row of 50 dots all at once: + + # long wait, then row of dots all at once + while( <> ) { + print "."; + print "\n" unless ++$count % 50; + + #... expensive line processing operations + } + +To get around this, you have to unbuffer the output filehandle, in this +case, C<STDOUT>. You can set the special variable C<$|> to a true value +(mnemonic: making your filehandles "piping hot"): + + $|++; + + # dot shown immediately + while( <> ) { + print "."; + print "\n" unless ++$count % 50; + + #... expensive line processing operations + } + +The C<$|> is one of the per-filehandle special variables, so each +filehandle has its own copy of its value. If you want to merge +standard output and standard error for instance, you have to unbuffer +each (although STDERR might be unbuffered by default): + + { + my $previous_default = select(STDOUT); # save previous default + $|++; # autoflush STDOUT + select(STDERR); + $|++; # autoflush STDERR, to be sure + select($previous_default); # restore previous default + } + + # now should alternate . and + + while( 1 ) + { + sleep 1; + print STDOUT "."; + print STDERR "+"; + print STDOUT "\n" unless ++$count % 25; + } + +Besides the C<$|> special variable, you can use C<binmode> to give +your filehandle a C<:unix> layer, which is unbuffered: + + binmode( STDOUT, ":unix" ); + + while( 1 ) { + sleep 1; + print "."; + print "\n" unless ++$count % 50; + } + +For more information on output layers, see the entries for C<binmode> +and C<open> in L<perlfunc>, and the C<PerlIO> module documentation. + +If you are using C<IO::Handle> or one of its subclasses, you can +call the C<autoflush> method to change the settings of the +filehandle: + + use IO::Handle; + open my( $io_fh ), ">", "output.txt"; + $io_fh->autoflush(1); + +The C<IO::Handle> objects also have a C<flush> method. You can flush +the buffer any time you want without auto-buffering + + $io_fh->flush; + +=head2 How do I change, delete, or insert a line in a file, or append to the beginning of a file? +X<file, editing> + +(contributed by brian d foy) + +The basic idea of inserting, changing, or deleting a line from a text +file involves reading and printing the file to the point you want to +make the change, making the change, then reading and printing the rest +of the file. Perl doesn't provide random access to lines (especially +since the record input separator, C<$/>, is mutable), although modules +such as C<Tie::File> can fake it. + +A Perl program to do these tasks takes the basic form of opening a +file, printing its lines, then closing the file: + + open my $in, '<', $file or die "Can't read old file: $!"; + open my $out, '>', "$file.new" or die "Can't write new file: $!"; + + while( <$in> ) + { + print $out $_; + } + + close $out; + +Within that basic form, add the parts that you need to insert, change, +or delete lines. + +To prepend lines to the beginning, print those lines before you enter +the loop that prints the existing lines. + + open my $in, '<', $file or die "Can't read old file: $!"; + open my $out, '>', "$file.new" or die "Can't write new file: $!"; + + print $out "# Add this line to the top\n"; # <--- HERE'S THE MAGIC + + while( <$in> ) + { + print $out $_; + } + + close $out; + +To change existing lines, insert the code to modify the lines inside +the C<while> loop. In this case, the code finds all lowercased +versions of "perl" and uppercases them. The happens for every line, so +be sure that you're supposed to do that on every line! + + open my $in, '<', $file or die "Can't read old file: $!"; + open my $out, '>', "$file.new" or die "Can't write new file: $!"; + + print $out "# Add this line to the top\n"; + + while( <$in> ) + { + s/\b(perl)\b/Perl/g; + print $out $_; + } + + close $out; + +To change only a particular line, the input line number, C<$.>, is +useful. First read and print the lines up to the one you want to +change. Next, read the single line you want to change, change it, and +print it. After that, read the rest of the lines and print those: + + while( <$in> ) # print the lines before the change + { + print $out $_; + last if $. == 4; # line number before change + } + + my $line = <$in>; + $line =~ s/\b(perl)\b/Perl/g; + print $out $line; + + while( <$in> ) # print the rest of the lines + { + print $out $_; + } + +To skip lines, use the looping controls. The C<next> in this example +skips comment lines, and the C<last> stops all processing once it +encounters either C<__END__> or C<__DATA__>. + + while( <$in> ) + { + next if /^\s+#/; # skip comment lines + last if /^__(END|DATA)__$/; # stop at end of code marker + print $out $_; + } + +Do the same sort of thing to delete a particular line by using C<next> +to skip the lines you don't want to show up in the output. This +example skips every fifth line: + + while( <$in> ) + { + next unless $. % 5; + print $out $_; + } + +If, for some odd reason, you really want to see the whole file at once +rather than processing line-by-line, you can slurp it in (as long as +you can fit the whole thing in memory!): + + open my $in, '<', $file or die "Can't read old file: $!" + open my $out, '>', "$file.new" or die "Can't write new file: $!"; + + my @lines = do { local $/; <$in> }; # slurp! + + # do your magic here + + print $out @lines; + +Modules such as C<File::Slurp> and C<Tie::File> can help with that +too. If you can, however, avoid reading the entire file at once. Perl +won't give that memory back to the operating system until the process +finishes. + +You can also use Perl one-liners to modify a file in-place. The +following changes all 'Fred' to 'Barney' in F<inFile.txt>, overwriting +the file with the new contents. With the C<-p> switch, Perl wraps a +C<while> loop around the code you specify with C<-e>, and C<-i> turns +on in-place editing. The current line is in C<$_>. With C<-p>, Perl +automatically prints the value of C<$_> at the end of the loop. See +L<perlrun> for more details. + + perl -pi -e 's/Fred/Barney/' inFile.txt + +To make a backup of C<inFile.txt>, give C<-i> a file extension to add: + + perl -pi.bak -e 's/Fred/Barney/' inFile.txt + +To change only the fifth line, you can add a test checking C<$.>, the +input line number, then only perform the operation when the test +passes: + + perl -pi -e 's/Fred/Barney/ if $. == 5' inFile.txt + +To add lines before a certain line, you can add a line (or lines!) +before Perl prints C<$_>: + + perl -pi -e 'print "Put before third line\n" if $. == 3' inFile.txt + +You can even add a line to the beginning of a file, since the current +line prints at the end of the loop: + + perl -pi -e 'print "Put before first line\n" if $. == 1' inFile.txt + +To insert a line after one already in the file, use the C<-n> switch. +It's just like C<-p> except that it doesn't print C<$_> at the end of +the loop, so you have to do that yourself. In this case, print C<$_> +first, then print the line that you want to add. + + perl -ni -e 'print; print "Put after fifth line\n" if $. == 5' inFile.txt + +To delete lines, only print the ones that you want. + + perl -ni -e 'print unless /d/' inFile.txt + + ... or ... + + perl -pi -e 'next unless /d/' inFile.txt + +=head2 How do I count the number of lines in a file? +X<file, counting lines> X<lines> X<line> + +(contributed by brian d foy) + +Conceptually, the easiest way to count the lines in a file is to +simply read them and count them: + + my $count = 0; + while( <$fh> ) { $count++; } + +You don't really have to count them yourself, though, since Perl +already does that with the C<$.> variable, which is the current line +number from the last filehandle read: + + 1 while( <$fh> ); + my $count = $.; + +If you want to use C<$.>, you can reduce it to a simple one-liner, +like one of these: + + % perl -lne '} print $.; {' file + + % perl -lne 'END { print $. }' file + +Those can be rather inefficient though. If they aren't fast enough for +you, you might just read chunks of data and count the number of +newlines: + + my $lines = 0; + open my($fh), '<:raw', $filename or die "Can't open $filename: $!"; + while( sysread $fh, $buffer, 4096 ) { + $lines += ( $buffer =~ tr/\n// ); + } + close FILE; + +However, that doesn't work if the line ending isn't a newline. You +might change that C<tr///> to a C<s///> so you can count the number of +times the input record separator, C<$/>, shows up: + + my $lines = 0; + open my($fh), '<:raw', $filename or die "Can't open $filename: $!"; + while( sysread $fh, $buffer, 4096 ) { + $lines += ( $buffer =~ s|$/||g; ); + } + close FILE; + +If you don't mind shelling out, the C<wc> command is usually the +fastest, even with the extra interprocess overhead. Ensure that you +have an untainted filename though: + + #!perl -T + + $ENV{PATH} = undef; + + my $lines; + if( $filename =~ /^([0-9a-z_.]+)\z/ ) { + $lines = `/usr/bin/wc -l $1` + chomp $lines; + } + +=head2 How do I delete the last N lines from a file? +X<lines> X<file> + +(contributed by brian d foy) + +The easiest conceptual solution is to count the lines in the +file then start at the beginning and print the number of lines +(minus the last N) to a new file. + +Most often, the real question is how you can delete the last N lines +without making more than one pass over the file, or how to do it +without a lot of copying. The easy concept is the hard reality when +you might have millions of lines in your file. + +One trick is to use C<File::ReadBackwards>, which starts at the end of +the file. That module provides an object that wraps the real filehandle +to make it easy for you to move around the file. Once you get to the +spot you need, you can get the actual filehandle and work with it as +normal. In this case, you get the file position at the end of the last +line you want to keep and truncate the file to that point: + + use File::ReadBackwards; + + my $filename = 'test.txt'; + my $Lines_to_truncate = 2; + + my $bw = File::ReadBackwards->new( $filename ) + or die "Could not read backwards in [$filename]: $!"; + + my $lines_from_end = 0; + until( $bw->eof or $lines_from_end == $Lines_to_truncate ) + { + print "Got: ", $bw->readline; + $lines_from_end++; + } + + truncate( $filename, $bw->tell ); + +The C<File::ReadBackwards> module also has the advantage of setting +the input record separator to a regular expression. + +You can also use the C<Tie::File> module which lets you access +the lines through a tied array. You can use normal array operations +to modify your file, including setting the last index and using +C<splice>. + +=head2 How can I use Perl's C<-i> option from within a program? +X<-i> X<in-place> + +C<-i> sets the value of Perl's C<$^I> variable, which in turn affects +the behavior of C<< <> >>; see L<perlrun> for more details. By +modifying the appropriate variables directly, you can get the same +behavior within a larger program. For example: + + # ... + { + local($^I, @ARGV) = ('.orig', glob("*.c")); + while (<>) { + if ($. == 1) { + print "This line should appear at the top of each file\n"; + } + s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case + print; + close ARGV if eof; # Reset $. + } + } + # $^I and @ARGV return to their old values here + +This block modifies all the C<.c> files in the current directory, +leaving a backup of the original data from each file in a new +C<.c.orig> file. + +=head2 How can I copy a file? +X<copy> X<file, copy> X<File::Copy> + +(contributed by brian d foy) + +Use the C<File::Copy> module. It comes with Perl and can do a +true copy across file systems, and it does its magic in +a portable fashion. + + use File::Copy; + + copy( $original, $new_copy ) or die "Copy failed: $!"; + +If you can't use C<File::Copy>, you'll have to do the work yourself: +open the original file, open the destination file, then print +to the destination file as you read the original. You also have to +remember to copy the permissions, owner, and group to the new file. + +=head2 How do I make a temporary file name? +X<file, temporary> + +If you don't need to know the name of the file, you can use C<open()> +with C<undef> in place of the file name. In Perl 5.8 or later, the +C<open()> function creates an anonymous temporary file: + + open my $tmp, '+>', undef or die $!; + +Otherwise, you can use the File::Temp module. + + use File::Temp qw/ tempfile tempdir /; + + my $dir = tempdir( CLEANUP => 1 ); + ($fh, $filename) = tempfile( DIR => $dir ); + + # or if you don't need to know the filename + + my $fh = tempfile( DIR => $dir ); + +The File::Temp has been a standard module since Perl 5.6.1. If you +don't have a modern enough Perl installed, use the C<new_tmpfile> +class method from the IO::File module to get a filehandle opened for +reading and writing. Use it if you don't need to know the file's name: + + use IO::File; + my $fh = IO::File->new_tmpfile() + or die "Unable to make new temporary file: $!"; + +If you're committed to creating a temporary file by hand, use the +process ID and/or the current time-value. If you need to have many +temporary files in one process, use a counter: + + BEGIN { + use Fcntl; + my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; + my $base_name = sprintf "%s/%d-%d-0000", $temp_dir, $$, time; + + sub temp_file { + local *FH; + my $count = 0; + until( defined(fileno(FH)) || $count++ > 100 ) { + $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; + # O_EXCL is required for security reasons. + sysopen my($fh), $base_name, O_WRONLY|O_EXCL|O_CREAT; + } + + if( defined fileno($fh) ) { + return ($fh, $base_name); + } + else { + return (); + } + } + + } + +=head2 How can I manipulate fixed-record-length files? +X<fixed-length> X<file, fixed-length records> + +The most efficient way is using L<pack()|perlfunc/"pack"> and +L<unpack()|perlfunc/"unpack">. This is faster than using +L<substr()|perlfunc/"substr"> when taking many, many strings. It is +slower for just a few. + +Here is a sample chunk of code to break up and put back together again +some fixed-format input lines, in this case from the output of a normal, +Berkeley-style ps: + + # sample input line: + # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what + my $PS_T = 'A6 A4 A7 A5 A*'; + open my $ps, '-|', 'ps'; + print scalar <$ps>; + my @fields = qw( pid tt stat time command ); + while (<$ps>) { + my %process; + @process{@fields} = unpack($PS_T, $_); + for my $field ( @fields ) { + print "$field: <$process{$field}>\n"; + } + print 'line=', pack($PS_T, @process{@fields} ), "\n"; + } + +We've used a hash slice in order to easily handle the fields of each row. +Storing the keys in an array means it's easy to operate on them as a +group or loop over them with for. It also avoids polluting the program +with global variables and using symbolic references. + +=head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? +X<filehandle, local> X<filehandle, passing> X<filehandle, reference> + +As of perl5.6, open() autovivifies file and directory handles +as references if you pass it an uninitialized scalar variable. +You can then pass these references just like any other scalar, +and use them in the place of named handles. + + open my $fh, $file_name; + + open local $fh, $file_name; + + print $fh "Hello World!\n"; + + process_file( $fh ); + +If you like, you can store these filehandles in an array or a hash. +If you access them directly, they aren't simple scalars and you +need to give C<print> a little help by placing the filehandle +reference in braces. Perl can only figure it out on its own when +the filehandle reference is a simple scalar. + + my @fhs = ( $fh1, $fh2, $fh3 ); + + for( $i = 0; $i <= $#fhs; $i++ ) { + print {$fhs[$i]} "just another Perl answer, \n"; + } + +Before perl5.6, you had to deal with various typeglob idioms +which you may see in older code. + + open FILE, "> $filename"; + process_typeglob( *FILE ); + process_reference( \*FILE ); + + sub process_typeglob { local *FH = shift; print FH "Typeglob!" } + sub process_reference { local $fh = shift; print $fh "Reference!" } + +If you want to create many anonymous handles, you should +check out the Symbol or IO::Handle modules. + +=head2 How can I use a filehandle indirectly? +X<filehandle, indirect> + +An indirect filehandle is using something other than a symbol +in a place that a filehandle is expected. Here are ways +to get indirect filehandles: + + $fh = SOME_FH; # bareword is strict-subs hostile + $fh = "SOME_FH"; # strict-refs hostile; same package only + $fh = *SOME_FH; # typeglob + $fh = \*SOME_FH; # ref to typeglob (bless-able) + $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob + +Or, you can use the C<new> method from one of the IO::* modules to +create an anonymous filehandle, store that in a scalar variable, +and use it as though it were a normal filehandle. + + use IO::Handle; # 5.004 or higher + my $fh = IO::Handle->new(); + +Then use any of those as you would a normal filehandle. Anywhere that +Perl is expecting a filehandle, an indirect filehandle may be used +instead. An indirect filehandle is just a scalar variable that contains +a filehandle. Functions like C<print>, C<open>, C<seek>, or +the C<< <FH> >> diamond operator will accept either a named filehandle +or a scalar variable containing one: + + ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); + print $ofh "Type it: "; + my $got = <$ifh> + print $efh "What was that: $got"; + +If you're passing a filehandle to a function, you can write +the function in two ways: + + sub accept_fh { + my $fh = shift; + print $fh "Sending to indirect filehandle\n"; + } + +Or it can localize a typeglob and use the filehandle directly: + + sub accept_fh { + local *FH = shift; + print FH "Sending to localized filehandle\n"; + } + +Both styles work with either objects or typeglobs of real filehandles. +(They might also work with strings under some circumstances, but this +is risky.) + + accept_fh(*STDOUT); + accept_fh($handle); + +In the examples above, we assigned the filehandle to a scalar variable +before using it. That is because only simple scalar variables, not +expressions or subscripts of hashes or arrays, can be used with +built-ins like C<print>, C<printf>, or the diamond operator. Using +something other than a simple scalar variable as a filehandle is +illegal and won't even compile: + + my @fd = (*STDIN, *STDOUT, *STDERR); + print $fd[1] "Type it: "; # WRONG + my $got = <$fd[0]> # WRONG + print $fd[2] "What was that: $got"; # WRONG + +With C<print> and C<printf>, you get around this by using a block and +an expression where you would place the filehandle: + + print { $fd[1] } "funny stuff\n"; + printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; + # Pity the poor deadbeef. + +That block is a proper block like any other, so you can put more +complicated code there. This sends the message out to one of two places: + + my $ok = -x "/bin/cat"; + print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; + print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; + +This approach of treating C<print> and C<printf> like object methods +calls doesn't work for the diamond operator. That's because it's a +real operator, not just a function with a comma-less argument. Assuming +you've been storing typeglobs in your structure as we did above, you +can use the built-in function named C<readline> to read a record just +as C<< <> >> does. Given the initialization shown above for @fd, this +would work, but only because readline() requires a typeglob. It doesn't +work with objects or strings, which might be a bug we haven't fixed yet. + + $got = readline($fd[0]); + +Let it be noted that the flakiness of indirect filehandles is not +related to whether they're strings, typeglobs, objects, or anything else. +It's the syntax of the fundamental operators. Playing the object +game doesn't help you at all here. + +=head2 How can I set up a footer format to be used with write()? +X<footer> + +There's no builtin way to do this, but L<perlform> has a couple of +techniques to make it possible for the intrepid hacker. + +=head2 How can I write() into a string? +X<write, into a string> + +See L<perlform/"Accessing Formatting Internals"> for an C<swrite()> function. + +=head2 How can I open a filehandle to a string? +X<string> X<open> X<IO::String> X<filehandle> + +(contributed by Peter J. Holzer, hjp-usenet2@hjp.at) + +Since Perl 5.8.0 a file handle referring to a string can be created by +calling open with a reference to that string instead of the filename. +This file handle can then be used to read from or write to the string: + + open(my $fh, '>', \$string) or die "Could not open string for writing"; + print $fh "foo\n"; + print $fh "bar\n"; # $string now contains "foo\nbar\n" + + open(my $fh, '<', \$string) or die "Could not open string for reading"; + my $x = <$fh>; # $x now contains "foo\n" + +With older versions of Perl, the C<IO::String> module provides similar +functionality. + +=head2 How can I output my numbers with commas added? +X<number, commify> + +(contributed by brian d foy and Benjamin Goldberg) + +You can use L<Number::Format> to separate places in a number. +It handles locale information for those of you who want to insert +full stops instead (or anything else that they want to use, +really). + +This subroutine will add commas to your number: + + sub commify { + local $_ = shift; + 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; + return $_; + } + +This regex from Benjamin Goldberg will add commas to numbers: + + s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; + +It is easier to see with comments: + + s/( + ^[-+]? # beginning of number. + \d+? # first digits before first comma + (?= # followed by, (but not included in the match) : + (?>(?:\d{3})+) # some positive multiple of three digits. + (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. + ) + | # or: + \G\d{3} # after the last group, get three digits + (?=\d) # but they have to have more digits after them. + )/$1,/xg; + +=head2 How can I translate tildes (~) in a filename? +X<tilde> X<tilde expansion> + +Use the E<lt>E<gt> (C<glob()>) operator, documented in L<perlfunc>. +Versions of Perl older than 5.6 require that you have a shell +installed that groks tildes. Later versions of Perl have this feature +built in. The C<File::KGlob> module (available from CPAN) gives more +portable glob functionality. + +Within Perl, you may use this directly: + + $filename =~ s{ + ^ ~ # find a leading tilde + ( # save this in $1 + [^/] # a non-slash character + * # repeated 0 or more times (0 means me) + ) + }{ + $1 + ? (getpwnam($1))[7] + : ( $ENV{HOME} || $ENV{LOGDIR} ) + }ex; + +=head2 How come when I open a file read-write it wipes it out? +X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating> + +Because you're using something like this, which truncates the file and +I<then> gives you read-write access: + + open my $fh, '+>', '/path/name'; # WRONG (almost always) + +Whoops. You should instead use this, which will fail if the file +doesn't exist. + + open my $fh, '+<', '/path/name'; # open for update + +Using ">" always clobbers or creates. Using "<" never does +either. The "+" doesn't change this. + +Here are examples of many kinds of file opens. Those using sysopen() +all assume + + use Fcntl; + +To open file for reading: + + open my $fh, '<', $path or die $!; + sysopen my $fh, $path, O_RDONLY or die $!; + +To open file for writing, create new file if needed or else truncate old file: + + open my $fh, '>', $path or die $!; + sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT or die $!; + sysopen my $fh, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666 or die $!; + +To open file for writing, create new file, file must not exist: + + sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT or die $!; + sysopen my $fh, $path, O_WRONLY|O_EXCL|O_CREAT, 0666 or die $!; + +To open file for appending, create if necessary: + + open my $fh, '>>' $path or die $!; + sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT or die $!; + sysopen my $fh, $path, O_WRONLY|O_APPEND|O_CREAT, 0666 or die $!; + +To open file for appending, file must exist: + + sysopen my $fh, $path, O_WRONLY|O_APPEND or die $!; + +To open file for update, file must exist: + + open my $fh, '+<', $path or die $!; + sysopen my $fh, $path, O_RDWR or die $!; + +To open file for update, create file if necessary: + + sysopen my $fh, $path, O_RDWR|O_CREAT or die $!; + sysopen my $fh, $path, O_RDWR|O_CREAT, 0666 or die $!; + +To open file for update, file must not exist: + + sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT or die $!; + sysopen my $fh, $path, O_RDWR|O_EXCL|O_CREAT, 0666 or die $!; + +To open a file without blocking, creating if necessary: + + sysopen my $fh, '/foo/somefile', O_WRONLY|O_NDELAY|O_CREAT + or die "can't open /foo/somefile: $!": + +Be warned that neither creation nor deletion of files is guaranteed to +be an atomic operation over NFS. That is, two processes might both +successfully create or unlink the same file! Therefore O_EXCL +isn't as exclusive as you might wish. + +See also the new L<perlopentut>. + +=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? +X<argument list too long> + +The C<< <> >> operator performs a globbing operation (see above). +In Perl versions earlier than v5.6.0, the internal glob() operator forks +csh(1) to do the actual glob expansion, but +csh can't handle more than 127 items and so gives the error message +C<Argument list too long>. People who installed tcsh as csh won't +have this problem, but their users may be surprised by it. + +To get around this, either upgrade to Perl v5.6.0 or later, do the glob +yourself with readdir() and patterns, or use a module like File::KGlob, +one that doesn't use the shell to do globbing. + +=head2 Is there a leak/bug in glob()? +X<glob> + +(contributed by brian d foy) + +Starting with Perl 5.6.0, C<glob> is implemented internally rather +than relying on an external resource. As such, memory issues with +C<glob> aren't a problem in modern perls. + +=head2 How can I open a file with a leading ">" or trailing blanks? +X<filename, special characters> + +(contributed by Brian McCauley) + +The special two argument form of Perl's open() function ignores +trailing blanks in filenames and infers the mode from certain leading +characters (or a trailing "|"). In older versions of Perl this was the +only version of open() and so it is prevalent in old code and books. + +Unless you have a particular reason to use the two argument form you +should use the three argument form of open() which does not treat any +characters in the filename as special. + + open my $fh, "<", " file "; # filename is " file " + open my $fh, ">", ">file"; # filename is ">file" + +=head2 How can I reliably rename a file? +X<rename> X<mv> X<move> X<file, rename> + +If your operating system supports a proper mv(1) utility or its +functional equivalent, this works: + + rename($old, $new) or system("mv", $old, $new); + +It may be more portable to use the C<File::Copy> module instead. +You just copy to the new file to the new name (checking return +values), then delete the old one. This isn't really the same +semantically as a C<rename()>, which preserves meta-information like +permissions, timestamps, inode info, etc. + +=head2 How can I lock a file? +X<lock> X<file, lock> X<flock> + +Perl's builtin flock() function (see L<perlfunc> for details) will call +flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and +later), and lockf(3) if neither of the two previous system calls exists. +On some systems, it may even use a different form of native locking. +Here are some gotchas with Perl's flock(): + +=over 4 + +=item 1 + +Produces a fatal error if none of the three system calls (or their +close equivalent) exists. + +=item 2 + +lockf(3) does not provide shared locking, and requires that the +filehandle be open for writing (or appending, or read/writing). + +=item 3 + +Some versions of flock() can't lock files over a network (e.g. on NFS file +systems), so you'd need to force the use of fcntl(2) when you build Perl. +But even this is dubious at best. See the flock entry of L<perlfunc> +and the F<INSTALL> file in the source distribution for information on +building Perl to do this. + +Two potentially non-obvious but traditional flock semantics are that +it waits indefinitely until the lock is granted, and that its locks are +I<merely advisory>. Such discretionary locks are more flexible, but +offer fewer guarantees. This means that files locked with flock() may +be modified by programs that do not also use flock(). Cars that stop +for red lights get on well with each other, but not with cars that don't +stop for red lights. See the perlport manpage, your port's specific +documentation, or your system-specific local manpages for details. It's +best to assume traditional behavior if you're writing portable programs. +(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.) + +For more information on file locking, see also +L<perlopentut/"File Locking"> if you have it (new for 5.6). + +=back + +=head2 Why can't I just open(FH, "E<gt>file.lock")? +X<lock, lockfile race condition> + +A common bit of code B<NOT TO USE> is this: + + sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE + open my $lock, '>', 'file.lock'; # THIS BROKEN CODE + +This is a classic race condition: you take two steps to do something +which must be done in one. That's why computer hardware provides an +atomic test-and-set instruction. In theory, this "ought" to work: + + sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT + or die "can't open file.lock: $!"; + +except that lamentably, file creation (and deletion) is not atomic +over NFS, so this won't work (at least, not every time) over the net. +Various schemes involving link() have been suggested, but +these tend to involve busy-wait, which is also less than desirable. + +=head2 I still don't get locking. I just want to increment the number in the file. How can I do this? +X<counter> X<file, counter> + +Didn't anyone ever tell you web-page hit counters were useless? +They don't count number of hits, they're a waste of time, and they serve +only to stroke the writer's vanity. It's better to pick a random number; +they're more realistic. + +Anyway, this is what you can do if you can't help yourself. + + use Fcntl qw(:DEFAULT :flock); + sysopen my $fh, "numfile", O_RDWR|O_CREAT or die "can't open numfile: $!"; + flock $fh, LOCK_EX or die "can't flock numfile: $!"; + my $num = <$fh> || 0; + seek $fh, 0, 0 or die "can't rewind numfile: $!"; + truncate $fh, 0 or die "can't truncate numfile: $!"; + (print $fh $num+1, "\n") or die "can't write numfile: $!"; + close $fh or die "can't close numfile: $!"; + +Here's a much better web-page hit counter: + + $hits = int( (time() - 850_000_000) / rand(1_000) ); + +If the count doesn't impress your friends, then the code might. :-) + +=head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking? +X<append> X<file, append> + +If you are on a system that correctly implements C<flock> and you use +the example appending code from "perldoc -f flock" everything will be +OK even if the OS you are on doesn't implement append mode correctly +(if such a system exists.) So if you are happy to restrict yourself to +OSs that implement C<flock> (and that's not really much of a +restriction) then that is what you should do. + +If you know you are only going to use a system that does correctly +implement appending (i.e. not Win32) then you can omit the C<seek> +from the code in the previous answer. + +If you know you are only writing code to run on an OS and filesystem +that does implement append mode correctly (a local filesystem on a +modern Unix for example), and you keep the file in block-buffered mode +and you write less than one buffer-full of output between each manual +flushing of the buffer then each bufferload is almost guaranteed to be +written to the end of the file in one chunk without getting +intermingled with anyone else's output. You can also use the +C<syswrite> function which is simply a wrapper around your system's +C<write(2)> system call. + +There is still a small theoretical chance that a signal will interrupt +the system level C<write()> operation before completion. There is also +a possibility that some STDIO implementations may call multiple system +level C<write()>s even if the buffer was empty to start. There may be +some systems where this probability is reduced to zero, and this is +not a concern when using C<:perlio> instead of your system's STDIO. + +=head2 How do I randomly update a binary file? +X<file, binary patch> + +If you're just trying to patch a binary, in many cases something as +simple as this works: + + perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs + +However, if you have fixed sized records, then you might do something more +like this: + + $RECSIZE = 220; # size of record, in bytes + $recno = 37; # which record to update + open my $fh, '+<', 'somewhere' or die "can't update somewhere: $!"; + seek $fh, $recno * $RECSIZE, 0; + read $fh, $record, $RECSIZE == $RECSIZE or die "can't read record $recno: $!"; + # munge the record + seek $fh, -$RECSIZE, 1; + print $fh $record; + close $fh; + +Locking and error checking are left as an exercise for the reader. +Don't forget them or you'll be quite sorry. + +=head2 How do I get a file's timestamp in perl? +X<timestamp> X<file, timestamp> + +If you want to retrieve the time at which the file was last read, +written, or had its meta-data (owner, etc) changed, you use the B<-A>, +B<-M>, or B<-C> file test operations as documented in L<perlfunc>. +These retrieve the age of the file (measured against the start-time of +your program) in days as a floating point number. Some platforms may +not have all of these times. See L<perlport> for details. To retrieve +the "raw" time in seconds since the epoch, you would call the stat +function, then use C<localtime()>, C<gmtime()>, or +C<POSIX::strftime()> to convert this into human-readable form. + +Here's an example: + + my $write_secs = (stat($file))[9]; + printf "file %s updated at %s\n", $file, + scalar localtime($write_secs); + +If you prefer something more legible, use the File::stat module +(part of the standard distribution in version 5.004 and later): + + # error checking left as an exercise for reader. + use File::stat; + use Time::localtime; + my $date_string = ctime(stat($file)->mtime); + print "file $file updated at $date_string\n"; + +The POSIX::strftime() approach has the benefit of being, +in theory, independent of the current locale. See L<perllocale> +for details. + +=head2 How do I set a file's timestamp in perl? +X<timestamp> X<file, timestamp> + +You use the utime() function documented in L<perlfunc/utime>. +By way of example, here's a little program that copies the +read and write times from its first argument to all the rest +of them. + + if (@ARGV < 2) { + die "usage: cptimes timestamp_file other_files ...\n"; + } + my $timestamp = shift; + my($atime, $mtime) = (stat($timestamp))[8,9]; + utime $atime, $mtime, @ARGV; + +Error checking is, as usual, left as an exercise for the reader. + +The perldoc for utime also has an example that has the same +effect as touch(1) on files that I<already exist>. + +Certain file systems have a limited ability to store the times +on a file at the expected level of precision. For example, the +FAT and HPFS filesystem are unable to create dates on files with +a finer granularity than two seconds. This is a limitation of +the filesystems, not of utime(). + +=head2 How do I print to more than one file at once? +X<print, to multiple files> + +To connect one filehandle to several output filehandles, +you can use the IO::Tee or Tie::FileHandle::Multiplex modules. + +If you only have to do this once, you can print individually +to each filehandle. + + for my $fh (FH1, FH2, FH3) { print $fh "whatever\n" } + +=head2 How can I read in an entire file all at once? +X<slurp> X<file, slurping> + +Are you sure you want to read the entire file and store it in memory? +If you mmap the file, you can virtually load the entire file into a +string without actually storing it in memory: + + use File::Map qw(map_file); + + map_file my $string, $filename; + +Once mapped, you can treat C<$string> as you would any other string. +Since you don't actually load the data, mmap-ing is very fast and does +not increase your memory footprint. + +If you really want to load the entire file, you can use the +C<File::Slurp> module to do it in one step. + + use File::Slurp; + + my $all_of_it = read_file($filename); # entire file in scalar + my @all_lines = read_file($filename); # one line per element + +The customary Perl approach for processing all the lines in a file is to +do so one line at a time: + + open my $input, '<', $file or die "can't open $file: $!"; + while (<$input>) { + chomp; + # do something with $_ + } + close $input or die "can't close $file: $!"; + +This is tremendously more efficient than reading the entire file into +memory as an array of lines and then processing it one element at a time, +which is often--if not almost always--the wrong approach. Whenever +you see someone do this: + + my @lines = <INPUT>; + +You should think long and hard about why you need everything loaded at +once. It's just not a scalable solution. You might also find it more +fun to use the standard Tie::File module, or the DB_File module's +$DB_RECNO bindings, which allow you to tie an array to a file so that +accessing an element the array actually accesses the corresponding +line in the file. + +You can read the entire filehandle contents into a scalar. + + { + local $/; + open my $fh, '<', $file or die "can't open $file: $!"; + $var = <$fh>; + } + +That temporarily undefs your record separator, and will automatically +close the file at block exit. If the file is already open, just use this: + + $var = do { local $/; <$fh> }; + +For ordinary files you can also use the read function. + + read( $fh, $var, -s $fh ); + +The third argument tests the byte size of the data on the INPUT filehandle +and reads that many bytes into the buffer $var. + +=head2 How can I read in a file by paragraphs? +X<file, reading by paragraphs> + +Use the C<$/> variable (see L<perlvar> for details). You can either +set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, +for instance, gets treated as two paragraphs and not three), or +C<"\n\n"> to accept empty paragraphs. + +Note that a blank line must have no blanks in it. Thus +S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. + +=head2 How can I read a single character from a file? From the keyboard? +X<getc> X<file, reading one character at a time> + +You can use the builtin C<getc()> function for most filehandles, but +it won't (easily) work on a terminal device. For STDIN, either use +the Term::ReadKey module from CPAN or use the sample code in +L<perlfunc/getc>. + +If your system supports the portable operating system programming +interface (POSIX), you can use the following code, which you'll note +turns off echo processing as well. + + #!/usr/bin/perl -w + use strict; + $| = 1; + for (1..4) { + print "gimme: "; + my $got = getone(); + print "--> $got\n"; + } + exit; + + BEGIN { + use POSIX qw(:termios_h); + + my ($term, $oterm, $echo, $noecho, $fd_stdin); + + my $fd_stdin = fileno(STDIN); + + $term = POSIX::Termios->new(); + $term->getattr($fd_stdin); + $oterm = $term->getlflag(); + + $echo = ECHO | ECHOK | ICANON; + $noecho = $oterm & ~$echo; + + sub cbreak { + $term->setlflag($noecho); + $term->setcc(VTIME, 1); + $term->setattr($fd_stdin, TCSANOW); + } + + sub cooked { + $term->setlflag($oterm); + $term->setcc(VTIME, 0); + $term->setattr($fd_stdin, TCSANOW); + } + + sub getone { + my $key = ''; + cbreak(); + sysread(STDIN, $key, 1); + cooked(); + return $key; + } + + } + + END { cooked() } + +The Term::ReadKey module from CPAN may be easier to use. Recent versions +include also support for non-portable systems as well. + + use Term::ReadKey; + open my $tty, '<', '/dev/tty'; + print "Gimme a char: "; + ReadMode "raw"; + my $key = ReadKey 0, $tty; + ReadMode "normal"; + printf "\nYou said %s, char number %03d\n", + $key, ord $key; + +=head2 How can I tell whether there's a character waiting on a filehandle? + +The very first thing you should do is look into getting the Term::ReadKey +extension from CPAN. As we mentioned earlier, it now even has limited +support for non-portable (read: not open systems, closed, proprietary, +not POSIX, not Unix, etc.) systems. + +You should also check out the Frequently Asked Questions list in +comp.unix.* for things like this: the answer is essentially the same. +It's very system dependent. Here's one solution that works on BSD +systems: + + sub key_ready { + my($rin, $nfd); + vec($rin, fileno(STDIN), 1) = 1; + return $nfd = select($rin,undef,undef,0); + } + +If you want to find out how many characters are waiting, there's +also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that +comes with Perl tries to convert C include files to Perl code, which +can be C<require>d. FIONREAD ends up defined as a function in the +I<sys/ioctl.ph> file: + + require 'sys/ioctl.ph'; + + $size = pack("L", 0); + ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; + $size = unpack("L", $size); + +If I<h2ph> wasn't installed or doesn't work for you, you can +I<grep> the include files by hand: + + % grep FIONREAD /usr/include/*/* + /usr/include/asm/ioctls.h:#define FIONREAD 0x541B + +Or write a small C program using the editor of champions: + + % cat > fionread.c + #include <sys/ioctl.h> + main() { + printf("%#08x\n", FIONREAD); + } + ^D + % cc -o fionread fionread.c + % ./fionread + 0x4004667f + +And then hard code it, leaving porting as an exercise to your successor. + + $FIONREAD = 0x4004667f; # XXX: opsys dependent + + $size = pack("L", 0); + ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; + $size = unpack("L", $size); + +FIONREAD requires a filehandle connected to a stream, meaning that sockets, +pipes, and tty devices work, but I<not> files. + +=head2 How do I do a C<tail -f> in perl? +X<tail> X<IO::Handle> X<File::Tail> X<clearerr> + +First try + + seek(GWFILE, 0, 1); + +The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, +but it does clear the end-of-file condition on the handle, so that the +next C<< <GWFILE> >> makes Perl try again to read something. + +If that doesn't work (it relies on features of your stdio implementation), +then you need something more like this: + + for (;;) { + for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { + # search for some stuff and put it into files + } + # sleep for a while + seek(GWFILE, $curpos, 0); # seek to where we had been + } + +If this still doesn't work, look into the C<clearerr> method +from C<IO::Handle>, which resets the error and end-of-file states +on the handle. + +There's also a C<File::Tail> module from CPAN. + +=head2 How do I dup() a filehandle in Perl? +X<dup> + +If you check L<perlfunc/open>, you'll see that several of the ways +to call open() should do the trick. For example: + + open my $log, '>>', '/foo/logfile'; + open STDERR, '>&LOG'; + +Or even with a literal numeric descriptor: + + my $fd = $ENV{MHCONTEXTFD}; + open $mhcontext, "<&=$fd"; # like fdopen(3S) + +Note that "<&STDIN" makes a copy, but "<&=STDIN" make +an alias. That means if you close an aliased handle, all +aliases become inaccessible. This is not true with +a copied one. + +Error checking, as always, has been left as an exercise for the reader. + +=head2 How do I close a file descriptor by number? +X<file, closing file descriptors> X<POSIX> X<close> + +If, for some reason, you have a file descriptor instead of a +filehandle (perhaps you used C<POSIX::open>), you can use the +C<close()> function from the C<POSIX> module: + + use POSIX (); + + POSIX::close( $fd ); + +This should rarely be necessary, as the Perl C<close()> function is to be +used for things that Perl opened itself, even if it was a dup of a +numeric descriptor as with C<MHCONTEXT> above. But if you really have +to, you may be able to do this: + + require 'sys/syscall.ph'; + my $rc = syscall(&SYS_close, $fd + 0); # must force numeric + die "can't sysclose $fd: $!" unless $rc == -1; + +Or, just use the fdopen(3S) feature of C<open()>: + + { + open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!"; + close $fh; + } + +=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? +X<filename, DOS issues> + +Whoops! You just put a tab and a formfeed into that filename! +Remember that within double quoted strings ("like\this"), the +backslash is an escape character. The full list of these is in +L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't +have a file called "c:(tab)emp(formfeed)oo" or +"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. + +Either single-quote your strings, or (preferably) use forward slashes. +Since all DOS and Windows versions since something like MS-DOS 2.0 or so +have treated C</> and C<\> the same in a path, you might as well use the +one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, +awk, Tcl, Java, or Python, just to mention a few. POSIX paths +are more portable, too. + +=head2 Why doesn't glob("*.*") get all the files? +X<glob> + +Because even on non-Unix ports, Perl's glob function follows standard +Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) +files. This makes glob() portable even to legacy systems. Your +port may include proprietary globbing functions as well. Check its +documentation for details. + +=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? + +This is elaborately and painstakingly described in the +F<file-dir-perms> article in the "Far More Than You Ever Wanted To +Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . + +The executive summary: learn how your filesystem works. The +permissions on a file say what can happen to the data in that file. +The permissions on a directory say what can happen to the list of +files in that directory. If you delete a file, you're removing its +name from the directory (so the operation depends on the permissions +of the directory, not of the file). If you try to write to the file, +the permissions of the file govern whether you're allowed to. + +=head2 How do I select a random line from a file? +X<file, selecting a random line> + +Short of loading the file into a database or pre-indexing the lines in +the file, there are a couple of things that you can do. + +Here's a reservoir-sampling algorithm from the Camel Book: + + srand; + rand($.) < 1 && ($line = $_) while <>; + +This has a significant advantage in space over reading the whole file +in. You can find a proof of this method in I<The Art of Computer +Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. + +You can use the C<File::Random> module which provides a function +for that algorithm: + + use File::Random qw/random_line/; + my $line = random_line($filename); + +Another way is to use the C<Tie::File> module, which treats the entire +file as an array. Simply access a random array element. + +=head2 Why do I get weird spaces when I print an array of lines? + +(contributed by brian d foy) + +If you are seeing spaces between the elements of your array when +you print the array, you are probably interpolating the array in +double quotes: + + my @animals = qw(camel llama alpaca vicuna); + print "animals are: @animals\n"; + +It's the double quotes, not the C<print>, doing this. Whenever you +interpolate an array in a double quote context, Perl joins the +elements with spaces (or whatever is in C<$">, which is a space by +default): + + animals are: camel llama alpaca vicuna + +This is different than printing the array without the interpolation: + + my @animals = qw(camel llama alpaca vicuna); + print "animals are: ", @animals, "\n"; + +Now the output doesn't have the spaces between the elements because +the elements of C<@animals> simply become part of the list to +C<print>: + + animals are: camelllamaalpacavicuna + +You might notice this when each of the elements of C<@array> end with +a newline. You expect to print one element per line, but notice that +every line after the first is indented: + + this is a line + this is another line + this is the third line + +That extra space comes from the interpolation of the array. If you +don't want to put anything between your array elements, don't use the +array in double quotes. You can send it to print without them: + + print @lines; + +=head2 How do I traverse a directory tree? + +(contributed by brian d foy) + +The C<File::Find> module, which comes with Perl, does all of the hard +work to traverse a directory structure. It comes with Perl. You simply +call the C<find> subroutine with a callback subroutine and the +directories you want to traverse: + + use File::Find; + + find( \&wanted, @directories ); + + sub wanted { + # full path in $File::Find::name + # just filename in $_ + ... do whatever you want to do ... + } + +The C<File::Find::Closures>, which you can download from CPAN, provides +many ready-to-use subroutines that you can use with C<File::Find>. + +The C<File::Finder>, which you can download from CPAN, can help you +create the callback subroutine using something closer to the syntax of +the C<find> command-line utility: + + use File::Find; + use File::Finder; + + my $deep_dirs = File::Finder->depth->type('d')->ls->exec('rmdir','{}'); + + find( $deep_dirs->as_options, @places ); + +The C<File::Find::Rule> module, which you can download from CPAN, has +a similar interface, but does the traversal for you too: + + use File::Find::Rule; + + my @files = File::Find::Rule->file() + ->name( '*.pm' ) + ->in( @INC ); + +=head2 How do I delete a directory tree? + +(contributed by brian d foy) + +If you have an empty directory, you can use Perl's built-in C<rmdir>. +If the directory is not empty (so, no files or subdirectories), you +either have to empty it yourself (a lot of work) or use a module to +help you. + +The C<File::Path> module, which comes with Perl, has a C<remove_tree> +which can take care of all of the hard work for you: + + use File::Path qw(remove_tree); + + remove_tree( @directories ); + +The C<File::Path> module also has a legacy interface to the older +C<rmtree> subroutine. + +=head2 How do I copy an entire directory? + +(contributed by Shlomi Fish) + +To do the equivalent of C<cp -R> (i.e. copy an entire directory tree +recursively) in portable Perl, you'll either need to write something yourself +or find a good CPAN module such as L<File::Copy::Recursive>. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and +other authors as noted. All rights reserved. + +This documentation is free; you can redistribute it and/or modify it +under the same terms as Perl itself. + +Irrespective of its distribution, all code examples here are in the public +domain. You are permitted and encouraged to use this code and any +derivatives thereof in your own programs for fun or for profit as you +see fit. A simple comment in the code giving credit to the FAQ would +be courteous but is not required. |