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diff --git a/Master/tlpkg/tlperl0/lib/pods/perlfork.pod b/Master/tlpkg/tlperl0/lib/pods/perlfork.pod new file mode 100755 index 00000000000..48d65ed4c58 --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perlfork.pod @@ -0,0 +1,323 @@ +=head1 NAME + +perlfork - Perl's fork() emulation + +=head1 SYNOPSIS + + NOTE: As of the 5.8.0 release, fork() emulation has considerably + matured. However, there are still a few known bugs and differences + from real fork() that might affect you. See the "BUGS" and + "CAVEATS AND LIMITATIONS" sections below. + +Perl provides a fork() keyword that corresponds to the Unix system call +of the same name. On most Unix-like platforms where the fork() system +call is available, Perl's fork() simply calls it. + +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. +While the emulation is designed to be as compatible as possible with the +real fork() at the level of the Perl program, there are certain +important differences that stem from the fact that all the pseudo child +"processes" created this way live in the same real process as far as the +operating system is concerned. + +This document provides a general overview of the capabilities and +limitations of the fork() emulation. Note that the issues discussed here +are not applicable to platforms where a real fork() is available and Perl +has been configured to use it. + +=head1 DESCRIPTION + +The fork() emulation is implemented at the level of the Perl interpreter. +What this means in general is that running fork() will actually clone the +running interpreter and all its state, and run the cloned interpreter in +a separate thread, beginning execution in the new thread just after the +point where the fork() was called in the parent. We will refer to the +thread that implements this child "process" as the pseudo-process. + +To the Perl program that called fork(), all this is designed to be +transparent. The parent returns from the fork() with a pseudo-process +ID that can be subsequently used in any process manipulation functions; +the child returns from the fork() with a value of C<0> to signify that +it is the child pseudo-process. + +=head2 Behavior of other Perl features in forked pseudo-processes + +Most Perl features behave in a natural way within pseudo-processes. + +=over 8 + +=item $$ or $PROCESS_ID + +This special variable is correctly set to the pseudo-process ID. +It can be used to identify pseudo-processes within a particular +session. Note that this value is subject to recycling if any +pseudo-processes are launched after others have been wait()-ed on. + +=item %ENV + +Each pseudo-process maintains its own virtual environment. Modifications +to %ENV affect the virtual environment, and are only visible within that +pseudo-process, and in any processes (or pseudo-processes) launched from +it. + +=item chdir() and all other builtins that accept filenames + +Each pseudo-process maintains its own virtual idea of the current directory. +Modifications to the current directory using chdir() are only visible within +that pseudo-process, and in any processes (or pseudo-processes) launched from +it. All file and directory accesses from the pseudo-process will correctly +map the virtual working directory to the real working directory appropriately. + +=item wait() and waitpid() + +wait() and waitpid() can be passed a pseudo-process ID returned by fork(). +These calls will properly wait for the termination of the pseudo-process +and return its status. + +=item kill() + +kill() can be used to terminate a pseudo-process by passing it the ID returned +by fork(). This should not be used except under dire circumstances, because +the operating system may not guarantee integrity of the process resources +when a running thread is terminated. Note that using kill() on a +pseudo-process() may typically cause memory leaks, because the thread that +implements the pseudo-process does not get a chance to clean up its resources. + +=item exec() + +Calling exec() within a pseudo-process actually spawns the requested +executable in a separate process and waits for it to complete before +exiting with the same exit status as that process. This means that the +process ID reported within the running executable will be different from +what the earlier Perl fork() might have returned. Similarly, any process +manipulation functions applied to the ID returned by fork() will affect the +waiting pseudo-process that called exec(), not the real process it is +waiting for after the exec(). + +When exec() is called inside a pseudo-process then DESTROY methods and +END blocks will still be called after the external process returns. + +=item exit() + +exit() always exits just the executing pseudo-process, after automatically +wait()-ing for any outstanding child pseudo-processes. Note that this means +that the process as a whole will not exit unless all running pseudo-processes +have exited. See below for some limitations with open filehandles. + +=item Open handles to files, directories and network sockets + +All open handles are dup()-ed in pseudo-processes, so that closing +any handles in one process does not affect the others. See below for +some limitations. + +=back + +=head2 Resource limits + +In the eyes of the operating system, pseudo-processes created via the fork() +emulation are simply threads in the same process. This means that any +process-level limits imposed by the operating system apply to all +pseudo-processes taken together. This includes any limits imposed by the +operating system on the number of open file, directory and socket handles, +limits on disk space usage, limits on memory size, limits on CPU utilization +etc. + +=head2 Killing the parent process + +If the parent process is killed (either using Perl's kill() builtin, or +using some external means) all the pseudo-processes are killed as well, +and the whole process exits. + +=head2 Lifetime of the parent process and pseudo-processes + +During the normal course of events, the parent process and every +pseudo-process started by it will wait for their respective pseudo-children +to complete before they exit. This means that the parent and every +pseudo-child created by it that is also a pseudo-parent will only exit +after their pseudo-children have exited. + +A way to mark a pseudo-processes as running detached from their parent (so +that the parent would not have to wait() for them if it doesn't want to) +will be provided in future. + +=head2 CAVEATS AND LIMITATIONS + +=over 8 + +=item BEGIN blocks + +The fork() emulation will not work entirely correctly when called from +within a BEGIN block. The forked copy will run the contents of the +BEGIN block, but will not continue parsing the source stream after the +BEGIN block. For example, consider the following code: + + BEGIN { + fork and exit; # fork child and exit the parent + print "inner\n"; + } + print "outer\n"; + +This will print: + + inner + +rather than the expected: + + inner + outer + +This limitation arises from fundamental technical difficulties in +cloning and restarting the stacks used by the Perl parser in the +middle of a parse. + +=item Open filehandles + +Any filehandles open at the time of the fork() will be dup()-ed. Thus, +the files can be closed independently in the parent and child, but beware +that the dup()-ed handles will still share the same seek pointer. Changing +the seek position in the parent will change it in the child and vice-versa. +One can avoid this by opening files that need distinct seek pointers +separately in the child. + +On some operating systems, notably Solaris and Unixware, calling C<exit()> +from a child process will flush and close open filehandles in the parent, +thereby corrupting the filehandles. On these systems, calling C<_exit()> +is suggested instead. C<_exit()> is available in Perl through the +C<POSIX> module. Please consult your systems manpages for more information +on this. + +=item Forking pipe open() not yet implemented + +The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet +implemented. This limitation can be easily worked around in new code +by creating a pipe explicitly. The following example shows how to +write to a forked child: + + # simulate open(FOO, "|-") + sub pipe_to_fork ($) { + my $parent = shift; + pipe my $child, $parent or die; + my $pid = fork(); + die "fork() failed: $!" unless defined $pid; + if ($pid) { + close $child; + } + else { + close $parent; + open(STDIN, "<&=" . fileno($child)) or die; + } + $pid; + } + + if (pipe_to_fork('FOO')) { + # parent + print FOO "pipe_to_fork\n"; + close FOO; + } + else { + # child + while (<STDIN>) { print; } + exit(0); + } + +And this one reads from the child: + + # simulate open(FOO, "-|") + sub pipe_from_fork ($) { + my $parent = shift; + pipe $parent, my $child or die; + my $pid = fork(); + die "fork() failed: $!" unless defined $pid; + if ($pid) { + close $child; + } + else { + close $parent; + open(STDOUT, ">&=" . fileno($child)) or die; + } + $pid; + } + + if (pipe_from_fork('BAR')) { + # parent + while (<BAR>) { print; } + close BAR; + } + else { + # child + print "pipe_from_fork\n"; + exit(0); + } + +Forking pipe open() constructs will be supported in future. + +=item Global state maintained by XSUBs + +External subroutines (XSUBs) that maintain their own global state may +not work correctly. Such XSUBs will either need to maintain locks to +protect simultaneous access to global data from different pseudo-processes, +or maintain all their state on the Perl symbol table, which is copied +naturally when fork() is called. A callback mechanism that provides +extensions an opportunity to clone their state will be provided in the +near future. + +=item Interpreter embedded in larger application + +The fork() emulation may not behave as expected when it is executed in an +application which embeds a Perl interpreter and calls Perl APIs that can +evaluate bits of Perl code. This stems from the fact that the emulation +only has knowledge about the Perl interpreter's own data structures and +knows nothing about the containing application's state. For example, any +state carried on the application's own call stack is out of reach. + +=item Thread-safety of extensions + +Since the fork() emulation runs code in multiple threads, extensions +calling into non-thread-safe libraries may not work reliably when +calling fork(). As Perl's threading support gradually becomes more +widely adopted even on platforms with a native fork(), such extensions +are expected to be fixed for thread-safety. + +=back + +=head1 BUGS + +=over 8 + +=item * + +Having pseudo-process IDs be negative integers breaks down for the integer +C<-1> because the wait() and waitpid() functions treat this number as +being special. The tacit assumption in the current implementation is that +the system never allocates a thread ID of C<1> for user threads. A better +representation for pseudo-process IDs will be implemented in future. + +=item * + +In certain cases, the OS-level handles created by the pipe(), socket(), +and accept() operators are apparently not duplicated accurately in +pseudo-processes. This only happens in some situations, but where it +does happen, it may result in deadlocks between the read and write ends +of pipe handles, or inability to send or receive data across socket +handles. + +=item * + +This document may be incomplete in some respects. + +=back + +=head1 AUTHOR + +Support for concurrent interpreters and the fork() emulation was implemented +by ActiveState, with funding from Microsoft Corporation. + +This document is authored and maintained by Gurusamy Sarathy +E<lt>gsar@activestate.comE<gt>. + +=head1 SEE ALSO + +L<perlfunc/"fork">, L<perlipc> + +=cut |