perlfork.1

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.\" ========================================================================
.\"
.IX Title "PERLFORK 1"
.TH PERLFORK 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlfork \- Perl's fork() emulation
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 4
\&    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.
.Ve
.PP
Perl provides a \fIfork()\fR keyword that corresponds to the Unix system call
of the same name.  On most Unix-like platforms where the \fIfork()\fR system
call is available, Perl's \fIfork()\fR simply calls it.
.PP
On some platforms such as Windows where the \fIfork()\fR system call is not
available, Perl can be built to emulate \fIfork()\fR at the interpreter level.
While the emulation is designed to be as compatible as possible with the
real \fIfork()\fR at the level of the Perl program, there are certain
important differences that stem from the fact that all the pseudo child
\&\*(L"processes\*(R" created this way live in the same real process as far as the
operating system is concerned.
.PP
This document provides a general overview of the capabilities and
limitations of the \fIfork()\fR emulation.  Note that the issues discussed here
are not applicable to platforms where a real \fIfork()\fR is available and Perl
has been configured to use it.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fIfork()\fR emulation is implemented at the level of the Perl interpreter.
What this means in general is that running \fIfork()\fR 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 \fIfork()\fR was called in the parent.  We will refer to the
thread that implements this child \*(L"process\*(R" as the pseudo-process.
.PP
To the Perl program that called \fIfork()\fR, all this is designed to be
transparent.  The parent returns from the \fIfork()\fR with a pseudo-process
\&\s-1ID\s0 that can be subsequently used in any process manipulation functions;
the child returns from the \fIfork()\fR with a value of \f(CW0\fR to signify that
it is the child pseudo-process.
.Sh "Behavior of other Perl features in forked pseudo-processes"
.IX Subsection "Behavior of other Perl features in forked pseudo-processes"
Most Perl features behave in a natural way within pseudo-processes.
.ie n .IP "$$ or $PROCESS_ID" 8
.el .IP "$$ or \f(CW$PROCESS_ID\fR" 8
.IX Item "$$ or $PROCESS_ID"
This special variable is correctly set to the pseudo-process \s-1ID\s0.
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 \fIwait()\fR\-ed on.
.ie n .IP "%ENV" 8
.el .IP "\f(CW%ENV\fR" 8
.IX Item "%ENV"
Each pseudo-process maintains its own virtual environment.  Modifications
to \f(CW%ENV\fR affect the virtual environment, and are only visible within that
pseudo-process, and in any processes (or pseudo-processes) launched from
it.
.IP "\fIchdir()\fR and all other builtins that accept filenames" 8
.IX 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 \fIchdir()\fR 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.
.IP "\fIwait()\fR and \fIwaitpid()\fR" 8
.IX Item "wait() and waitpid()"
\&\fIwait()\fR and \fIwaitpid()\fR can be passed a pseudo-process \s-1ID\s0 returned by \fIfork()\fR.
These calls will properly wait for the termination of the pseudo-process
and return its status.
.IP "\fIkill()\fR" 8
.IX Item "kill()"
\&\fIkill()\fR can be used to terminate a pseudo-process by passing it the \s-1ID\s0 returned
by \fIfork()\fR.  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 \fIkill()\fR on a
pseudo\-\fIprocess()\fR may typically cause memory leaks, because the thread that
implements the pseudo-process does not get a chance to clean up its resources.