{
    "mode": "man",
    "parameter": "perlfork",
    "section": "1",
    "url": "https://www.chedong.com/phpMan.php/man/perlfork/1/json",
    "generated": "2026-06-10T21:12:57Z",
    "synopsis": "NOTE:  As of the 5.8.0 release, fork() emulation has considerably\nmatured.  However, there are still a few known bugs and differences\nfrom real fork() that might affect you.  See the \"BUGS\" and\n\"CAVEATS AND LIMITATIONS\" sections below.\nPerl provides a fork() keyword that corresponds to the Unix system call of the same name.  On\nmost Unix-like platforms where the fork() system call is available, Perl's fork() simply\ncalls it.\nOn some platforms such as Windows where the fork() system call is not available, Perl can be\nbuilt to emulate fork() at the interpreter level.  While the emulation is designed to be as\ncompatible as possible with the real fork() at the level of the Perl program, there are\ncertain important differences that stem from the fact that all the pseudo child \"processes\"\ncreated this way live in the same real process as far as the operating system is concerned.\nThis document provides a general overview of the capabilities and limitations of the fork()\nemulation.  Note that the issues discussed here are not applicable to platforms where a real\nfork() is available and Perl has been configured to use it.",
    "sections": {
        "NAME": {
            "content": "perlfork - Perl's fork() emulation\n",
            "subsections": []
        },
        "SYNOPSIS": {
            "content": "NOTE:  As of the 5.8.0 release, fork() emulation has considerably\nmatured.  However, there are still a few known bugs and differences\nfrom real fork() that might affect you.  See the \"BUGS\" and\n\"CAVEATS AND LIMITATIONS\" sections below.\n\nPerl provides a fork() keyword that corresponds to the Unix system call of the same name.  On\nmost Unix-like platforms where the fork() system call is available, Perl's fork() simply\ncalls it.\n\nOn some platforms such as Windows where the fork() system call is not available, Perl can be\nbuilt to emulate fork() at the interpreter level.  While the emulation is designed to be as\ncompatible as possible with the real fork() at the level of the Perl program, there are\ncertain important differences that stem from the fact that all the pseudo child \"processes\"\ncreated this way live in the same real process as far as the operating system is concerned.\n\nThis document provides a general overview of the capabilities and limitations of the fork()\nemulation.  Note that the issues discussed here are not applicable to platforms where a real\nfork() is available and Perl has been configured to use it.\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "The fork() emulation is implemented at the level of the Perl interpreter.  What this means in\ngeneral is that running fork() will actually clone the running interpreter and all its state,\nand run the cloned interpreter in a separate thread, beginning execution in the new thread\njust after the point where the fork() was called in the parent.  We will refer to the thread\nthat implements this child \"process\" as the pseudo-process.\n\nTo the Perl program that called fork(), all this is designed to be transparent.  The parent\nreturns from the fork() with a pseudo-process ID that can be subsequently used in any\nprocess-manipulation functions; the child returns from the fork() with a value of 0 to\nsignify that it is the child pseudo-process.\n",
            "subsections": [
                {
                    "name": "Behavior of other Perl features in forked pseudo-processes",
                    "content": "Most Perl features behave in a natural way within pseudo-processes.\n\n$$ or $PROCESSID\nThis special variable is correctly set to the pseudo-process ID.  It can be used to\nidentify pseudo-processes within a particular session.  Note that this value is\nsubject to recycling if any pseudo-processes are launched after others have been\nwait()-ed on.\n\n%ENV    Each pseudo-process maintains its own virtual environment.  Modifications to %ENV\naffect the virtual environment, and are only visible within that pseudo-process, and\nin any processes (or pseudo-processes) launched from it.\n\nchdir() and all other builtins that accept filenames\nEach pseudo-process maintains its own virtual idea of the current directory.\nModifications to the current directory using chdir() are only visible within that\npseudo-process, and in any processes (or pseudo-processes) launched from it.  All\nfile and directory accesses from the pseudo-process will correctly map the virtual\nworking directory to the real working directory appropriately.\n\nwait() and waitpid()\nwait() and waitpid() can be passed a pseudo-process ID returned by fork().  These\ncalls will properly wait for the termination of the pseudo-process and return its\nstatus.\n\nkill()  \"kill('KILL', ...)\" can be used to terminate a pseudo-process by passing it the ID\nreturned by fork(). The outcome of kill on a pseudo-process is unpredictable and it\nshould not be used except under dire circumstances, because the operating system may\nnot guarantee integrity of the process resources when a running thread is terminated.\nThe process which implements the pseudo-processes can be blocked and the Perl\ninterpreter hangs. Note that using \"kill('KILL', ...)\" on a pseudo-process() may\ntypically cause memory leaks, because the thread that implements the pseudo-process\ndoes not get a chance to clean up its resources.\n\n\"kill('TERM', ...)\" can also be used on pseudo-processes, but the signal will not be\ndelivered while the pseudo-process is blocked by a system call, e.g. waiting for a\nsocket to connect, or trying to read from a socket with no data available.  Starting\nin Perl 5.14 the parent process will not wait for children to exit once they have\nbeen signalled with \"kill('TERM', ...)\" to avoid deadlock during process exit.  You\nwill have to explicitly call waitpid() to make sure the child has time to clean-up\nitself, but you are then also responsible that the child is not blocking on I/O\neither.\n\nexec()  Calling exec() within a pseudo-process actually spawns the requested executable in a\nseparate process and waits for it to complete before exiting with the same exit\nstatus as that process.  This means that the process ID reported within the running\nexecutable will be different from what the earlier Perl fork() might have returned.\nSimilarly, any process manipulation functions applied to the ID returned by fork()\nwill affect the waiting pseudo-process that called exec(), not the real process it is\nwaiting for after the exec().\n\nWhen exec() is called inside a pseudo-process then DESTROY methods and END blocks\nwill still be called after the external process returns.\n\nexit()  exit() always exits just the executing pseudo-process, after automatically wait()-ing\nfor any outstanding child pseudo-processes.  Note that this means that the process as\na whole will not exit unless all running pseudo-processes have exited.  See below for\nsome limitations with open filehandles.\n\nOpen handles to files, directories and network sockets\nAll open handles are dup()-ed in pseudo-processes, so that closing any handles in one\nprocess does not affect the others.  See below for some limitations.\n"
                },
                {
                    "name": "Resource limits",
                    "content": "In the eyes of the operating system, pseudo-processes created via the fork() emulation are\nsimply threads in the same process.  This means that any process-level limits imposed by the\noperating system apply to all pseudo-processes taken together.  This includes any limits\nimposed by the operating system on the number of open file, directory and socket handles,\nlimits on disk space usage, limits on memory size, limits on CPU utilization etc.\n"
                },
                {
                    "name": "Killing the parent process",
                    "content": "If the parent process is killed (either using Perl's kill() builtin, or using some external\nmeans) all the pseudo-processes are killed as well, and the whole process exits.\n"
                },
                {
                    "name": "Lifetime of the parent process and pseudo-processes",
                    "content": "During the normal course of events, the parent process and every pseudo-process started by it\nwill wait for their respective pseudo-children to complete before they exit.  This means that\nthe parent and every pseudo-child created by it that is also a pseudo-parent will only exit\nafter their pseudo-children have exited.\n\nStarting with Perl 5.14 a parent will not wait() automatically for any child that has been\nsignalled with \"kill('TERM', ...)\"  to avoid a deadlock in case the child is blocking on I/O\nand never receives the signal.\n"
                }
            ]
        },
        "CAVEATS AND LIMITATIONS": {
            "content": "BEGIN blocks\nThe fork() emulation will not work entirely correctly when called from within a BEGIN\nblock.  The forked copy will run the contents of the BEGIN block, but will not\ncontinue parsing the source stream after the BEGIN block.  For example, consider the\nfollowing code:\n\nBEGIN {\nfork and exit;          # fork child and exit the parent\nprint \"inner\\n\";\n}\nprint \"outer\\n\";\n\nThis will print:\n\ninner\n\nrather than the expected:\n\ninner\nouter\n\nThis limitation arises from fundamental technical difficulties in cloning and\nrestarting the stacks used by the Perl parser in the middle of a parse.\n\nOpen filehandles\nAny filehandles open at the time of the fork() will be dup()-ed.  Thus, the files can\nbe closed independently in the parent and child, but beware that the dup()-ed handles\nwill still share the same seek pointer.  Changing the seek position in the parent\nwill change it in the child and vice-versa.  One can avoid this by opening files that\nneed distinct seek pointers separately in the child.\n\nOn some operating systems, notably Solaris and Unixware, calling \"exit()\" from a\nchild process will flush and close open filehandles in the parent, thereby corrupting\nthe filehandles.  On these systems, calling \"exit()\" is suggested instead.\n\"exit()\" is available in Perl through the \"POSIX\" module.  Please consult your\nsystem's manpages for more information on this.\n\nOpen directory handles\nPerl will completely read from all open directory handles until they reach the end of\nthe stream.  It will then seekdir() back to the original location and all future\nreaddir() requests will be fulfilled from the cache buffer.  That means that neither\nthe directory handle held by the parent process nor the one held by the child process\nwill see any changes made to the directory after the fork() call.\n\nNote that rewinddir() has a similar limitation on Windows and will not force\nreaddir() to read the directory again either.  Only a newly opened directory handle\nwill reflect changes to the directory.\n\nForking pipe open() not yet implemented\nThe \"open(FOO, \"|-\")\" and \"open(BAR, \"-|\")\" constructs are not yet implemented.  This\nlimitation can be easily worked around in new code by creating a pipe explicitly.\nThe following example shows how to write to a forked child:\n\n# simulate open(FOO, \"|-\")\nsub pipetofork ($) {\nmy $parent = shift;\npipe my $child, $parent or die;\nmy $pid = fork();\ndie \"fork() failed: $!\" unless defined $pid;\nif ($pid) {\nclose $child;\n}\nelse {\nclose $parent;\nopen(STDIN, \"<&=\" . fileno($child)) or die;\n}\n$pid;\n}\n\nif (pipetofork('FOO')) {\n# parent\nprint FOO \"pipetofork\\n\";\nclose FOO;\n}\nelse {\n# child\nwhile (<STDIN>) { print; }\nexit(0);\n}\n\nAnd this one reads from the child:\n\n# simulate open(FOO, \"-|\")\nsub pipefromfork ($) {\nmy $parent = shift;\npipe $parent, my $child or die;\nmy $pid = fork();\ndie \"fork() failed: $!\" unless defined $pid;\nif ($pid) {\nclose $child;\n}\nelse {\nclose $parent;\nopen(STDOUT, \">&=\" . fileno($child)) or die;\n}\n$pid;\n}\n\nif (pipefromfork('BAR')) {\n# parent\nwhile (<BAR>) { print; }\nclose BAR;\n}\nelse {\n# child\nprint \"pipefromfork\\n\";\nexit(0);\n}\n\nForking pipe open() constructs will be supported in future.\n\nGlobal state maintained by XSUBs\nExternal subroutines (XSUBs) that maintain their own global state may not work\ncorrectly.  Such XSUBs will either need to maintain locks to protect simultaneous\naccess to global data from different pseudo-processes, or maintain all their state on\nthe Perl symbol table, which is copied naturally when fork() is called.  A callback\nmechanism that provides extensions an opportunity to clone their state will be\nprovided in the near future.\n\nInterpreter embedded in larger application\nThe fork() emulation may not behave as expected when it is executed in an application\nwhich embeds a Perl interpreter and calls Perl APIs that can evaluate bits of Perl\ncode.  This stems from the fact that the emulation only has knowledge about the Perl\ninterpreter's own data structures and knows nothing about the containing\napplication's state.  For example, any state carried on the application's own call\nstack is out of reach.\n\nThread-safety of extensions\nSince the fork() emulation runs code in multiple threads, extensions calling into\nnon-thread-safe libraries may not work reliably when calling fork().  As Perl's\nthreading support gradually becomes more widely adopted even on platforms with a\nnative fork(), such extensions are expected to be fixed for thread-safety.\n",
            "subsections": []
        },
        "PORTABILITY CAVEATS": {
            "content": "In portable Perl code, \"kill(9, $child)\" must not be used on forked processes.  Killing a\nforked process is unsafe and has unpredictable results.  See \"kill()\", above.\n",
            "subsections": []
        },
        "BUGS": {
            "content": "•       Having pseudo-process IDs be negative integers breaks down for the integer \"-1\"\nbecause the wait() and waitpid() functions treat this number as being special.  The\ntacit assumption in the current implementation is that the system never allocates a\nthread ID of 1 for user threads.  A better representation for pseudo-process IDs will\nbe implemented in future.\n\n•       In certain cases, the OS-level handles created by the pipe(), socket(), and accept()\noperators are apparently not duplicated accurately in pseudo-processes.  This only\nhappens in some situations, but where it does happen, it may result in deadlocks\nbetween the read and write ends of pipe handles, or inability to send or receive data\nacross socket handles.\n\n•       This document may be incomplete in some respects.\n",
            "subsections": []
        },
        "AUTHOR": {
            "content": "Support for concurrent interpreters and the fork() emulation was implemented by ActiveState,\nwith funding from Microsoft Corporation.\n\nThis document is authored and maintained by Gurusamy Sarathy <gsar@activestate.com>.\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "\"fork\" in perlfunc, perlipc\n\n\n\nperl v5.34.0                                 2025-07-25                                  PERLFORK(1)",
            "subsections": []
        }
    },
    "summary": "perlfork - Perl's fork() emulation",
    "flags": [],
    "examples": [],
    "see_also": []
}