[MAN] fork

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FORK

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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NAME

fork - create a child process  

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

pid_t fork(void);  

DESCRIPTION

fork() creates a new process by duplicating the calling process. The new process is referred to as the child process. The calling process is referred to as the parent process.

The child process and the parent process run in separate memory spaces. At the time of fork() both memory spaces have the same content. Memory writes, file mappings (mmap(2)), and unmappings (munmap(2)) performed by one of the processes do not affect the other.

The child process is an exact duplicate of the parent process except for the following points:

*

The child has its own unique process ID, and this PID does not match the ID of any existing process group (setpgid(2)) or session.

*

The child's parent process ID is the same as the parent's process ID.

*

The child does not inherit its parent's memory locks (mlock(2), mlockall(2)).

*

Process resource utilizations (getrusage(2)) and CPU time counters (times(2)) are reset to zero in the child.

*

The child's set of pending signals is initially empty (sigpending(2)).

*

The child does not inherit semaphore adjustments from its parent (semop(2)).

*

The child does not inherit process-associated record locks from its parent (fcntl(2)). (On the other hand, it does inherit fcntl(2) open file description locks and flock(2) locks from its parent.)

*

The child does not inherit timers from its parent (setitimer(2), alarm(2), timer_create(2)).

*

The child does not inherit outstanding asynchronous I/O operations from its parent (aio_read(3), aio_write(3)), nor does it inherit any asynchronous I/O contexts from its parent (see io_setup(2)).

The process attributes in the preceding list are all specified in POSIX.1. The parent and child also differ with respect to the following Linux-specific process attributes:

*

The child does not inherit directory change notifications (dnotify) from its parent (see the description of F_NOTIFY in fcntl(2)).

*

The prctl(2) PR_SET_PDEATHSIG setting is reset so that the child does not receive a signal when its parent terminates.

*

The default timer slack value is set to the parent's current timer slack value. See the description of PR_SET_TIMERSLACK in prctl(2).

*

Memory mappings that have been marked with the madvise(2) MADV_DONTFORK flag are not inherited across a fork().

*

Memory in address ranges that have been marked with the madvise(2) MADV_WIPEONFORK flag is zeroed in the child after a fork(). (The MADV_WIPEONFORK setting remains in place for those address ranges in the child.)

*

The termination signal of the child is always SIGCHLD (see clone(2)).

*

The port access permission bits set by ioperm(2) are not inherited by the child; the child must turn on any bits that it requires using ioperm(2).

Note the following further points:

*

The child process is created with a single thread---the one that called fork(). The entire virtual address space of the parent is replicated in the child, including the states of mutexes, condition variables, and other pthreads objects; the use of pthread_atfork(3) may be helpful for dealing with problems that this can cause.

*

After a fork() in a multithreaded program, the child can safely call only async-signal-safe functions (see signal-safety(7)) until such time as it calls execve(2).

*

The child inherits copies of the parent's set of open file descriptors. Each file descriptor in the child refers to the same open file description (see open(2)) as the corresponding file descriptor in the parent. This means that the two file descriptors share open file status flags, file offset, and signal-driven I/O attributes (see the description of F_SETOWN and F_SETSIG in fcntl(2)).

*

The child inherits copies of the parent's set of open message queue descriptors (see mq_overview(7)). Each file descriptor in the child refers to the same open message queue description as the corresponding file descriptor in the parent. This means that the two file descriptors share the same flags (mq_flags).

*

The child inherits copies of the parent's set of open directory streams (see opendir(3)). POSIX.1 says that the corresponding directory streams in the parent and child may share the directory stream positioning; on Linux/glibc they do not.

 

RETURN VALUE

On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately.  

ERRORS

EAGAIN

A system-imposed limit on the number of threads was encountered. There are a number of limits that may trigger this error:

*

the RLIMIT_NPROC soft resource limit (set via setrlimit(2)), which limits the number of processes and threads for a real user ID, was reached;

*

the kernel's system-wide limit on the number of processes and threads, /proc/sys/kernel/threads-max, was reached (see proc(5));

*

the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached (see proc(5)); or

*

the PID limit (pids.max) imposed by the cgroup "process number" (PIDs) controller was reached.

EAGAIN

The caller is operating under the SCHED_DEADLINE scheduling policy and does not have the reset-on-fork flag set. See sched(7).

ENOMEM

fork() failed to allocate the necessary kernel structures because memory is tight.

ENOMEM

An attempt was made to create a child process in a PID namespace whose "init" process has terminated. See pid_namespaces(7).

ENOSYS

fork() is not supported on this platform (for example, hardware without a Memory-Management Unit).

ERESTARTNOINTR (since Linux 2.6.17)

System call was interrupted by a signal and will be restarted. (This can be seen only during a trace.)

 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  

NOTES

Under Linux, fork() is implemented using copy-on-write pages, so the only penalty that it incurs is the time and memory required to duplicate the parent's page tables, and to create a unique task structure for the child.  

C library/kernel differences

Since version 2.3.3, rather than invoking the kernel's fork() system call, the glibc fork() wrapper that is provided as part of the NPTL threading implementation invokes clone(2) with flags that provide the same effect as the traditional system call. (A call to fork() is equivalent to a call to clone(2) specifying flags as just SIGCHLD.) The glibc wrapper invokes any fork handlers that have been established using pthread_atfork(3).  

EXAMPLE

See pipe(2) and wait(2).  

SEE ALSO

clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3), pthread_atfork(3), capabilities(7), credentials(7)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

CONFORMING TO

NOTES

C library/kernel differences

EXAMPLE

SEE ALSO

COLOPHON

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FORK

Section: GNU Awk Extension Modules (3am)
Updated: Jan 15 2013
Index Return to Main Contents

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NAME

fork, wait, waitpid - basic process management  

SYNOPSIS

@load "fork"

pid = fork()

ret = waitpid(pid)

ret = wait();  

DESCRIPTION

The fork extension adds three functions, as follows.

fork()

This function creates a new process. The return value is the zero in the child and the process-id number of the child in the parent, or -1 upon error. In the latter case, ERRNO indicates the problem. In the child, PROCINFO["pid"] and PROCINFO["ppid"] are updated to reflect the correct values.

waitpid()

This function takes a numeric argument, which is the process-id to wait for. The return value is that of the waitpid(2) system call.

wait()

This function waits for the first child to die. The return value is that of the wait(2) system call.

 

BUGS

There is no corresponding exec() function.

The interfaces could be enhanced to provide more facilities, including pulling out the various bits of the return status.  

EXAMPLE

@load "fork"
...
if ((pid = fork()) == 0)
    print "hello from the child"
else
    print "hello from the parent"

 

SEE ALSO

GAWK: Effective AWK Programming, filefuncs(3am), fnmatch(3am), inplace(3am), ordchr(3am), readdir(3am), readfile(3am), revoutput(3am), rwarray(3am), time(3am).

fork(2), wait(2), waitpid(2).  

AUTHOR

Arnold Robbins, arnold@skeeve.com.  

COPYING PERMISSIONS

Copyright © 2012, 2013, Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual page into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

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Index

NAME

SYNOPSIS

DESCRIPTION

BUGS

EXAMPLE

SEE ALSO

AUTHOR

COPYING PERMISSIONS

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This document was created by man2html, using the manual pages.
Time: 04:45:44 GMT, September 16, 2022