fork — create a child process
#include <unistd.h>
pid_t
fork( |
void); |
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)).
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().
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(2) in a multithreaded program, the child can safely call only async-signal-safe functions (see signal(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.
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.
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)); or the
maximum number of PIDs, /proc/sys/kernel/pid_max, was reached
(see proc(5)).
The caller is operating under the SCHED_DEADLINE scheduling policy and
does not have the reset-on-fork flag set. See sched(7).
fork() failed to
allocate the necessary kernel structures because memory
is tight.
An attempt was made to create a child process in a PID namespace whose "init" process has terminated. See pid_namespaces(7).
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.)
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.
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).
clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3), capabilities(7), credentials(7)
This page is part of release 4.07 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/.
|
Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> A few fragments remain from an earlier (1992) page by Drew Eckhardt (drewcs.colorado.edu), %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual 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 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. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified by Michael Haardt (michaelmoria.de) Modified Sat Jul 24 13:22:07 1993 by Rik Faith (faithcs.unc.edu) Modified 21 Aug 1994 by Michael Chastain (mecshell.portal.com): Referenced 'clone(2)'. Modified 1995-06-10, 1996-04-18, 1999-11-01, 2000-12-24 by Andries Brouwer (aebcwi.nl) Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements 2006-09-04, Michael Kerrisk Greatly expanded, to describe all attributes that differ parent and child. |