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, referred
to as the child, is
an exact duplicate of the calling process, referred to as the
parent, 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 record locks from its parent (fcntl(2)).
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 (seeio_setup(2)).
The process attributes in the preceding list are all specified in POSIX.1-2001. 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.
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)).
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.
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 descriptors share open file status flags,
current 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
descriptor in the child refers to the same open message
queue description as the corresponding descriptor in
the parent. This means that the two descriptors share
the same flags (mq_flags).
The child inherits copies of the parent's set of
open directory streams (see opendir(3)).
POSIX.1-2001 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.
fork() cannot allocate
sufficient memory to copy the parent's page tables and
allocate a task structure for the child.
It was not possible to create a new process because
the caller's RLIMIT_NPROC
resource limit was encountered. To exceed this limit,
the process must have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE capability.
fork() failed to
allocate the necessary kernel structures because memory
is tight.
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. The
glibc wrapper invokes any fork handlers that have been
established using pthread_atfork(3).
clone(2), execve(2), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3), capabilities(7), credentials(7)
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> A few fragments remain from an earlier (1992) page by Drew Eckhardt (drewcs.colorado.edu), 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. 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. |