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[thirdparty/man-pages.git] / man2 / fork.2

.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\" A few fragments remain from an earlier (1992) page by
.\" Drew Eckhardt (drew@cs.colorado.edu),
.\"
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.\" Modified by Michael Haardt (michael@moria.de)
.\" Modified Sat Jul 24 13:22:07 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 21 Aug 1994 by Michael Chastain (mec@shell.portal.com):
.\"   Referenced 'clone(2)'.
.\" Modified 1995-06-10, 1996-04-18, 1999-11-01, 2000-12-24
.\"   by Andries Brouwer (aeb@cwi.nl)
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" 2006-09-04, Michael Kerrisk
.\"     Greatly expanded, to describe all attributes that differ
.\"	parent and child.
.\"
.TH FORK 2 2013-02-12 "Linux" "Linux Programmer's Manual"
.SH NAME
fork \- create a child process
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.B pid_t fork(void);
.SH DESCRIPTION
.BR fork ()
creates a new process by duplicating the calling process.
The new process, referred to as the \fIchild\fP,
is an exact duplicate of the calling process,
referred to as the \fIparent\fP, except for the following points:
.IP * 3
The child has its own unique process ID,
and this PID does not match the ID of any existing process group
.RB ( setpgid (2)).
.IP *
The child's parent process ID is the same as the parent's process ID.
.IP *
The child does not inherit its parent's memory locks
.RB ( mlock (2),
.BR mlockall (2)).
.IP *
Process resource utilizations
.RB ( getrusage (2))
and CPU time counters
.RB ( times (2))
are reset to zero in the child.
.IP *
The child's set of pending signals is initially empty
.RB ( sigpending (2)).
.IP *
The child does not inherit semaphore adjustments from its parent
.RB ( semop (2)).
.IP *
The child does not inherit record locks from its parent
.RB ( fcntl (2)).
.IP *
The child does not inherit timers from its parent
.RB ( setitimer (2),
.BR alarm (2),
.BR timer_create (2)).
.IP *
The child does not inherit outstanding asynchronous I/O operations
from its parent
.RB ( aio_read (3),
.BR aio_write (3)),
nor does it inherit any asynchronous I/O contexts from its parent (see
.BR io_setup (2)).
.PP
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:
.IP * 3
The child does not inherit directory change notifications (dnotify)
from its parent
(see the description of
.B F_NOTIFY
in
.BR fcntl (2)).
.IP *
The
.BR prctl (2)
.B PR_SET_PDEATHSIG
setting is reset so that the child does not receive a signal
when its parent terminates.
.IP *
The default timer slack value is set to the parent's
current timer slack value.
See the description of
.BR PR_SET_TIMERSLACK
in
.BR prctl (2).
.IP *
Memory mappings that have been marked with the
.BR madvise (2)
.B MADV_DONTFORK
flag are not inherited across a
.BR fork ().
.IP *
The termination signal of the child is always
.B SIGCHLD
(see
.BR clone (2)).
.PP
Note the following further points:
.IP * 3
The child process is created with a single thread\(emthe
one that called
.BR 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
.BR pthread_atfork (3)
may be helpful for dealing with problems that this can cause.
.IP *
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
.BR 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
.B F_SETOWN
and
.B F_SETSIG
in
.BR fcntl (2)).
.IP *
The child inherits copies of the parent's set of open message
queue descriptors (see
.BR 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
.RI ( mq_flags ).
.IP *
The child inherits copies of the parent's set of open directory streams (see
.BR opendir (3)).
POSIX.1-2001 says that the corresponding directory streams
in the parent and child
.I may
share the directory stream positioning;
on Linux/glibc they do not.
.SH 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
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EAGAIN
.BR fork ()
cannot allocate sufficient memory to copy the parent's page tables and
allocate a task structure for the child.
.TP
.B EAGAIN
It was not possible to create a new process because the caller's
.B RLIMIT_NPROC
resource limit was encountered.
To exceed this limit, the process must have either the
.B CAP_SYS_ADMIN
or the
.B CAP_SYS_RESOURCE
capability.
.TP
.B ENOMEM
.BR fork ()
failed to allocate the necessary kernel structures because memory is tight.
.TP
.B ENOSYS
.BR fork ()
is not supported on this platform (for example,
.\" e.g., arm (optionally), blackfin, c6x, frv, h8300, microblaze, xtensa
hardware without a Memory-Management Unit).
.SH CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
.PP
Under Linux,
.BR 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,
.\" nptl/sysdeps/unix/sysv/linux/fork.c
rather than invoking the kernel's
.BR fork ()
system call,
the glibc
.BR fork ()
wrapper that is provided as part of the
NPTL threading implementation invokes
.BR clone (2)
with flags that provide the same effect as the traditional system call.
(A call to
.BR fork ()
is equivalent to a call to
.BR clone (2)
specifying
.I flags
as just
.BR SIGCHLD .)
The glibc wrapper invokes any fork handlers that have been
established using
.BR pthread_atfork (3).
.\" and does some magic to ensure that getpid(2) returns the right value.
.SH EXAMPLE
See
.BR pipe (2)
and
.BR wait (2).
.SH SEE ALSO
.BR clone (2),
.BR execve (2),
.BR exit (2),
.BR setrlimit (2),
.BR unshare (2),
.BR vfork (2),
.BR wait (2),
.BR daemon (3),
.BR capabilities (7),
.BR credentials (7)