.\" Copyright (C) 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pthread_atfork 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
pthread_atfork \- register fork handlers
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B  #include <pthread.h>
.PP
.BI "int pthread_atfork(void (*" prepare ")(void), void (*" parent ")(void),"
.BI "                   void (*" child ")(void));"
.fi
.SH DESCRIPTION
The
.BR pthread_atfork ()
function registers fork handlers that are to be executed when
.BR fork (2)
is called by any thread in a process.
The handlers are executed in the context of the thread that calls
.BR fork (2).
.PP
Three kinds of handler can be registered:
.IP \[bu] 3
.I prepare
specifies a handler that is executed in the parent process before
.BR fork (2)
processing starts.
.IP \[bu]
.I parent
specifies a handler that is executed in the parent process after
.BR fork (2)
processing completes.
.IP \[bu]
.I child
specifies a handler that is executed in the child process after
.BR fork (2)
processing completes.
.PP
Any of the three arguments may be NULL if no handler is needed
in the corresponding phase of
.BR fork (2)
processing.
.SH RETURN VALUE
On success,
.BR pthread_atfork ()
returns zero.
On error, it returns an error number.
.BR pthread_atfork ()
may be called multiple times by a process
to register additional handlers.
The handlers for each phase are called in a specified order: the
.I prepare
handlers are called in reverse order of registration; the
.I parent
and
.I child
handlers are called in the order of registration.
.SH ERRORS
.TP
.B ENOMEM
Could not allocate memory to record the fork handler list entry.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH NOTES
When
.BR fork (2)
is called in a multithreaded process,
only the calling thread is duplicated in the child process.
The original intention of
.BR pthread_atfork ()
was to allow the child process to be returned to a consistent state.
For example, at the time of the call to
.BR fork (2),
other threads may have locked mutexes that are visible in the
user-space memory duplicated in the child.
Such mutexes would never be unlocked,
since the threads that placed the locks are not duplicated in the child.
The intent of
.BR pthread_atfork ()
was to provide a mechanism whereby the application (or a library)
could ensure that mutexes and other process and thread state would be
restored to a consistent state.
In practice, this task is generally too difficult to be practicable.
.PP
After a
.BR fork (2)
in a multithreaded process returns in the child,
the child should call only async-signal-safe functions (see
.BR signal\-safety (7))
until such time as it calls
.BR execve (2)
to execute a new program.
.PP
POSIX.1 specifies that
.BR pthread_atfork ()
shall not fail with the error
.BR EINTR .
.SH SEE ALSO
.BR fork (2),
.BR atexit (3),
.BR pthreads (7)