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.\" $OpenBSD: pthread_atfork.3,v 1.12 2015/05/15 18:47:53 jmc Exp $
.\"
.\" David Leonard <d@openbsd.org>, 1999. Public domain.
.\"
.Dd $Mdocdate: May 15 2015 $
.Dt PTHREAD_ATFORK 3
.Os
.Sh NAME
.Nm pthread_atfork
.Nd specify handler functions to call when the process forks
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_atfork "void (*prepare)(void)" "void (*parent)(void)" "void (*child)(void)"
.Sh DESCRIPTION
The
.Fn pthread_atfork
function declares fork handlers to be called before and after
.Xr fork 2 ,
in the context of the thread that called
.Xr fork 2 .
The
.Fa prepare
fork handler will be called before
.Xr fork 2
processing commences.
The
.Fa parent
fork handler will be called after
.Xr fork 2
processing completes in the parent process.
The
.Fa child
fork handler will be called after
.Xr fork 2
processing completes in the child process.
If no handling is desired at
one or more of these three points,
the corresponding fork handler
address(es) may be set to
.Dv NULL .
.Pp
The order of calls to
.Fn pthread_atfork
is significant.
The
.Fa parent
and
.Fa child
fork handlers will be called in the order in which they were established
by calls to
.Fn pthread_atfork .
The
.Fa prepare
fork handlers will be called in the opposite order.
.Pp
If a shared object is unloaded from process memory using
.Xr dlclose 3 ,
then any functions registered by calling
.Fn pthread_atfork
from that shared object will be unregistered without being invoked.
Note that it is the source of the call to
.Fn pthread_atfork
that matters, not the source of the functions that were registered.
.Sh RETURN VALUES
Upon successful completion,
.Fn pthread_atfork
will return a value of zero.
Otherwise, an error number will be
returned to indicate the error.
.Sh ERRORS
.Fn pthread_atfork
will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
Insufficient table space exists to record the fork handler addresses.
None of the handler lists are modified.
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr atexit 3 ,
.Xr dlclose 3
.Sh STANDARDS
.Fn pthread_atfork
conforms to
.St -p1003.1-2004 .
.Pp
The behavior when a shared object is unloaded is an extension to
that standard.
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