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Dd July 18, 2014
Dt PTHREAD_ATFORK 3
Os
Sh NAME
Nm pthread_atfork
Nd register handlers to be called when process forks
Sh LIBRARY
Lb libc
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 registers the provided handler functions to be called when the
Xr fork 2
function is called.
Each of the three handlers is called at a different place in the
Xr fork 2
sequence.
The
Ar prepare
handler is called in the parent process before the fork happens, the
Ar parent
handler is called in the parent process after the fork has happened, and the
Ar child
handler is called in the child process after the fork has happened.
The
Ar parent
and
Ar child
handlers are called in the order in which they were registered, while the
Ar prepare
handlers are called in reverse of the order in which they were registered.
Pp
Any of the handlers given may be
Dv NULL .
Pp
The intended use of
Fn pthread_atfork
is to provide a consistent state to a child process from a multithreaded parent
process where locks may be acquired and released asynchronously with respect to the
Xr fork 2
call.
Each subsystem with locks that are used in a child process should register
handlers with
Fn pthread_atfork
that acquires those locks in the
Ar prepare
handler and releases them in the
Ar parent
handler.
Sh RETURN VALUES
The
Fn pthread_atfork
function returns 0 on success and an error number on failure.
Sh ERRORS
The following error code may be returned:
Bl -tag -width Er
It Bq Er ENOMEM
Insufficient memory exists to register the fork handlers.
El
Sh SEE ALSO
Xr fork 2 ,
Xr pthread_mutex 3 ,
Xr signal 7
Sh STANDARDS
The
Fn pthread_atfork
function conforms to
St -p1003.1c-95 .
Sh HISTORY
The
Fn pthread_atfork
function first appeared in
Nx 2.0 .
Sh CAVEATS
After calling
Xr fork 2
from a multithreaded process, it is only safe to call
async-signal-safe functions until calling one of the
Xr exec 3
functions.
The
Fn pthread_*
functions are not async-signal-safe, so it is not safe to use such functions
in the
Ar child
handler.
POSIX does not mandate that
Fn pthread_mutex_unlock
be async-signal-safe, but it is in
Nx
and thus safe to use within the
Ar child
handler.
Sh BUGS
There is no way to unregister a handler registered with
Fn pthread_atfork .
