Tribblix: manual page: pthread

PTHREAD_ATFORK(3C) Standard C Library Functions PTHREAD_ATFORK(3C)

NAME

pthread_atfork - register fork handlers

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

int pthread_atfork(void (*prepare) (void), void (*parent) (void),
void (*child) (void));

DESCRIPTION

The pthread_atfork() function declares fork handlers to be called
prior to and following fork(2), within the thread that called fork().
The order of calls to pthread_atfork() is significant.

Before fork() processing begins, the prepare fork handler is called.
The prepare handler is not called if its address is NULL.

The parent fork handler is called after fork() processing finishes in
the parent process, and the child fork handler is called after fork()
processing finishes in the child process. If the address of parent or
child is NULL, then its handler is not called.

The prepare fork handler is called in LIFO (last-in first-out)
order, whereas the parent and child fork handlers are called in FIFO
(first-in first-out) order. This calling order allows applications to
preserve locking order.

RETURN VALUES

Upon successful completion, pthread_atfork() returns 0. Otherwise, an
error number is returned.

ERRORS

The pthread_atfork() function will fail if:

ENOMEM
Insufficient table space exists to record the fork handler
addresses.

USAGE

Solaris threads do not offer pthread_atfork() functionality (there is
no thr_atfork() interface). However, a Solaris threads application
can call pthread_atfork() to ensure fork()-safety, since the two
thread APIs are interoperable. See fork(2) for information relating
to fork() in a Solaris threads environment in Solaris 10 relative to
previous releases.

EXAMPLES

Example 1: Make a library safe with respect to fork().

All multithreaded applications that call fork() in a POSIX threads
program and do more than simply call exec(2) in the child of the fork
need to ensure that the child is protected from deadlock.

Since the "fork-one" model results in duplicating only the thread
that called fork(), it is possible that at the time of the call
another thread in the parent owns a lock. This thread is not
duplicated in the child, so no thread will unlock this lock in the
child. Deadlock occurs if the single thread in the child needs this
lock.

The problem is more serious with locks in libraries. Since a library
writer does not know if the application using the library calls
fork(), the library must protect itself from such a deadlock
scenario. If the application that links with this library calls
fork() and does not call exec() in the child, and if it needs a
library lock that may be held by some other thread in the parent that
is inside the library at the time of the fork, the application
deadlocks inside the library.

The following describes how to make a library safe with respect to
fork() by using pthread_atfork().

1. Identify all locks used by the library (for example
{L1,...Ln}). Identify also the locking order for these
locks (for example {L1...Ln}, as well.)

2. Add a call to pthread_atfork(f1, f2, f3) in the library's
.init section. f1, f2, f3 are defined as follows:

f1()
{
/* ordered in lock order */
pthread_mutex_lock(L1);
pthread_mutex_lock(...);
pthread_mutex_lock(Ln);
}

f2()
{
pthread_mutex_unlock(L1);
pthread_mutex_unlock(...);
pthread_mutex_unlock(Ln);
}

f3()
{
pthread_mutex_unlock(L1);
pthread_mutex_unlock(...);
pthread_mutex_unlock(Ln);
}

ATTRIBUTES