man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

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. Seefork(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

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

See Also

exec(2), fork(2), atexit(3C), attributes(5), standards(5)