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. 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	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

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

SunOS 5.11							    12 Dec 2003 						pthread_atfork(3C)