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| Threads Library Functions | pthread_atfork(3THR) |
| | pthread_atfork - register fork handlers |
SYNOPSIS
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cc -mt [ flag... ] file... -lpthread [ -lrt library... ]
#include <sys/types.h>
#include <unistd.h> int pthread_atfork(void (*prepare) (void), void (*parent) (void), void (*child) (void)); |
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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.
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Upon successful completion, pthread_atfork() returns 0. Otherwise, an error number is returned.
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The pthread_atfork() function will fail if:
-
ENOMEM
- Insufficient table space exists to record the fork handler addresses.
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Solaris threads do not offer pthread_atfork() functionality, though a Solaris threads application can call this interface to ensure fork1()-safety, since the two thread APIs are interoperable. If the application is linked with -lpthread, fork() is defined to be the same as fork1(); otherwise fork() and fork1() behave differently. The pthread_atfork() function affects only fork1(). See fork(2).
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| | Example 1. make a library safe with respect to fork
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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().
- 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.)
- Add a call to pthread_atfork(f1, f2, f3) in the library's .init section. f1, f2, f3 are defined as follows:
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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);
}
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See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | MT-Safe |
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