05-30-2001
fork() and IPC
Hey all. I recently ported a multithreaded program from WinNT to LINUX using pthreads. Everything works fine, except that it turns out the drivers for the hardware I'm working with aren't thread safe. So, now I have to find a way to make this multi-process instead of multi-threaded.
I just started working with fork(), and am having a little trouble following the interprocess communication models. Is there an "easy" way to allow a child process(es) to have access to the global variables in the parent process? This is the way the original programmer did it under NT multithreaded, and I'd rather not rewrite the entire app (it's big).
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LEARN ABOUT SUNOS
pthread_atfork
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: mMake 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 5.10 12 Dec 2003 pthread_atfork(3C)