POSIX Thread Tutorial

NULL for this argument. The third argument is the name of the function in which the thread should start -- this function is expected to return a void *. The last argument is passed untouched to the start routine. In this case, we just pass the thread's number (loop count) to the start routine. In more complicated situations, you can pass a pointer to a block of thread-specific data. pthread_create() returns 0 on success and an error number on failure. If it fails, we print a message with perror() and exit (line 58). As a general rule, you should always check for system and library call errors. When programming with pthreads, it is suicidal not to check for error conditions. Henry Spencer's Sixth Commandment for C programmers sums it up nicely:

If a function be advertised to return an error code in the event of difficulties, thou shalt check for that

 code, yea, even though the checks triple the size of thy code and produce aches in thy typing fingers, for

 if thou thinkest "it cannot happen to me," the gods shall surely punish thee for thy arrogance. 

The main thread next waits for each of the workers to finish in lines 62-72. It calls pthread_join() to wait for the thread to terminate. The thread we're waiting on is specified in the first argument and its exit code is placed into the second argument (declared on line 49 as type int *). We check the exit code and issue a message (and exit) if it's not what we expect.

The code that each worker thread executes occupies lines 34-40. First, we cast the argument to the start routine to an int (since we passed a pointer to an int to pthread_create()). The thread then prints its message and returns its thread ID. At this point, the thread terminates and is reaped when the master thread calls pthread_join(). You can also have a thread terminate itself with the pthread_exit() call.

The call pthread_join() causes the calling thread to go to sleep until the specified thread terminates. In the examples given in this document, the master thread cannot proceed until all the workers complete their tasks. Once a thread has been joined, its system resources are reclaimed. If you need to terminate a running thread, use pthread_cancel() and pthread_testcancel().

Once you join a thread, you cannot reassign it a new task. The reason is that once a thread has been joined, it has (by definition) terminated. If you need more threads to perform another task, you have to call pthread_create() again.

Aside from returning a thread's exit status, the major function of pthread_join() is to reclaim the system resources used by the terminated thread. If you forget to call pthread_join() in a program that creates many threads, you will run into problems sooner or later (like what happens when you call malloc() many times and never call free()).

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