pthread_join - wait for thread termination

#include <pthread.h>

int pthread_join(pthread_t thread, void **value_ptr);

The pthread_join() function shall suspend execution of the calling thread until the target thread terminates, unless the target thread has already terminated. On return from a successful pthread_join() call with a non-NULL value_ptr argument, the value passed to pthread_exit() by the terminating thread shall be made available in the location referenced by value_ptr. When a pthread_join() returns successfully, the target thread has been terminated. The results of multiple simultaneous calls to pthread_join() specifying the same target thread are undefined. If the thread calling pthread_join() is canceled, then the target thread shall not be detached.

It is unspecified whether a thread that has exited but remains unjoined counts against {PTHREAD_THREADS_MAX}.

If successful, the pthread_join() function shall return zero; otherwise, an error number shall be returned to indicate the error.

The pthread_join() function shall fail if:

EINVAL

The implementation has detected that the value specified by thread does not refer to a joinable thread.

ESRCH

No thread could be found corresponding to that specified by the given thread ID.

The pthread_join() function may fail if:

EDEADLK

A deadlock was detected or the value of thread specifies the calling thread.

The pthread_join() function shall not return an error code of [EINTR].

The following sections are informative.

An example of thread creation and deletion follows:

typedef struct { int *ar; long n; } subarray;

void * incer(void *arg) { long i;

for (i = 0; i < ((subarray *)arg)->n; i++) ((subarray *)arg)->ar[i]++; }

int main(void) { int ar[1000000]; pthread_t th1, th2; subarray sb1, sb2;

sb1.ar = &ar[0]; sb1.n = 500000; (void) pthread_create(&th1, NULL, incer, &sb1);

sb2.ar = &ar[500000]; sb2.n = 500000; (void) pthread_create(&th2, NULL, incer, &sb2);

(void) pthread_join(th1, NULL); (void) pthread_join(th2, NULL); return 0; }

None.

The pthread_join() function is a convenience that has proven useful in multi-threaded applications. It is true that a programmer could simulate this function if it were not provided by passing extra state as part of the argument to the start_routine(). The terminating thread would set a flag to indicate termination and broadcast a condition that is part of that state; a joining thread would wait on that condition variable. While such a technique would allow a thread to wait on more complex conditions (for example, waiting for multiple threads to terminate), waiting on individual thread termination is considered widely useful. Also, including the pthread_join() function in no way precludes a programmer from coding such complex waits. Thus, while not a primitive, including pthread_join() in this volume of IEEE Std 1003.1-2001 was considered valuable.

The pthread_join() function provides a simple mechanism allowing an application to wait for a thread to terminate. After the thread terminates, the application may then choose to clean up resources that were used by the thread. For instance, after pthread_join() returns, any application-provided stack storage could be reclaimed.

The pthread_join() or pthread_detach() function should eventually be called for every thread that is created with the detachstate attribute set to PTHREAD_CREATE_JOINABLE so that storage associated with the thread may be reclaimed.

The interaction between pthread_join() and cancellation is well-defined for the following reasons:

*

The pthread_join() function, like all other non-async-cancel-safe functions, can only be called with deferred cancelability type.

*

Cancellation cannot occur in the disabled cancelability state.

Thus, only the default cancelability state need be considered. As specified, either the pthread_join() call is canceled, or it succeeds, but not both. The difference is obvious to the application, since either a cancellation handler is run or pthread_join() returns. There are no race conditions since pthread_join() was called in the deferred cancelability state.

None.

pthread_create() , wait() , the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .