PTHREAD_JOIN(3)            POSIX Programmer's Manual           PTHREAD_JOIN(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       pthread_join -- wait for thread termination

SYNOPSIS
       #include <pthread.h>

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

DESCRIPTION
       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}.

       The behavior is undefined if the value specified by the thread argument
       to pthread__join() does not refer to a joinable thread.

       The behavior is undefined if the value specified by the thread argument
       to pthread__join() refers to the calling thread.

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

ERRORS
       The pthread__join() function may fail if:

       EDEADLK
              A deadlock was detected.

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

       The following sections are informative.

EXAMPLES
       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;
           }

APPLICATION USAGE
       None.

RATIONALE
       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 POSIX.1-2017
       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.

       If an implementation detects that the value specified by the thread
       argument to pthread__join() does not refer to a joinable thread, it is
       recommended that the function should fail and report an [EINVAL] error.

       If an implementation detects that the value specified by the thread
       argument to pthread__join() refers to the calling thread, it is
       recommended that the function should fail and report an [EDEADLK]
       error.

       If an implementation detects use of a thread ID after the end of its
       lifetime, it is recommended that the function should fail and report an
       [ESRCH] error.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread__create(), wait()

       The Base Definitions volume of POSIX.1-2017, Section 4.12, Memory
       Synchronization, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                      PTHREAD_JOIN(3)
