PTHREAD_JOIN(3P) POSIX Programmer's Manual PTHREAD_JOIN(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEpthread_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 ref‐
erenced 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 unde‐
fined. If the thread calling pthread_join() is canceled, then the tar‐
get 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; other‐
wise, 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 simu‐
late 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 termi‐
nate), 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 primi‐
tive, including pthread_join() in this volume of POSIX.1‐2008 was con‐
sidered valuable.
The pthread_join() function provides a simple mechanism allowing an
application to wait for a thread to terminate. After the thread termi‐
nates, 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 spec‐
ified, 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 cance‐
lability 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 recom‐
mended 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 ALSOpthread_create(), wait()
The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory Syn‐
chronization, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_JOIN(3P)
