bionic (3) pthread_join.3posix.gz
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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.
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.1‐2008 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.1‐2008, Section 4.11, Memory Synchronization, <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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .