'\" t
.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pthread_join 3 2023-03-30 "Linux man-pages 6.04"
.SH NAME
pthread_join \- join with a terminated thread
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.PP
.BI "int pthread_join(pthread_t " thread ", void **" retval );
.fi
.SH DESCRIPTION
The
.BR pthread_join ()
function waits for the thread specified by
.I thread
to terminate.
If that thread has already terminated, then
.BR pthread_join ()
returns immediately.
The thread specified by
.I thread
must be joinable.
.PP
If
.I retval
is not NULL, then
.BR pthread_join ()
copies the exit status of the target thread
(i.e., the value that the target thread supplied to
.BR pthread_exit (3))
into the location pointed to by
.IR retval .
If the target thread was canceled, then
.B PTHREAD_CANCELED
is placed in the location pointed to by
.IR retval .
.PP
If multiple threads simultaneously try to join with the same thread,
the results are undefined.
If the thread calling
.BR pthread_join ()
is canceled, then the target thread will remain joinable
(i.e., it will not be detached).
.SH RETURN VALUE
On success,
.BR pthread_join ()
returns 0;
on error, it returns an error number.
.SH ERRORS
.TP
.B EDEADLK
A deadlock was detected
.\" The following verified by testing on glibc 2.8/NPTL:
(e.g., two threads tried to join with each other);
or
.\" The following verified by testing on glibc 2.8/NPTL:
.I thread
specifies the calling thread.
.TP
.B EINVAL
.I thread
is not a joinable thread.
.TP
.B EINVAL
Another thread is already waiting to join with this thread.
.\" POSIX.1-2001 does not specify this error case.
.TP
.B ESRCH
No thread with the ID
.I thread
could be found.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR pthread_join ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SH NOTES
After a successful call to
.BR pthread_join (),
the caller is guaranteed that the target thread has terminated.
The caller may then choose to do any clean-up that is required
after termination of the thread (e.g., freeing memory or other
resources that were allocated to the target thread).
.PP
Joining with a thread that has previously been joined results in
undefined behavior.
.PP
Failure to join with a thread that is joinable
(i.e., one that is not detached),
produces a "zombie thread".
Avoid doing this,
since each zombie thread consumes some system resources,
and when enough zombie threads have accumulated,
it will no longer be possible to create new threads (or processes).
.PP
There is no pthreads analog of
.IR "waitpid(\-1,\ &status,\ 0)" ,
that is, "join with any terminated thread".
If you believe you need this functionality,
you probably need to rethink your application design.
.PP
All of the threads in a process are peers:
any thread can join with any other thread in the process.
.SH EXAMPLES
See
.BR pthread_create (3).
.SH SEE ALSO
.BR pthread_cancel (3),
.BR pthread_create (3),
.BR pthread_detach (3),
.BR pthread_exit (3),
.BR pthread_tryjoin_np (3),
.BR pthreads (7)