#include <pthread.h> int pthread_join(pthread_t thread, void **retval);
Compile and link with -pthread.
If retval is not NULL, then pthread_join() copies the exit status of the target thread (i.e., the value that the target thread supplied to pthread_exit(3)) into the location pointed to by *retval. If the target thread was canceled, then PTHREAD_CANCELED is placed in *retval.
If multiple threads simultaneously try to join with the same thread, the results are undefined. If the thread calling pthread_join() is canceled, then the target thread will remain joinable (i.e., it will not be detached).
Joining with a thread that has previously been joined results in undefined behavior.
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).
There is no pthreads analog of 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.
All of the threads in a process are peers: any thread can join with any other thread in the process.