Why one can't do multiple pthread_joins on the same thread?

Question

From https://computing.llnl.gov/tutorials/pthreads/:

A joining thread can match one pthread_join() call. It is a logical error to attempt multiple joins on the same thread.

Also from "man pthread_join":

If multiple threads simultaneously try to join with the same thread, the results are undefined.

However, from programmer's point of view, it makes a perfect sense that multiple threads may want to wait for a single thread to complete (similar to barriers).

For example, we may have thread1, thread2 running independently, and we may want both threads to wait until thread3 is completed.

Is there any technical reason behind this restriction?

Answer 1

I believe the technical reason is that pthread_join is a POSIX standard which, for multithreading, tries to specify only necessary primitives for implementers. Any richer semantics would introduce a costlier implementation and perhaps a more difficult API.

Indeed, POSIX already considers this function to be a convenience, rather than a primitive, in support of a very, very common use case: one thread waits for the termination of one other.

The first paragraph of the POSIX.1-2008 pthread_join RATIONALE is a bit lengthy, but makes many germane observations:

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.