pthread_join and pthread_detach - glibc

Split from the chapter glibc: Process, Threads and Signals into the per-function layout; see the method page and the glibc ledger for coverage status.

Semantics – pthread_join(thread, &retval) waits for a joinable thread and fetches its exit value (PTHREAD_CANCELED = (void *) -1 if it was canceled). pthread_detach(thread) makes termination self-cleaning; every thread must be joined or detached exactly once or it lingers as a “zombie thread” – and there is deliberately no waitpid(-1)-style join-any. All threads are peers: any thread can join any other.

Return and errors – 0 or error number (not errno): join – EDEADLK (self-join or detected cycle), EINVAL (not joinable or already being joined), ESRCH; detach – EINVAL, ESRCH. Joining a detached or already-joined thread is undefined.

glibc layer – GNU extensions relax the blocking: pthread_tryjoin_np returns EBUSY immediately, pthread_timedjoin_np adds an absolute CLOCK_REALTIME deadline returning ETIMEDOUT, and pthread_clockjoin_np (2.30) takes an explicit clockid (CLOCK_MONOTONIC or CLOCK_REALTIME). Note the glibc manual’s Waiting with Explicit Clocks text says these “return EBUSY” on timeout – the man page (pthread_tryjoin_np(3)) documents ETIMEDOUT and that is what the implementation returns; the manual wording is a defect.

References – pthread_join(3), pthread_detach(3), pthread_tryjoin_np(3).