Is there a way to ‘join’ (block) in POSIX threads, without exiting the joinee?

Use a pthread_cond_t wait on it on one thread and signal or broadcast it in the other.

Sounds like you actually want an implementation of the Thread Pool Pattern. It makes for a fairly simple conceptual model, without repeated thread creation & tear down costs. Some OS's directly support it, on others it should be reasonably simple to implement using a queue and a semaphore.

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