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

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

I’m buried in multithreading / parallelism documents, trying to figure out how to implement a threading implementation in a programming language I’ve been designing.

I’m trying to map a mental model to the pthreads.h library, but I’m having trouble with one thing: I need my interpreter instances to continue to exist after they complete interpretation of a routine (the language’s closure/function data type), because I want to later assign other routines to them for interpretation, thus saving me the thread and interpreter setup/teardown time.

This would be fine, except that pthread_join(3) requires that I call pthread_exit(3) to ‘unblock’ the original thread. How can I block the original thread (when it needs the result of executing the routine), and then unblock it when interpretation of the child routine is complete?

+1 A:

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

rpetrich 2010-05-01 23:43:40

Just a semaphore might be sufficient in this case - init with value 0, wait on it in one thread and post it in the other. Same principle, though.

Steve Jessop 2010-05-01 23:46:30

When you say ‘wait on it,’ do you mean simply a tight loop against it in the original thread? That’s … well, I was trying to avoid such a hacky/lame solution. I want to actually freeze the thread using the `pthread.h` library if I can, so that it doesn’t even get scheduled: that should save a ton of execution time, no?

elliottcable 2010-05-01 23:50:13

@elliotcable: I was referring to pthread_cond_wait: http://opengroup.org/onlinepubs/007908799/xsh/pthread_cond_wait.html

rpetrich 2010-05-02 00:18:19

@Steve Jessop: semaphores would likely work just as well and are simpler, but aren't technically part of pthreads though :)

rpetrich 2010-05-02 00:19:58

Yeah, are they "not part of pthreads" in a troublesome way, or just in the sense that they're in a different header? Is it legal for a POSIX implementation to provide pthreads, but not provide semaphores?

Steve Jessop 2010-05-02 00:49:05

@Steve Jessop: I can't imagine any POSIX implementation that would provide pthreads and not semaphores. Consider my earlier reply overly pedantic :)

rpetrich 2010-05-02 08:42:52

+1 A:

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.

torak 2010-05-02 00:08:01

Yes, pretty much. I’m just looking for the proper `pthread.h` calls, or whatever else I need to investigate, to create this.

elliottcable 2010-05-02 00:46:53

ansaurus

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

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