Hang when shutting down blocking concurrent queue via interrupt

When attempting to interrupt a blocking concurrent queue that is waiting on a time out, I'm experiencing a hang (until the timeout) in this line

        do {
#ifdef MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC
          rc = sem_clockwait(&m_sema, CLOCK_MONOTONIC, &ts);
#else
          rc = sem_timedwait(&m_sema, &ts);
#endif
        } while (rc == -1 && errno == EINTR);
        return rc == 0;

If I change it to respect the interrupt, the timeout doesn't work. However, I'm unable to ctrl-c in my program and shut it down cleanly. What can I do here short of implementing some sort of poll and a smaller timeout?

Also, out of curiosity, is there a benefit to using MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC?

That would be due to the while (errno == EINTR) loop. Signals for all sorts of (non-ctrl+c) reasons (including from I/O and debuggers) will interrupt the sem_timedwait, and this needs to be handled gracefully.

Redesign your application to enqueue something on SIGINT in order to unblock the waiting thread, which can then exit gracefully.

Yes, sem_clockwait is superior to sem_timedwait as it allows a monotonic clock to be used. This is only available in later versions of glibc. When compiling against such a version, MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC is automatically enabled.

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Hang when shutting down blocking concurrent queue via interrupt #411

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Hang when shutting down blocking concurrent queue via interrupt #411

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cameron314 commented on Mar 30, 2025

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