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Editorial Team
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Asked: 2026-05-20T10:36:41+00:00

Suppose some condition variable cond is associated with a mutex variable mutex. If a

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Suppose some condition variable “cond” is associated with a mutex variable “mutex”. If a thread is sleeping on cond after calling pthread_cond_wait(&cond,&mutex), and another thread that has mutex locked is finished, does it matter whether that thread calls pthread_cond_signal(&cond) before or after calling pthread_mutex_unlock(&mutex) ? Does it even need to unlock the mutex at all if it calls pthread_cond_signal(&cond), since the sleeping thread will acquire the mutex anyway?

EDIT: According to https://computing.llnl.gov/tutorials/pthreads/#ConVarOverview, “Failing to unlock the mutex after calling pthread_cond_signal() may not allow a matching pthread_cond_wait() routine to complete (it will remain blocked).” I guess then, unlocking, and perhaps only afterwards, is required.

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1 Answer

  1. Editorial Team

    You should always unlock the mutex after calling pthread_cond_signal. Here are some good questions/answers to read:

    Calling pthread_cond_signal without locking mutex

    It won’t come to me right now, but I’m pretty sure there’s a good reason (in terms of race conditions) that you don’t want to unlock the mutex before signalling.

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