当pthread_cond_wait被调用后，它解锁互斥量并停止线程的执行。

When invoked, pthread_cond_wait unlocks the mutex and then pauses execution of its thread.

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之后它创建一个新的线程同时增量threadcount；在完成之后，它解锁互斥量。

After locking that, it creates a new thread and increments threadcount; after it's done, it unlocks the mutex.

youdao

当一个互斥锁被锁定后，它必须被解锁。

After a mutex is locked, it must be unlocked.

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