The function int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex); shall block on a condition variable. It shall be called with mutex locked by the calling thread or undefined behavior results. Upon successful return, the mutex shall have been locked and shall be owned by the calling thread. Upon successful completion, a value of zero shall be returned; The pthread_cond_wait() function may fail if: [EINVAL] The value specified by cond, mutex,or abstime is invalid. [EINVAL] Different mutexes were supplied for concurrent pthread_cond_timedwait() or pthread_cond_wait() operations on the same condition variable. [EPERM] The mutex was not owned by the current thread at the time of the call. These functions shall not return an error code of [EINTR].