A mutex may be configured with rather exotic options such as recursive, unsafe, etc. The availability of these mutex options is controlled by configuation settings. When each option is enabled, additional fields are managed inside of the mutex structure.
pthread_cond_wait() and pthread_timed_wait() do the following atomically: (1) unlock the mutex, (2) wait for the condition, and (3) restore the mutex lock. When that lock is restored, pthread_cond_[timed]wait() must also restore the exact configuration of the mutex data structure if these "exotic" features are enabled.
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