On successful completion, pthread_mutexattr_getprotocol() returns 0. Any other returned value indicates that an error occurred.
If the following condition occurs, pthread_mutexattr_getprotocol() fails and returns the corresponding value.
Neither of the options, _POSIX_THREAD_PRIO_INHERIT nor _POSIX_THREAD_PRIO_PROTECT is defined and the implementation does not support the function.
If either of the following conditions occurs, pthread_mutexattr_getprotocol() might fail and return the corresponding value.