that's not "zero = owner dead", but it returns 0 if call was succesfull or the value of EOWNERDEAD if something was wrong:

  If mutex is a robust mutex and the process containing the owning
  thread terminated while holding the mutex lock, a call to
  pthread_mutex_lock() shall return the error value [EOWNERDEAD].

as in "the current owner of the lock is dead without releasing it"

(edit: fix formatting)

Thanks for updating for me. I was just looking at "The mutex object referenced by mutex shall be locked by a call to pthread_mutex_lock() that returns zero or [EOWNERDEAD]." and saw that it returns zero or EOWNERDEAD and thought that zero = EOWNERDEAD. Thanks for all your comments. They were helpful and I learned new things by reading your posted links.