The pthread_spin_unlock( ) function shall release the spin lock referenced by lock which was locked via the pthread_spin_lock( ) or pthread_spin_trylock( ) functions. Upon successful completion, the pthread_spin_unlock( ) function shall return zero; otherwise, an error number shall be returned to indicate the error. The pthread_spin_unlock( ) function may fail if: [EINVAL] An invalid argument was specified. [EPERM] The calling thread does not hold the lock. These functions shall not return an error code of EINTR.