It turns out that SUSv3:

http://www.opengroup.org/onlinepubs/000095399/toc.htm

states that following variables must be defined in sys/types.h (their definitions in pthread.h must be identical)

pthread_attr_t pthread_barrier_t pthread_barrierattr_t pthread_cond_t pthread_condattr_t pthread_key_t pthread_mutex_t pthread_mutexattr_t pthread_once_t pthread_rwlock_t pthread_rwlockattr_t pthread_spinlock_t pthread_t

It has been reported that Sun Grid Engine relies on this behavior.

-- Brooks