The OS/400 implementation of pthread_rwlock_tryrdlock() for example does not completely honor the Single UNIX Specification in its treatment of reader/writer contention.

The standard says: "The function pthread_rwlock_tryrdlock() applies a read lock as in the pthread_rwlock_rdlock() function with the exception that the function fails if any thread holds a write lock on rwlock or there are writers blocked on rwlock. "

In the OS/400 implementation, if pthread_rwlock_tryrdlock() function is used on a read/write lock that has multiple readers holding the lock, and multiple waiting writers blocked on the lock, the pthread_rwlock_tryrdlock() will be allowed to complete successfully.