#include <synch.h> (or #include <thread.h>) int rw_tryrdlock(rwlock_t *rwlp);
Use rw_tryrdlock(3THR) to attempt to acquire a read lock on the read-write lock pointed to by rwlp. When the read-write lock is already locked for writing, it returns an error. Otherwise, the read lock is acquired. (For POSIX threads, see pthread_rwlock_tryrdlock(3THR).)
rw_tryrdlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.