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