Use rw_rdlock(3C) to acquire a read lock on the read-write lock pointed to by rwlp.
#include <synch.h> (or #include <thread.h>) int rw_rdlock(rwlock_t *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 Syntax.
rw_rdlock() 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.
rwlp points to an illegal address.