#include <synch.h> (or #include <thread.h>) int rw_wrlock(rwlock_t *rwlp);
Use rw_wrlock(3THR) to acquire a write lock on the read-write lock pointed to by rwlp. When the read-write lock is already locked for reading or writing, the calling thread blocks until all the read locks and write locks are released. Only one thread at a time can hold a write lock on a read-write lock. (For POSIX threads, see "pthread_rwlock_wrlock(3THR)".)
rw_wrlock() 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.