Use rw_wrlock(3C) to acquire a write lock on the read-write lock pointed to by rwlp.
#include <synch.h> (or #include <thread.h>) int rw_wrlock(rwlock_t *rwlp);
When the read-write lock is already locked for reading or writing, the calling thread blocks until all 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 Syntax.
rw_wrlock() 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.