#include <synch.h> (or #include <thread.h>) int rw_trywrlock(rwlock_t *rwlp);
Use rw_trywrlock() to attempt to acquire a write lock on the readers/writer lock pointed to by rwlp. When the readers/writer lock is already locked for reading or writing, it returns an error.
rw_trywrlock() returns zero after completing successfully. Any other returned value indicates that an error occurred. When any of the following conditions occur, the function fails and returns the corresponding value.