pthread_rwlock_wrlock, pthread_rwlock_trywrlock - lock a read-write lock
object for writing
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
Read-write locks allow a thread to exclusively lock some shared data
while updating that data, or allow any number of threads to have
simultaneous read-only access to the data. The pthread_rwlock_wrlock()
and pthread_rwlock_trywrlock() functions apply a write lock to the readwrite
lock object referenced by rwlock. The calling thread will only
acquire the lock if no other thread (reader or writer) holds the lock. A
thread that calls pthread_rwlock_wrlock() will block until it can gain
the read-write lock for writing, and retains ownership until it calls
pthread_rwlock_unlock(). Only the thread that locked a read-write lock
should unlock it. When unlocked, if there are both readers and writers
of the same priority waiting for the read-write lock, writers will be
scheduled before readers.
If a thread waiting for a read-write lock receives a signal, upon return
from the signal handler, the thread resumes waiting for the read-write
lock as if there was no interrupt.
All of the read-write lock functions return zero if successful;
otherwise, an error number is returned.
pthread_rwlock_trywrlock() can return the following error:
[EBUSY] The read-write lock is currently held by another thread.
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