NAME
pthread_rwlock_unlock - unlock a read-write lock object
SYNOPSIS
#include <pthread.h>
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
DESCRIPTION
The pthread_rwlock_unlock() function is called to release a lock held on the read-write lock object referenced by rwlock. Results are undefined if the read-write lock rwlock is not held by the calling thread. (pthread_rwlock_unlock)If this function is called to release a read lock from the read-write lock object and there are other read locks currently held on this read-write lock object, the read-write lock object remains in the read locked state. If this function releases the calling thread's last read lock on this read-write lock object, then the calling thread is no longer one of the owners of the object. If this function releases the last read lock for this read-write lock object, the read-write lock object will be put in the unlocked state with no owners.
If this function is called to release a write lock for this read-write lock object, the read-write lock object will be put in the unlocked state with no owners.
If the call to the pthread_rwlock_unlock() function results in the read-write lock object becoming unlocked and there are multiple threads waiting to acquire the read-write lock object for writing, the scheduling policy is used to determine which thread acquires the read-write lock object for writing. If there are multiple threads waiting to acquire the read-write lock object for reading, the scheduling policy is used to determine the order in which the waiting threads acquire the read-write lock object for reading. If there are multiple threads blocked on rwlock for both read locks and write locks, it is unspecified whether the readers acquire the lock first or whether a writer acquires the lock first.
Results are undefined if any of these functions are called with an uninitialised read-write lock.
RETURN VALUE
If successful, the pthread_rwlock_unlock() function returns zero. Otherwise, an error number is returned to indicate the error.
ERRORS
The pthread_rwlock_unlock() function may fail if:
- [EINVAL]
- The value specified by rwlock does not refer to an initialised read-write lock object.
- [EPERM]
- The current thread does not own the read-write lock.
EXAMPLES of pthread_rwlock_unlock
#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include "check.h"
pthread_rwlock_t rwlock;
void *rdlockThread(void *arg)
{
int rc;
printf("Entered thread, getting read lock\n");
rc = pthread_rwlock_rdlock(&rwlock);
checkResults("pthread_rwlock_rdlock()\n", rc);
printf("got the rwlock read lock\n");
sleep(5);
printf("unlock the read lock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Secondary thread unlocked\n");
return NULL;
}
void *wrlockThread(void *arg)
{
int rc;
printf("Entered thread, getting write lock\n");
rc = pthread_rwlock_wrlock(&rwlock);
checkResults("pthread_rwlock_wrlock()\n", rc);
printf("Got the rwlock write lock, now unlock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Secondary thread unlocked\n");
return NULL;
}
int main(int argc, char **argv)
{
int rc=0;
pthread_t thread, thread1;
printf("Enter Testcase - %s\n", argv[0]);
printf("Main, initialize the read write lock\n");
rc = pthread_rwlock_init(&rwlock, NULL);
checkResults("pthread_rwlock_init()\n", rc);
printf("Main, grab a read lock\n");
rc = pthread_rwlock_rdlock(&rwlock);
checkResults("pthread_rwlock_rdlock()\n",rc);
printf("Main, grab the same read lock again\n");
rc = pthread_rwlock_rdlock(&rwlock);
checkResults("pthread_rwlock_rdlock() second\n", rc);
printf("Main, create the read lock thread\n");
rc = pthread_create(&thread, NULL, rdlockThread, NULL);
checkResults("pthread_create\n", rc);
printf("Main - unlock the first read lock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Main, create the write lock thread\n");
rc = pthread_create(&thread1, NULL, wrlockThread, NULL);
checkResults("pthread_create\n", rc);
sleep(5);
printf("Main - unlock the second read lock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Main, wait for the threads\n");
rc = pthread_join(thread, NULL);
checkResults("pthread_join\n", rc);
rc = pthread_join(thread1, NULL);
checkResults("pthread_join\n", rc);
rc = pthread_rwlock_destroy(&rwlock);
checkResults("pthread_rwlock_destroy()\n", rc);
printf("Main completed\n");
return 0;
}
Output:
Enter Testcase - QP0WTEST/TPRWLINI0
Main, initialize the read write lock
Main, grab a read lock
Main, grab the same read lock again
Main, create the read lock thread
Main - unlock the first read lock
Main, create the write lock thread
Entered thread, getting read lock
got the rwlock read lock
Entered thread, getting write lock
Main - unlock the second read lock
Main, wait for the threads
unlock the read lock
Secondary thread unlocked
Got the rwlock write lock, now unlock
Secondary thread unlocked
Main completed
