NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write lock object for reading
SYNOPSIS
#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
DESCRIPTION of (pthread_rwlock_rdlock, pthread_rwlock_tryrdlock)
The pthread_rwlock_rdlock() function applies a read lock to the read-write lock referenced by rwlock. The calling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock. It is unspecified whether the calling thread acquires the lock when a writer does not hold the lock and there are writers waiting for the lock. If a writer holds the lock, the calling thread will not acquire the read lock. If the read lock is not acquired, the calling thread blocks (that is, it does not return from the pthread_rwlock_rdlock() call) until it can acquire the lock. Results are undefined if the calling thread holds a write lock on rwlockat the time the call is made.Implementations are allowed to favour writers over readers to avoid writer starvation.
A thread may hold multiple concurrent read locks on rwlock (that is, successfully call the pthread_rwlock_rdlock() function ntimes). If so, the thread must perform matching unlocks (that is, it must call the pthread_rwlock_unlock() function n times).
The function pthread_rwlock_tryrdlock() applies a read lock as in the pthread_rwlock_rdlock() function with the exception that the function fails if any thread holds a write lock on rwlock or there are writers blocked on rwlock.
Results are undefined if any of these functions are called with an uninitialised read-write lock.
If a signal is delivered to a thread waiting for a read-write lock for reading, upon return from the signal handler the thread resumes waiting for the read-write lock for reading as if it was not interrupted.
RETURN VALUE
If successful, the pthread_rwlock_rdlock() function returns zero. Otherwise, an error number is returned to indicate the error.The function pthread_rwlock_tryrdlock() returns zero if the lock for reading on the read-write lock object referenced by rwlock is acquired. Otherwise an error number is returned to indicate the error.
ERRORS
The pthread_rwlock_tryrdlock() function will fail if:
- [EBUSY]
- The read-write lock could not be acquired for reading because a writer holds the lock or was blocked on it.
The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if:
- [EINVAL]
- The value specified by rwlock does not refer to an initialised read-write lock object.
- [EDEADLK]
- The current thread already owns the read-write lock for writing.
- [EAGAIN]
- The read lock could not be acquired because the maximum number of read locks for rwlock has been exceeded.
Example of (pthread_rwlock_rdlock,
pthread_rwlock_tryrdlock)
#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include "check.h"
pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
void *rdlockThread(void *arg)
{
int rc;
int count=0;
printf("Entered thread, getting read lock with mp wait\n");
Retry:
rc = pthread_rwlock_tryrdlock(&rwlock);
if (rc == EBUSY) {
if (count >= 10) {
printf("Retried too many times, failure!\n");
exit(EXIT_FAILURE);
}
++count;
printf("Could not get lock, do other work, then RETRY...\n");
sleep(1);
goto Retry;
}
checkResults("pthread_rwlock_tryrdlock() 1\n", rc);
sleep(2);
printf("unlock the read lock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Secondary thread complete\n");
return NULL;
}
int main(int argc, char **argv)
{
int rc=0;
pthread_t thread;
printf("Enter Testcase - %s\n", argv[0]);
printf("Main, get the write lock\n");
rc = pthread_rwlock_wrlock(&rwlock);
checkResults("pthread_rwlock_wrlock()\n", rc);
printf("Main, create the try read lock thread\n");
rc = pthread_create(&thread, NULL, rdlockThread, NULL);
checkResults("pthread_create\n", rc);
printf("Main, wait a bit holding the write lock\n");
sleep(5);
printf("Main, Now unlock the write lock\n");
rc = pthread_rwlock_unlock(&rwlock);
checkResults("pthread_rwlock_unlock()\n", rc);
printf("Main, wait for the thread to end\n");
rc = pthread_join(thread, 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/TPRWLRD1
Main, get the write lock
Main, create the try read lock thread
Main, wait a bit holding the write lock
Entered thread, getting read lock with mp wait
Could not get lock, do other work, then RETRY...
Could not get lock, do other work, then RETRY...
Could not get lock, do other work, then RETRY...
Could not get lock, do other work, then RETRY...
Could not get lock, do other work, then RETRY...
Main, Now unlock the write lock
Main, wait for the thread to end
unlock the read lock
Secondary thread complete
Main completed
