pthread_rwlock_init example c c++

NAME

pthread_rwlock_destroy, pthread_rwlock_init - destroy and initialize a read-write lock object

SYNOPSIS

[THR]  #include <pthread.h>

int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock,
       const pthread_rwlockattr_t *restrict attr); 

DESCRIPTION

The pthread_rwlock_destroy() function shall destroy the read-write lock object referenced by rwlock and release any resources used by the lock. The effect of subsequent use of the lock is undefined until the lock is reinitialized by another call to pthread_rwlock_init(). An implementation may cause pthread_rwlock_destroy() to set the object referenced by rwlock to an invalid value. Results are undefined ifpthread_rwlock_destroy() is called when any thread holds rwlock. Attempting to destroy an uninitialized read-write lock results in undefined behavior.
The pthread_rwlock_init() function shall allocate any resources required to use the read-write lock referenced by rwlock and initializes the lock to an unlocked state with attributes referenced by attr. If attr is NULL, the default read-write lock attributes shall be used; the effect is the same as passing the address of a default read-write lock attributes object. Once initialized, the lock can be used any number of times without being reinitialized. Results are undefined if pthread_rwlock_init() is called specifying an already initialized read-write lock. Results are undefined if a read-write lock is used without first being initialized.
If the pthread_rwlock_init() function fails, rwlock shall not be initialized and the contents of rwlock are undefined.
Only the object referenced by rwlock may be used for performing synchronization. The result of referring to copies of that object in calls topthread_rwlock_destroy(), pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), pthread_rwlock_timedwrlock(),pthread_rwlock_tryrdlock(), pthread_rwlock_trywrlock(), pthread_rwlock_unlock(), or pthread_rwlock_wrlock() is undefined.

RETURN VALUE

If successful, the pthread_rwlock_destroy() and pthread_rwlock_init() functions shall return zero; otherwise, an error number shall be returned to indicate the error.
The [EBUSY] and [EINVAL] error checks, if implemented, act as if they were performed immediately at the beginning of processing for the function and caused an error return prior to modifying the state of the read-write lock specified by rwlock.

ERRORS

The pthread_rwlock_destroy() function may fail if:

[EBUSY]

The implementation has detected an attempt to destroy the object referenced by rwlock while it is locked.

[EINVAL]

The value specified by rwlock is invalid.

The pthread_rwlock_init() function shall fail if:

[EAGAIN]

The system lacked the necessary resources (other than memory) to initialize another read-write lock.

[ENOMEM]

Insufficient memory exists to initialize the read-write lock.

[EPERM]

The caller does not have the privilege to perform the operation.

The pthread_rwlock_init() function may fail if:

[EBUSY]

The implementation has detected an attempt to reinitialize the object referenced by rwlock, a previously initialized but not yet destroyed read-write lock.

[EINVAL]

The value specified by attr is invalid.

These functions shall not return an error code of [EINTR].

Example of (pthread_rwlock_destroy, pthread_rwlock_init)
 #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