pthread_rwlock_init()--Initialize Read/Write Lock

 #include <pthread.h>
 int pthread_rwlock_init(pthread_rwlock_t *rwlock,   
 const pthread_rwlockattr_t *attr);

The pthread_rwlock_init() function initializes a new read/write lock with the specified attributes for use. The new read/write lock may be used immediately for serializing critical resources. If attr is specified as NULL, all attributes are set to the default read/write lock attributes for the newly created read/write lock.

With these declarations and initializations:

pthread_rwlock_t        rwlock2;
pthread_rwlock_t        rwlock3;
pthread_rwlockattr_t    attr;
pthread_rwlockattr_init(&attr);

The following three read/write lock initialization mechanisms have equivalent function.

pthread_rwlock_t        rwlock1 = PTHREAD_RWLOCK_INITIALIZER;
pthread_rwlock_init(&rwlock2, NULL);
pthread_rwlock_init(&rwlock, &attr);

All three read/write locks are created with the default read/write lock attributes.

Every read/write lock must eventually be destroyed with pthread_rwlock_destroy(). Always use pthread_rwlock_destroy() before freeing or reusing read/write lock storage.

Authorities and Locks

None.

Parameters

rwlock

(Output) The address of the variable to contain a read/write lock

attr

(Input) The address of the variable containing the read/write lock attributes object

Return Value

0

pthread_rwlock_init() was successful.

value

pthread_rwlock_init() was not successful. value is set to indicate the error condition.

Error Conditions

If pthread_rwlock_init() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.

[EINVAL]

The value specified for the argument is not correct.

Related Information

• The <pthread.h> header file. See Header files for Pthread functions.
  
  
• pthread_rwlockattr_init()--Initialize Read/Write Lock Attribute
  
  
• pthread_rwlock_destroy()--Destroy Read/Write Lock

Example

#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

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