#include <pthread.h>
int pthread_mutex_unlock(pthread_mutex_t *mutex);
Threadsafe: Yes
Signal Safe: Yes

The pthread_mutex_unlock() function unlocks the mutex specified. If the calling thread does not currently hold the mutex (via a previous call to pthread_mutex_lock() or pthread_mutex_trylock()) the unlock request will fail with the EPERM result.

Note that mutex initialization using the PTHREAD_MUTEX_INITIALIZER does not immediately initialize the mutex. Instead, on first use, pthread_mutex_lock() or pthread_mutex_trylock() branches into a slow path and causes the initialization of the mutex. Because a mutex isn't just a simple memory object, and requires that some resources be allocated by the system, an attempt to call pthread_mutex_destroy() or pthread_mutex_unlock() on a mutex that has was statically initialized using PTHREAD_MUTEX_INITIALER and was not yet locked will result in an EINVAL error.

Mutex Types

A recursive mutex can be locked repeatedly by the owner. The mutex doesn't become unlocked until the owner has called pthread_mutex_unlock() for each successful lock request that it has outstanding on the mutex.

An errorcheck mutex checks for deadlock conditions that occur when a thread re-locks an already held mutex. If a thread attempts to relock a mutex that it already holds, the lock request fails with the EDEADLK error

An ownerterm mutex is an OS/400 extension to the errorcheck mutex type. An ownerterm mutex checks for deadlock conditions that occur when a thread re-locks an already held mutex. If a thread attempts to relock a mutex that it already holds, the lock request fails with the EDEADLK error. An ownerterm mutex also checks for deadlock conditions that occur when a thread attempts to lock a mutex that was held by another thread when that thread terminated (an orphaned mutex). If a thread attempts to lock an orphaned mutex, the lock request fails with the EOWNERTERM error.

When a thread terminates holding a mutex lock on a normal or errorcheck mutex, other threads that wait for that mutex will block forever. The pthreads run-time simulates the deadlock that has occurred in your application. When attempting to debug these deadlock scenarios, the CL command WRKJOB, option 20 will show the thread as in a condition wait. Displaying the call stack will show that the function deadlockedOnOrphanedMutex is in the call stack.

When a thread attempts to acquire a normal mutex that it already holds, the thread will block forever. The pthreads run-time simulates the deadlock that has occurred in your application. When attempting to debug these deadlock scenarios, the CL command WRKJOB, option 20 will show the thread as in a condition wait. Displaying the call stack will show that the function deadlockedOnAlreadyLockedMutex is in the call stack.

In order to change these behaviors, use an errorcheck or ownerterm mutex type.

Parameters

mutex

(Input) Address of the mutex to unlock

For successful completion, the mutex lock must be held prior to calling pthread_mutex_unlock().

Return Value

0

pthread_mutex_unlock() was successful.

value

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

If pthread_mutex_unlock() 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.

[EPERM]

The mutex is not currently held by the caller

• The <pthread.h> header file. See Header files for Pthread functions
• pthread_mutex_destroy()--Destroy Mutex
• pthread_mutex_init()--Initialize Mutex
• pthread_mutex_lock()--Lock Mutex
• pthread_mutex_trylock()--Lock Mutex with No Wait

#include <pthread.h>
#include <stdio.h>
#include "check.h"

pthread_mutex_t    mutex = PTHREAD_MUTEX_INITIALIZER;

int main(int argc, char **argv)
{
  int                   rc=0;

  printf("Entering testcase\n");

  printf("Lock the mutex\n");
  rc = pthread_mutex_lock(&mutex);
  checkResults("pthread_mutex_lock()\n", rc);

  /* All other threads will be blocked from the resource here */
  
  printf("Unlock the mutex\n");
  rc = pthread_mutex_unlock(&mutex);
  checkResults("pthread_mutex_unlock()\n", rc);

  printf("Destroy the mutex\n");
  rc = pthread_mutex_destroy(&mutex);
  checkResults("pthread_mutex_destroy()\n", rc);
  
  printf("Main completed\n");
  return 0;
}

Entering testcase
Lock the mutex
Unlock the mutex
Destroy the mutex
Main completed