pthread_mutex_lock()

Lock a mutex

Synopsis:

#include <pthread.h>

int pthread_mutex_lock( pthread_mutex_t* mutex );

Arguments:

mutex
A pointer to the pthread_mutex_t object that you want to lock.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The pthread_mutex_lock() function locks the mutex object referenced by mutex. If the mutex is already locked, then the calling thread blocks until it has acquired the mutex. When the function returns, the mutex object is locked and owned by the calling thread.

This function's behavior when you try to lock a mutex that you already own depends on the type of the mutex. For more information, see the entry for pthread_mutexattr_settype().

By default, if a thread with a higher priority than the mutex owner attempts to lock a mutex, then the effective priority of the current owner is increased to that of the higher-priority blocked thread waiting for the mutex. The owner returns to its real priority when it unlocks the mutex. For more information, see Mutexes: mutual exclusion locks in the QNX Neutrino Microkernel chapter of the System Architecture guide.

If the mutex is recursive, you must call pthread_mutex_unlock() for each corresponding call to lock the mutex.

If a signal is delivered to a thread that's waiting for a mutex, the thread resumes waiting for the mutex on returning from the signal handler.

If, before initializing the mutex, you've called pthread_mutexattr_setwakeup_np() to enable wake-ups, you can later call pthread_mutex_wakeup_np(), to wake up any threads that are blocked on the mutex. The “np” in this function's name stands for “non-POSIX.”

Returns:

EOK
Success.
EAGAIN
The mutex couldn't be acquired because the maximum number of recursive locks for mutex has been exceeded.
EDEADLK
One of the following occurred:
  • The mutex type is PTHREAD_MUTEX_ERRORCHECK, and the current thread already owns the mutex.
  • A deadlock condition was detected.
EFAULT
A fault occurred when the kernel tried to access the buffers you provided.
EINTR
(QNX Neutrino extension) The thread was unblocked by a call to pthread_mutex_wakeup_np().
EINVAL
One of the following occurred:
  • The mutex was created with a protocol attribute of PTHREAD_PRIO_PROTECT, and the calling thread's priority is higher than the mutex's current priority ceiling.
  • The mutex is invalid, or it has died (see SyncMutexEvent()).
ENOTRECOVERABLE
The mutex is a robust mutex, and the state that it protects isn't recoverable. All you can do with the mutex is destroy it by calling pthread_mutex_destroy().
EOWNERDEAD
The mutex is a robust mutex, and the previous owning thread or the process containing the previous owning thread terminated while holding the mutex lock. The calling thread acquires the mutex lock; it's up to the new owner to make the state consistent (see pthread_mutex_consistent()).
ETIMEDOUT
A kernel timeout unblocked the call.

Examples:

This example shows how you can use a mutex to synchronize access to a shared variable. In this example, function1() and function2() both attempt to access and modify the global variable count. Either thread could be interrupted between modifying count and assigning its value to the local tmp variable. Locking mutex prevents this from happening; if one thread has mutex locked, the other thread waits until it's unlocked, before continuing.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int count = 0;

void* function1( void* arg )
{
    int tmp = 0, ret_code;
    
    while( 1 ) {
        ret_code = pthread_mutex_lock( &mutex );
        if (ret_code == EOK)
        {
            tmp = count++;
            ret_code = pthread_mutex_unlock( &mutex );
            if (ret_code != EOK)
            {
                printf ("pthread_mutex_unlock() failed: %s\n",
                        strerror(ret_code));
            }
                
            printf ("Count is %d\n", tmp );
        }
        else
        {
            printf ("pthread_mutex_lock() failed: %s\n",
                    strerror(ret_code));
        }
            
        /* Snooze for 1 second */
        sleep( 1 );
    }
    
    return 0;
}

void* function2( void* arg )
{
    int tmp = 0, ret_code;

    while( 1 ) {
        ret_code = pthread_mutex_lock( &mutex );
        if (ret_code == EOK)
        {
            tmp = count--;
            ret_code = pthread_mutex_unlock( &mutex );
            if (ret_code != EOK)
            {
                printf ("** pthread_mutex_unlock() failed: %s\n",
                        strerror (ret_code));
            }
            printf ("** Count is %d\n", tmp );
        }
        else
        {
            printf ("** pthread_mutex_lock() failed: %s\n",
                    strerror (ret_code));
        }
            
        /* Snooze for 2 seconds */
        sleep( 2 );
    }

    return 0;
}

int main( void )
{
	int ret_code;
	
    ret_code = pthread_create( NULL, NULL, &function1, NULL );
	if (ret_code != EOK)
	{
		printf ("Couldn't create first thread: %s\n",
				strerror (ret_code));
	}
    ret_code = pthread_create( NULL, NULL, &function2, NULL );
	if (ret_code != EOK)
	{
		printf ("Couldn't create second thread: %s\n",
				strerror (ret_code));
	}
    
    /* Let the threads run for 60 seconds. */
    sleep( 60 );

    return EXIT_SUCCESS;
}

Classification:

POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler No
Signal handler No
Thread Yes