Return Values

pthread_mutex_lock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EAGAIN

The mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.

EDEADLK

The current thread already owns the mutex.

If the symbol _POSIX_THREAD_PRIO_INHERIT is defined, the mutex is initialized with the protocol attribute value PTHREAD_PRIO_INHERIT, and the robustness argument of pthread_mutexattr_setrobust_np() is PTHREAD_MUTEX_ROBUST_NP the function fails and returns:

EOWNERDEAD

The last owner of this mutex died while holding the mutex. This mutex is now owned by the caller. The caller must attempt to make the state protected by the mutex consistent.

If the caller is able to make the state consistent, call pthread_mutex_consistent_np() for the mutex and unlock the mutex. Subsequent calls to pthread_mutex_lock() will behave normally.

If the caller is unable to make the state consistent, do not call pthread_mutex_init() for the mutex, but unlock the mutex. Subsequent calls to pthread_mutex_lock() fail to acquire the mutex and return an ENOTRECOVERABLE error code.

If the owner that acquired the lock with EOWNERDEAD dies, the next owner acquires the lock with EOWNERDEAD.

ENOTRECOVERABLE

The mutex you are trying to acquire is protecting state left irrecoverable by the mutex's previous owner that died while holding the lock. The mutex has not been acquired. This condition can occur when the lock was previously acquired with EOWNERDEAD and the owner was unable to cleanup the state and had unlocked the mutex without making the mutex state consistent.

ENOMEM

The limit on the number of simultaneously held mutexes has been exceeded.

Unlock a Mutex

pthread_mutex_unlock(3THR)

Use pthread_mutex_unlock(3THR) to unlock the mutex pointed to by mutex. (For Solaris threads, see "mutex_unlock(3THR)".)

Prototype:
int	pthread_mutex_unlock(pthread_mutex_t *mutex); 

#include <pthread.h>

pthread_mutex_t mutex;
int ret;

ret = pthread_mutex_unlock(&mutex); /* release the mutex */

pthread_mutex_unlock() releases the mutex object referenced by mutex. The manner in which a mutex is released is dependent upon the mutex's type attribute. If there are threads blocked on the mutex object referenced by mutex when pthread_mutex_unlock() is called, resulting in the mutex becoming available, the scheduling policy is used to determine which thread shall acquire the mutex. (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex becomes available when the count reaches zero and the calling thread no longer has any locks on this mutex).

Return Values

pthread_mutex_unlock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EPERM

The current thread does not own the mutex.

Lock With a Nonblocking Mutex

pthread_mutex_trylock(3THR)

Use pthread_mutex_trylock(3THR) to attempt to lock the mutex pointed to by mutex. (For Solaris threads, see "mutex_trylock(3THR)".)

Prototype:
int	pthread_mutex_trylock(pthread_mutex_t *mutex); 

#include <pthread.h>

pthread_mutex_t mutex;
int ret;

ret = pthread_mutex_trylock(&mutex); /* try to lock the mutex */

pthread_mutex_trylock() is a nonblocking version of pthread_mutex_lock(). If the mutex object referenced by mutex is currently locked (by any thread, including the current thread), the call returns immediately. Otherwise, the mutex is locked and the calling thread is the owner.

Return Values

pthread_mutex_trylock() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EBUSY

The mutex could not be acquired because the mutex pointed to by mutex was already locked.

EAGAIN

The mutex could not be acquired because the maximum number of recursive locks for mutex has been exceeded.

If the symbol _POSIX_THREAD_PRIO_INHERIT is defined, the mutex is initialized with the protocol attribute value PTHREAD_PRIO_INHERIT, and the robustness argument of pthread_mutexattr_setrobust_np() is PTHREAD_MUTEX_ROBUST_NP the function fails and returns:

EOWNERDEAD

The last owner of this mutex died while holding the mutex. This mutex is now owned by the caller. The caller must attempt to make the state protected by the mutex consistent.

If the caller is able to make the state consistent, call pthread_mutex_consistent_np() for the mutex and unlock the mutex. Subsequent calls to pthread_mutex_lock() will behave normally.

If the caller is unable to make the state consistent, do not call pthread_mutex_init() for the mutex, but unlock the mutex. Subsequent calls to pthread_mutex_trylock() fail to acquire the mutex and return an ENOTRECOVERABLE error code.

If the owner that acquired the lock with EOWNERDEAD dies, the next owner acquires the lock with EOWNERDEAD.

ENOTRECOVERABLE

The mutex you are trying to acquire is protecting state left irrecoverable by the mutex's previous owner that died while holding the lock. The mutex has not been acquired. This condition can occur when the lock was previously acquired with EOWNERDEAD and the owner was unable to cleanup the state and had unlocked the mutex without making the mutex state consistent.

ENOMEM

The limit on the number of simultaneously held mutexes has been exceeded.

Destroy a Mutex

pthread_mutex_destroy(3THR)

Use pthread_mutex_destroy(3THR) to destroy any state associated with the mutex pointed to by mp. (For Solaris threads, see "mutex_destroy(3THR)".)

Prototype:
int	pthread_mutex_destroy(pthread_mutex_t *mp); 

#include <pthread.h>

pthread_mutex_t mp;
int ret;

ret = pthread_mutex_destroy(&mp); /* mutex is destroyed */

Note that the space for storing the mutex is not freed.

Return Values

pthread_mutex_destroy() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occur, the function fails and returns the corresponding value.

EINVAL

The value specified by mp does not refer to an initialized mutex object.

Mutex Lock Code Examples

Example 4-1 shows some code fragments with mutex locking.

Example 4-1 Mutex Lock Example

#include <pthread.h>

pthread_mutex_t count_mutex;
long long count;

void
increment_count()
{
	    pthread_mutex_lock(&count_mutex);
    count = count + 1;
	    pthread_mutex_unlock(&count_mutex);
}

long long
get_count()
{
    long long c;
    
    pthread_mutex_lock(&count_mutex);
	    c = count;
    pthread_mutex_unlock(&count_mutex);
	    return (c);
}

The two functions in Example 4-1 use the mutex lock for different purposes. The increment_count() function uses the mutex lock simply to ensure an atomic update of the shared variable. The get_count() function uses the mutex lock to guarantee that the 64-bit quantity count is read atomically. On a 32-bit architecture, a long long is really two 32-bit quantities.

Reading an integer value is an atomic operation because integer is the common word size on most machines.