SYNOPSIS

#include <pthread.h>

pthread_mutex_t fastmutex = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t recmutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;

pthread_mutex_t errchkmutex = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;

int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t *mutexattr);

int pthread_mutex_lock(pthread_mutex_t *mutex);

int pthread_mutex_trylock(pthread_mutex_t *mutex);

int pthread_mutex_unlock(pthread_mutex_t *mutex);

int pthread_mutex_destroy(pthread_mutex_t *mutex);

DESCRIPTION

A mutex is a MUTual EXclusion device, and is useful for protecting shared data structures from concurrent modifications, and implementing critical sections and monitors.

A mutex has two possible states: unlocked (not owned by any thread), and locked (owned by one thread). A mutex can never be owned by two different threads simultaneously. A thread attempting to lock a mutex that is already locked by another thread is suspended until the owning thread unlocks the mutex first.

pthread_mutex_init initializes the mutex object pointed to by mutex according to the mutex attributes specified in mutexattr. If mutexattr is NULL, default attributes are used instead.

The LinuxThreads implementation supports only one mutex attributes, the mutex kind, which is either ``fast'', ``recursive'', or ``error checking''. The kind of a mutex determines whether it can be locked again by a thread that already owns it. The default kind is ``fast''. See pthread_mutexattr_init(3) for more information on mutex attributes.

Variables of type pthread_mutex_t can also be initialized statically, using the constants PTHREAD_MUTEX_INITIALIZER (for fast mutexes), PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP (for recursive mutexes), and PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP (for error checking mutexes).

pthread_mutex_lock locks the given mutex. If the mutex is currently unlocked, it becomes locked and owned by the calling thread, and pthread_mutex_lock returns immediately. If the mutex is already locked by another thread, pthread_mutex_lock suspends the calling thread until the mutex is unlocked.

If the mutex is already locked by the calling thread, the behavior of pthread_mutex_lock depends on the kind of the mutex. If the mutex is of the ``fast'' kind, the calling thread is suspended until the mutex is unlocked, thus effectively causing the calling thread to deadlock. If the mutex is of the ``error checking'' kind, pthread_mutex_lock returns immediately with the error code EDEADLK. If the mutex is of the ``recursive'' kind, pthread_mutex_lock succeeds and returns immediately, recording the number of times the calling thread has locked the mutex. An equal number of pthread_mutex_unlock operations must be performed before the mutex returns to the unlocked state.

pthread_mutex_trylock behaves identically to pthread_mutex_lock, except that it does not block the calling thread if the mutex is already locked by another thread (or by the calling thread in the case of a ``fast'' mutex). Instead, pthread_mutex_trylock returns immediately with the error code EBUSY.

pthread_mutex_unlock unlocks the given mutex. The mutex is assumed to be locked and owned by the calling thread on entrance to pthread_mutex_unlock. If the mutex is of the ``fast'' kind, pthread_mutex_unlock always returns it to the unlocked state. If it is of the ``recursive'' kind, it decrements the locking count of the mutex (number of pthread_mutex_lock operations performed on it by the calling thread), and only when this count reaches zero is the mutex actually unlocked.

On ``error checking'' and ``recursive'' mutexes, pthread_mutex_unlock actually checks at run-time that the mutex is locked on entrance, and that it was locked by the same thread that is now calling pthread_mutex_unlock. If these conditions are not met, an error code is returned and the mutex remains unchanged. ``Fast'' mutexes perform no such checks, thus allowing a locked mutex to be unlocked by a thread other than its owner. This is non-portable behavior and must not be relied upon.

pthread_mutex_destroy destroys a mutex object, freeing the resources it might hold. The mutex must be unlocked on entrance. In the LinuxThreads implementation, no resources are associated with mutex objects, thus pthread_mutex_destroy actually does nothing except checking that the mutex is unlocked.

CANCELLATION

None of the mutex functions is a cancellation point, not even pthread_mutex_lock, in spite of the fact that it can suspend a thread for arbitrary durations. This way, the status of mutexes at cancellation points is predictable, allowing cancellation handlers to unlock precisely those mutexes that need to be unlocked before the thread stops executing. Consequently, threads using deferred cancellation should never hold a mutex for extended periods of time.

ASYNC-SIGNAL SAFETY

The mutex functions are not async-signal safe. What this means is that they should not be called from a signal handler. In particular, calling pthread_mutex_lock or pthread_mutex_unlock from a signal handler may deadlock the calling thread.

RETURN VALUE

pthread_mutex_init always returns 0. The other mutex functions return 0 on success and a non-zero error code on error.

AUTHOR

Xavier Leroy <[email protected]>

EXAMPLE

A shared global variable x can be protected by a mutex as follows:

int x;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;

All accesses and modifications to x should be bracketed by calls to pthread_mutex_lock and pthread_mutex_unlock as follows:

pthread_mutex_lock(&mut);
/* operate on x */
pthread_mutex_unlock(&mut);