pthread_mutex_init

PTHREAD_MUTEX(3)           Library Functions Manual           PTHREAD_MUTEX(3)



NAME
       pthread_mutex_init, pthread_mutex_lock, pthread_mutex_trylock,
       pthread_mutex_unlock, pthread_mutex_destroy - operations on mutexes


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.


ERRORS
       The pthread_mutex_lock function returns the following error code on
       error:

              EINVAL the mutex has not been properly initialized.


              EDEADLK
                     the mutex is already locked by the calling thread
                     (``error checking'' mutexes only).

       The pthread_mutex_trylock function returns the following error codes on
       error:

              EBUSY  the mutex could not be acquired because it was currently
                     locked.


              EINVAL the mutex has not been properly initialized.

       The pthread_mutex_unlock function returns the following error code on
       error:

              EINVAL the mutex has not been properly initialized.


              EPERM  the calling thread does not own the mutex (``error
                     checking'' mutexes only).

       The pthread_mutex_destroy function returns the following error code on
       error:

              EBUSY  the mutex is currently locked.


AUTHOR
       Xavier Leroy <Xavier.Leroy@inria.fr>


SEE ALSO
       pthread_mutexattr_init(3), pthread_mutexattr_setkind_np(3),
       pthread_cancel(3).


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);

                                 LinuxThreads                 PTHREAD_MUTEX(3)