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)