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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)