pthread_mutex_unlock

       and unlock a mutex

SYNOPSIS
       #include <pthread.h>

       int pthread_mutex_lock(pthread_mutex_t *mutex);
       int pthread_mutex_trylock(pthread_mutex_t *mutex);
       int pthread_mutex_unlock(pthread_mutex_t *mutex);


DESCRIPTION
       The mutex object  referenced  by  mutex  shall  be  locked  by  calling
       pthread_mutex_lock().  If  the  mutex  is  already  locked, the calling
       thread shall block until the mutex becomes available.   This  operation
       shall  return  with  the mutex object referenced by mutex in the locked
       state with the calling thread as its owner.

       If the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall not
       be  provided.  Attempting  to  relock  the  mutex causes deadlock. If a
       thread attempts to unlock a mutex that it has not  locked  or  a  mutex
       which is unlocked, undefined behavior results.

       If  the  mutex  type  is  PTHREAD_MUTEX_ERRORCHECK, then error checking
       shall be provided. If a thread attempts to relock a mutex that  it  has
       already  locked,  an  error  shall be returned. If a thread attempts to
       unlock a mutex that it has not locked or a mutex which is unlocked,  an
       error shall be returned.

       If  the  mutex  type  is  PTHREAD_MUTEX_RECURSIVE, then the mutex shall
       maintain the concept of  a  lock  count.  When  a  thread  successfully
       acquires  a  mutex  for  the first time, the lock count shall be set to
       one. Every time a thread relocks this mutex, the lock  count  shall  be
       incremented  by  one.  Each time the thread unlocks the mutex, the lock
       count shall be decremented by one. When the lock  count  reaches  zero,
       the  mutex  shall  become  available for other threads to acquire. If a
       thread attempts to unlock a mutex that it has not  locked  or  a  mutex
       which is unlocked, an error shall be returned.

       If  the  mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively
       lock the mutex results in undefined behavior. Attempting to unlock  the
       mutex  if  it was not locked by the calling thread results in undefined
       behavior. Attempting to unlock the mutex if it is not locked results in
       undefined behavior.

       The   pthread_mutex_trylock()   function   shall   be   equivalent   to
       pthread_mutex_lock(), except that if the  mutex  object  referenced  by
       mutex  is  currently  locked  (by  any  thread,  including  the current
       thread), the call shall  return  immediately.  If  the  mutex  type  is
       PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling
       thread, the mutex lock count  shall  be  incremented  by  one  and  the
       pthread_mutex_trylock() function shall immediately return success.

       The pthread_mutex_unlock() function shall release the mutex object ref-
       erenced by mutex.    The manner in which a mutex is released is  depen-

RETURN VALUE
       If  successful,  the  pthread_mutex_lock()  and  pthread_mutex_unlock()
       functions  shall  return  zero;  otherwise,  an  error  number shall be
       returned to indicate the error.

       The pthread_mutex_trylock() function shall return zero if a lock on the
       mutex  object referenced by mutex is acquired. Otherwise, an error num-
       ber is returned to indicate the error.

ERRORS
       The pthread_mutex_lock() and  pthread_mutex_trylock()  functions  shall
       fail if:

       EINVAL The  mutex  was  created  with the protocol attribute having the
              value PTHREAD_PRIO_PROTECT and the calling thread's priority  is
              higher than the mutex's current priority ceiling.


       The pthread_mutex_trylock() function shall fail if:

       EBUSY  The mutex could not be acquired because it was already locked.


       The       pthread_mutex_lock(),       pthread_mutex_trylock(),      and
       pthread_mutex_unlock() functions may fail if:

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

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


       The pthread_mutex_lock() function may fail if:

       EDEADLK
              The current thread already owns the mutex.


       The pthread_mutex_unlock() function may fail if:

       EPERM  The current thread does not own the mutex.


       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.
       ior in order to avoid requiring more overhead in  the  basic  mechanism
       than  is  absolutely  necessary.  (More  "friendly" mutexes that detect
       deadlock or that allow multiple locking by the same thread  are  easily
       constructed by the user via the other mechanisms provided. For example,
       pthread_self() can be used to record mutex ownership.)  Implementations
       might also choose to provide such extended features as options via spe-
       cial mutex attributes.

       Since most attributes only need to be checked when a thread is going to
       be  blocked,  the  use  of attributes does not slow the (common) mutex-
       locking case.

       Likewise, while being able to extract the thread ID of the owner  of  a
       mutex  might  be desirable, it would require storing the current thread
       ID when each mutex is locked, and this could incur unacceptable  levels
       of overhead. Similar arguments apply to a mutex_tryunlock operation.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_mutex_destroy()  , pthread_mutex_timedlock() , the Base Defini-
       tions volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                PTHREAD_MUTEX_LOCK(P)