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NAME

       pthread_mutex_lock,  pthread_mutex_trylock, pthread_mutex_unlock - lock
       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
       referenced by mutex.    The manner in which  a  mutex  is  released  is
       dependent  upon  the  mutex’s  type  attribute.   If  there are threads
       blocked   on   the   mutex   object   referenced    by    mutex    when
       pthread_mutex_unlock()  is  called,  resulting  in  the  mutex becoming
       available, the scheduling policy shall  determine  which  thread  shall
       acquire the mutex.

       (In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex shall become
       available when the count reaches zero and the calling thread no  longer
       has any locks on this mutex.)

       If  a  signal is delivered to a thread waiting for a mutex, upon return
       from the signal handler the thread shall resume waiting for  the  mutex
       as if it was not interrupted.

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
       number 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.

RATIONALE

       Mutex objects are intended to serve as a low-level primitive from which
       other thread synchronization functions  can  be  built.  As  such,  the
       implementation  of mutexes should be as efficient as possible, and this
       has ramifications on the features available at the interface.

       The mutex functions and the particular default settings  of  the  mutex
       attributes  have  been  motivated  by  the desire to not preclude fast,
       inlined implementations of mutex locking and unlocking.

       For  example,  deadlocking  on  a  double-lock  is  explicitly  allowed
       behavior  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 special 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
       Definitions 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 .