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 .