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 .

IEEE/The Open Group                                   2003                                 PTHREAD_MUTEX_LOCK(P)