PTHREAD_MUTEX_LOCK(3P)                              POSIX Programmer's Manual                             PTHREAD_MUTEX_LOCK(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

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  refer-
       enced 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 decre-
       mented 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 refer-
       enced 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 num-
       ber 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 fea-
       tures 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  lock-
       ing  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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                        PTHREAD_MUTEX_LOCK(3P)