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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 (consult 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 a call to
       pthread_mutex_lock() that returns zero or [EOWNERDEAD].  If  the  mutex
       is  already  locked  by  another thread, 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 a thread attempts to relock a mutex that it has
       already  locked,  pthread_mutex_lock() shall behave as described in the
       Relock column of the following table. If a thread attempts to unlock  a
       mutex   that   it  has  not  locked  or  a  mutex  which  is  unlocked,
       pthread_mutex_unlock() shall behave as described in the Unlock When Not
       Owner column of the following table.
         +-----------+------------+----------------+-----------------------+
         |Mutex Type | Robustness |     Relock     | Unlock When Not Owner |
         +-----------+------------+----------------+-----------------------+
         |NORMAL     | non-robust | deadlock       | undefined behavior    |
         +-----------+------------+----------------+-----------------------+
         |NORMAL     | robust     | deadlock       | error returned        |
         +-----------+------------+----------------+-----------------------+
         |ERRORCHECK | either     | error returned | error returned        |
         +-----------+------------+----------------+-----------------------+
         |RECURSIVE  | either     | recursive      | error returned        |
         |           |            | (see below)    |                       |
         +-----------+------------+----------------+-----------------------+
         |DEFAULT    | non-robust | undefined      | undefined behavior-   |
         |           |            | behavior-      |                       |
         +-----------+------------+----------------+-----------------------+
         |DEFAULT    | robust     | undefined      | error returned        |
         |           |            | behavior-      |                       |
         +-----------+------------+----------------+-----------------------+
       -     If  the  mutex  type  is  PTHREAD_MUTEX_DEFAULT,  the behavior of
             pthread_mutex_lock() may correspond to one  of  the  three  other
             standard  mutex types as described in the table above. If it does
             not correspond to one of those three, the behavior  is  undefined
             for the cases marked -.
       Where  the table indicates recursive behavior, 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.
       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 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.
       If mutex is a robust mutex and the process containing the owning thread
       terminated while holding the mutex lock, a call to pthread_mutex_lock()
       shall return the error value [EOWNERDEAD].  If mutex is a robust  mutex
       and  the  owning thread terminated while holding the mutex lock, a call
       to pthread_mutex_lock() may return the error value [EOWNERDEAD] even if
       the  process  in which the owning thread resides has not terminated. In
       these cases, the mutex is locked by the thread but the  state  it  pro-
       tects is marked as inconsistent. The application should ensure that the
       state is made consistent for reuse  and  when  that  is  complete  call
       pthread_mutex_consistent().   If  the  application is unable to recover
       the state,  it  should  unlock  the  mutex  without  a  prior  call  to
       pthread_mutex_consistent(), after which the mutex is marked permanently
       unusable.
       If mutex does not refer to an initialized mutex object, the behavior of
       pthread_mutex_lock(),            pthread_mutex_trylock(),           and
       pthread_mutex_unlock() is undefined.
RETURN VALUE
       If successful, the pthread_mutex_lock(),  pthread_mutex_trylock(),  and
       pthread_mutex_unlock() functions shall return zero; otherwise, an error
       number shall be returned to indicate the error.
ERRORS
       The pthread_mutex_lock() and  pthread_mutex_trylock()  functions  shall
       fail if:
       EAGAIN The  mutex  could  not be acquired because the maximum number of
              recursive locks for mutex has been exceeded.
       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.
       ENOTRECOVERABLE
              The state protected by the mutex is not recoverable.
       EOWNERDEAD
              The mutex is a robust mutex and the process containing the  pre-
              vious owning thread terminated while holding the mutex lock. The
              mutex lock shall be acquired by the calling thread and it is  up
              to the new owner to make the state consistent.
       The pthread_mutex_lock() function shall fail if:
       EDEADLK
              The  mutex  type  is  PTHREAD_MUTEX_ERRORCHECK  and  the current
              thread already owns the mutex.
       The pthread_mutex_trylock() function shall fail if:
       EBUSY  The mutex could not be acquired because it was already locked.
       The pthread_mutex_unlock() function shall fail if:
       EPERM  The    mutex     type     is     PTHREAD_MUTEX_ERRORCHECK     or
              PTHREAD_MUTEX_RECURSIVE, or the mutex is a robust mutex, and the
              current thread does not own the mutex.
       The pthread_mutex_lock() and pthread_mutex_trylock() functions may fail
       if:
       EOWNERDEAD
              The  mutex is a robust mutex and the previous owning thread ter-
              minated while holding the mutex lock. The mutex  lock  shall  be
              acquired  by the calling thread and it is up to the new owner to
              make the state consistent.
       The pthread_mutex_lock() function may fail if:
       EDEADLK
              A deadlock condition was detected.
       These functions shall not return an error code of [EINTR].
       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       Applications that have assumed that non-zero return values  are  errors
       will  need  updating  for use with robust mutexes, since a valid return
       for a thread acquiring a mutex which is protecting a  currently  incon-
       sistent  state  is  [EOWNERDEAD].   Applications  that do not check the
       error returns, due to ruling out the possibility of such  errors  aris-
       ing,  should  not  use robust mutexes. If an application is supposed to
       work with normal and robust mutexes it should check all  return  values
       for error conditions and if necessary take appropriate action.
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.
       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.
       For  further  rationale  on the extended mutex types, see the Rationale
       (Informative) volume of POSIX.1-2008, Threads Extensions.
       If an implementation detects that the  value  specified  by  the  mutex
       argument  does  not  refer to an initialized mutex object, it is recom-
       mended that the function should fail and report an [EINVAL] error.
FUTURE DIRECTIONS
       None.
SEE ALSO
       pthread_mutex_consistent(), pthread_mutex_destroy(),
       pthread_mutex_timedlock(), pthread_mutexattr_getrobust()
       The  Base Definitions volume of POSIX.1-2008, Section 4.11, Memory Syn-
       chronization, <pthread.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013               PTHREAD_MUTEX_LOCK(3P)