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



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

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
       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.

       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
       protects 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
              previous 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
              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 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
       inconsistent state is [EOWNERDEAD].  Applications that do not check the
       error returns, due to ruling out the possibility of such errors
       arising, 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.1-2017, Threads Extensions.

       If an implementation detects that the value specified by the mutex
       argument does not refer to an initialized mutex object, it is
       recommended 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.1-2017, Section 4.12, Memory
       Synchronization, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                PTHREAD_MUTEX_LOCK(3)
