PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock —
       lock and unlock a mutex

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       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
       recommended that the function should fail and report an [EINVAL]
       error.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       pthread_mutex_consistent(3p), pthread_mutex_destroy(3p),
       pthread_mutex_timedlock(3p), pthread_mutexattr_getrobust(3p)

       The Base Definitions volume of  POSIX.1‐2008,  Section  4.11,  Memory
       Synchronization, pthread.h(0p)

COPYRIGHT         top

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

IEEE/The Open Group                 2013              PTHREAD_MUTEX_LOCK(3P)