pthread_mutex_lock(3p) — Linux manual page

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.1‐2017, 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         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.1‐2017, Section 4.12, Memory
       Synchronization, pthread.h(0p)

COPYRIGHT         top

       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:https://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(3P)

Pages that refer to this page: pthread.h(0p)pthread_mutexattr_getrobust(3p)pthread_mutexattr_gettype(3p)pthread_mutex_consistent(3p)pthread_mutex_destroy(3p)pthread_mutex_getprioceiling(3p)pthread_mutex_timedlock(3p)pthread_mutex_trylock(3p)