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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PTHREAD_MUTEX_LOCK(3P) POSIX Programmer's Manual PTHREAD_MUTEX_LOCK(3P)
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.
pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock —
lock and unlock a mutex
#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);
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.
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.
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.
None.
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.
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.
None.
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)
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(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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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