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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 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.1‐2008, 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
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 Synchronization,
<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 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 .