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PTHREAD_MUTEX_LOCK
Section: POSIX Programmer's Manual (3P)
Updated: 2003
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PROLOG
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 calling
pthread_mutex_lock(). If the mutex is already
locked, 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 the mutex type is PTHREAD_MUTEX_NORMAL, deadlock detection shall
not be provided. Attempting to relock the mutex causes
deadlock. If a thread attempts to unlock a mutex that it has not locked
or a mutex which is unlocked, undefined behavior
results.
If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking
shall be provided. If a thread attempts to relock a mutex
that it has already locked, an error shall be returned. If a thread
attempts to unlock a mutex that it has not locked or a mutex
which is unlocked, an error shall be returned.
If the mutex type is PTHREAD_MUTEX_RECURSIVE, then 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. If a thread attempts to unlock a mutex
that it has not locked or a mutex which is unlocked, an error shall
be returned.
If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively
lock the mutex results in undefined behavior. Attempting
to unlock the mutex if it was not locked by the calling thread results
in undefined behavior. Attempting to unlock the mutex if it
is not locked results in undefined behavior.
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.
RETURN VALUE
If successful, the pthread_mutex_lock() and pthread_mutex_unlock()
functions shall return zero; otherwise, an
error number shall be returned to indicate the error.
The pthread_mutex_trylock() function shall return zero if a
lock on the mutex object referenced by mutex is
acquired. Otherwise, an error number is returned to indicate the error.
ERRORS
The pthread_mutex_lock() and pthread_mutex_trylock() functions
shall fail if:
- 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.
The pthread_mutex_trylock() function shall fail if:
- EBUSY
-
The mutex could not be acquired because it was already locked.
The pthread_mutex_lock(), pthread_mutex_trylock(), and
pthread_mutex_unlock() functions may fail if:
- EINVAL
-
The value specified by mutex does not refer to an initialized
mutex object.
- EAGAIN
-
The mutex could not be acquired because the maximum number of recursive
locks for mutex has been exceeded.
The pthread_mutex_lock() function may fail if:
- EDEADLK
-
The current thread already owns the mutex.
The pthread_mutex_unlock() function may fail if:
- EPERM
-
The current thread does not own the mutex.
These functions shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
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.
For example, deadlocking on a double-lock is explicitly allowed behavior
in order to avoid requiring more overhead in the basic
mechanism than is absolutely necessary. (More "friendly" mutexes that
detect deadlock or that allow multiple locking by the same
thread are easily constructed by the user via the other mechanisms
provided. For example, pthread_self() can be used to record
mutex ownership.) Implementations might also
choose to provide such extended features as options via special mutex
attributes.
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.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_mutex_destroy(), pthread_mutex_timedlock(),
the Base Definitions volume of
IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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