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 .