PTHREAD_MUTEX(3)
NAME
pthread_mutex_init, pthread_mutex_lock, pthread_mutex_try-
lock, pthread_mutex_unlock, pthread_mutex_destroy - opera-
tions on mutexes
SYNOPSIS
#include <<pthread.h>>
pthread_mutex_t fastmutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t recmutex = PTHREAD_RECURSIVE_MUTEX_INI-
TIALIZER_NP;
pthread_mutex_t errchkmutex =
PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;
int pthread_mutex_init(pthread_mutex_t *mutex, const
pthread_mutexattr_t *mutexattr);
int pthread_mutex_lock(pthread_mutex_t *mutex));
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);
int pthread_mutex_destroy(pthread_mutex_t *mutex);
DESCRIPTION
A mutex is a MUTual EXclusion device, and is useful for
protecting shared data structures from concurrent modifi-
cations, and implementing critical sections and monitors.
A mutex has two possible states: unlocked (not owned by
any thread), and locked (owned by one thread). A mutex can
never be owned by two different threads simultaneously. A
thread attempting to lock a mutex that is already locked
by another thread is suspended until the owning thread
unlocks the mutex first.
pthread_mutex_init initializes the mutex object pointed to
by mutex according to the mutex attributes specified in
mutexattr. If mutexattr is NULL, default attributes are
used instead.
The LinuxThreads implementation supports only one mutex
attributes, the mutex kind, which is either ``fast'',
``recursive'', or ``error checking''. The kind of a mutex
determines whether it can be locked again by a thread that
already owns it. The default kind is ``fast''. See
pthread_mutexattr_init(3) for more information on mutex
attributes.
Variables of type pthread_mutex_t can also be initialized
statically, using the constants PTHREAD_MUTEX_INITIALIZER
(for fast mutexes), PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
(for recursive mutexes), and PTHREAD_ERRORCHECK_MUTEX_INI-
TIALIZER_NP (for error checking mutexes).
pthread_mutex_lock locks the given mutex. If the mutex is
currently unlocked, it becomes locked and owned by the
calling thread, and pthread_mutex_lock returns immedi-
ately. If the mutex is already locked by another thread,
pthread_mutex_lock suspends the calling thread until the
mutex is unlocked.
If the mutex is already locked by the calling thread, the
behavior of pthread_mutex_lock depends on the kind of the
mutex. If the mutex is of the ``fast'' kind, the calling
thread is suspended until the mutex is unlocked, thus
effectively causing the calling thread to deadlock. If the
mutex is of the ``error checking'' kind,
pthread_mutex_lock returns immediately with the error code
EDEADLK. If the mutex is of the ``recursive'' kind,
pthread_mutex_lock succeeds and returns immediately,
recording the number of times the calling thread has
locked the mutex. An equal number of pthread_mutex_unlock
operations must be performed before the mutex returns to
the unlocked state.
pthread_mutex_trylock behaves identically to
pthread_mutex_lock, except that it does not block the
calling thread if the mutex is already locked by another
thread (or by the calling thread in the case of a ``fast''
mutex). Instead, pthread_mutex_trylock returns immediately
with the error code EBUSY.
pthread_mutex_unlock unlocks the given mutex. The mutex is
assumed to be locked and owned by the calling thread on
entrance to pthread_mutex_unlock. If the mutex is of the
``fast'' kind, pthread_mutex_unlock always returns it to
the unlocked state. If it is of the ``recursive'' kind, it
decrements the locking count of the mutex (number of
pthread_mutex_lock operations performed on it by the call-
ing thread), and only when this count reaches zero is the
mutex actually unlocked.
On ``error checking'' mutexes, pthread_mutex_unlock actu-
ally checks at run-time that the mutex is locked on
entrance, and that it was locked by the same thread that
is now calling pthread_mutex_unlock. If these conditions
are not met, an error code is returned and the mutex
remains unchanged. ``Fast'' and ``recursive'' mutexes
perform no such checks, thus allowing a locked mutex to be
unlocked by a thread other than its owner. This is non-
portable behavior and must not be relied upon.
pthread_mutex_destroy destroys a mutex object, freeing the
resources it might hold. The mutex must be unlocked on
entrance. In the LinuxThreads implementation, no resources
are associated with mutex objects, thus
pthread_mutex_destroy actually does nothing except check-
ing that the mutex is unlocked.
CANCELLATION
None of the mutex functions is a cancellation point, not
even pthread_mutex_lock, in spite of the fact that it can
suspend a thread for arbitrary durations. This way, the
status of mutexes at cancellation points is predictable,
allowing cancellation handlers to unlock precisely those
mutexes that need to be unlocked before the thread stops
executing. Consequently, threads using deferred cancella-
tion should never hold a mutex for extended periods of
time.
ASYNC-SIGNAL SAFETY
The mutex functions are not async-signal safe. What this
means is that they should not be called from a signal han-
dler. In particular, calling pthread_mutex_lock or
pthread_mutex_unlock from a signal handler may deadlock
the calling thread.
RETURN VALUE
pthread_mutex_init always returns 0. The other mutex func-
tions return 0 on success and a non-zero error code on
error.
ERRORS
The pthread_mutex_lock function returns the following
error code on error:
EINVAL the mutex has not been properly initialized.
EDEADLK
the mutex is already locked by the calling
thread (``error checking'' mutexes only).
The pthread_mutex_trylock function returns the following
error codes on error:
EBUSY the mutex could not be acquired because it
was currently locked.
EINVAL the mutex has not been properly initialized.
The pthread_mutex_unlock function returns the following
error code on error:
EINVAL the mutex has not been properly initialized.
EPERM the calling thread does not own the mutex
(``error checking'' mutexes only).
The pthread_mutex_destroy function returns the following
error code on error:
EBUSY the mutex is currently locked.
AUTHOR
Xavier Leroy lt;Xavier.Leroy@inria.fr
SEE ALSO
pthread_mutexattr_init(3) pthread_mutex-
attr_setkind_np(3) pthread_cancel(3).
EXAMPLE
A shared global variable x can be protected by a mutex as
follows:
int x;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
All accesses and modifications to x should be bracketed by
calls to pthread_mutex_lock and pthread_mutex_unlock as
follows:
pthread_mutex_lock(&mut);
/* operate on x */
pthread_mutex_unlock(&mut);