NAME¶
pthread_mutex_init, pthread_mutex_lock, pthread_mutex_trylock,
pthread_mutex_unlock, pthread_mutex_destroy - operations on mutexes
SYNOPSIS¶
#include <pthread.h>
pthread_mutex_t fastmutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t recmutex =
PTHREAD_RECURSIVE_MUTEX_INITIALIZER_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 modifications, 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_INITIALIZER_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 immediately. 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 calling
thread), and only when this count reaches zero is the mutex actually unlocked.
On ``error checking'' and ``recursive'' mutexes,
pthread_mutex_unlock
actually 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'' 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 checking 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 cancellation 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 handler. 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 functions 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 <Xavier.Leroy@inria.fr>
SEE ALSO¶
pthread_mutexattr_init(3),
pthread_mutexattr_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);
