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
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.
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.
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