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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "PTHREAD_MUTEX_LOCK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_mutex_lock
.SH NAME
pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock \-
lock and unlock a mutex
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
int pthread_mutex_lock(pthread_mutex_t *\fP\fImutex\fP\fB);
.br
int pthread_mutex_trylock(pthread_mutex_t *\fP\fImutex\fP\fB);
.br
int pthread_mutex_unlock(pthread_mutex_t *\fP\fImutex\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The mutex object referenced by \fImutex\fP shall be locked by calling
\fIpthread_mutex_lock\fP(). 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 \fImutex\fP in the locked state with the calling thread
as its owner.
.LP
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.
.LP
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.
.LP
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.
.LP
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.
.LP
The \fIpthread_mutex_trylock\fP() function shall be equivalent to
\fIpthread_mutex_lock\fP(), except that if the mutex object
referenced by \fImutex\fP 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 \fIpthread_mutex_trylock\fP() function
shall immediately return success.
.LP
The \fIpthread_mutex_unlock\fP() function shall release the mutex
object referenced by \fImutex\fP. \ 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 \fImutex\fP
when \fIpthread_mutex_unlock\fP() is
called, resulting in the mutex becoming available, the scheduling
policy shall determine which thread shall acquire the mutex.
.LP
(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.)
.LP
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.
.SH RETURN VALUE
.LP
If successful, the \fIpthread_mutex_lock\fP() and \fIpthread_mutex_unlock\fP()
functions shall return zero; otherwise, an
error number shall be returned to indicate the error.
.LP
The \fIpthread_mutex_trylock\fP() function shall return zero if a
lock on the mutex object referenced by \fImutex\fP is
acquired. Otherwise, an error number is returned to indicate the error.
.SH ERRORS
.LP
The \fIpthread_mutex_lock\fP() and \fIpthread_mutex_trylock\fP() functions
shall fail if:
.TP 7
.B EINVAL
The \fImutex\fP 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.
.sp
.LP
The \fIpthread_mutex_trylock\fP() function shall fail if:
.TP 7
.B EBUSY
The \fImutex\fP could not be acquired because it was already locked.
.sp
.LP
The \fIpthread_mutex_lock\fP(), \fIpthread_mutex_trylock\fP(), and
\fIpthread_mutex_unlock\fP() functions may fail if:
.TP 7
.B EINVAL
The value specified by \fImutex\fP does not refer to an initialized
mutex object.
.TP 7
.B EAGAIN
The mutex could not be acquired because the maximum number of recursive
locks for \fImutex\fP has been exceeded.
.sp
.LP
The \fIpthread_mutex_lock\fP() function may fail if:
.TP 7
.B EDEADLK
The current thread already owns the mutex.
.sp
.LP
The \fIpthread_mutex_unlock\fP() function may fail if:
.TP 7
.B EPERM
The current thread does not own the mutex.
.sp
.LP
These functions shall not return an error code of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
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.
.LP
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.
.LP
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, \fIpthread_self\fP() can be used to record
mutex ownership.) Implementations might also
choose to provide such extended features as options via special mutex
attributes.
.LP
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.
.LP
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
\fImutex_tryunlock\fP operation.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpthread_mutex_destroy\fP() , \fIpthread_mutex_timedlock\fP() ,
the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP
.SH 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 .
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