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.TH MUTEX 5 "Jun 5, 2007"
.SH NAME
mutex \- concepts relating to mutual exclusion locks
.SH DESCRIPTION
.sp
.LP
Mutual exclusion locks (mutexes) prevent multiple threads from simultaneously
executing critical sections of code which access shared data (that is, mutexes
are used to serialize the execution of threads). All mutexes must be global. A
successful call to acquire a mutex will cause another thread that is also
trying to lock the same mutex to block until the owner thread unlocks the
mutex.
.sp
.LP
Mutexes can synchronize threads within the same process or in other processes.
Mutexes can be used to synchronize threads between processes if the mutexes are
allocated in writable memory and shared among the cooperating processes (see
\fBmmap\fR(2)), and have been initialized for this task.
.sp
.LP
The following table lists mutex functions and the actions they perform.
.sp
.sp
.TS
box;
c | c
l | l .
FUNCTION ACTION
_
\fBmutex_init\fR Initialize a mutex.
\fBmutex_destroy\fR Destroy a mutex.
\fBmutex_lock\fR Lock a mutex.
\fBmutex_trylock\fR Attempt to lock a mutex.
\fBmutex_unlock\fR Unlock a mutex.
\fBpthread_mutex_init\fR Initialize a mutex.
\fBpthread_mutex_destroy\fR Destroy a mutex.
\fBpthread_mutex_lock\fR Lock a mutex.
\fBpthread_mutex_trylock\fR Attempt to lock a mutex.
\fBpthread_mutex_unlock\fR Unlock a mutex.
.TE
.SS "Initialization"
.sp
.LP
Mutexes are either intra-process or inter-process, depending upon the argument
passed implicitly or explicitly to the initialization of that mutex. A
statically allocated mutex does not need to be explicitly initialized; by
default, a statically allocated mutex is initialized with all zeros and its
scope is set to be within the calling process.
.sp
.LP
For inter-process synchronization, a mutex needs to be allocated in memory
shared between these processes. Since the memory for such a mutex must be
allocated dynamically, the mutex needs to be explicitly initialized with the
appropriate attribute that indicates inter-process use.
.SS "Locking and Unlocking"
.sp
.LP
A critical section of code is enclosed by a call to lock the mutex and the call
to unlock the mutex to protect it from simultaneous access by multiple threads.
Only one thread at a time may possess mutually exclusive access to the critical
section of code that is enclosed by the mutex-locking call and the
mutex-unlocking call, whether the mutex's scope is intra-process or
inter-process. A thread calling to lock the mutex either gets exclusive access
to the code starting from the successful locking until its call to unlock the
mutex, or it waits until the mutex is unlocked by the thread that locked it.
.sp
.LP
Mutexes have ownership, unlike semaphores. Only the thread that locked a mutex,
(that is, the owner of the mutex), should unlock it.
.sp
.LP
If a thread waiting for a mutex receives a signal, upon return from the signal
handler, the thread resumes waiting for the mutex as if there was no interrupt.
.SS "Caveats"
.sp
.LP
Mutexes are almost like data - they can be embedded in data structures, files,
dynamic or static memory, and so forth. Hence, they are easy to introduce into
a program. However, too many mutexes can degrade performance and scalability of
the application. Because too few mutexes can hinder the concurrency of the
application, they should be introduced with care. Also, incorrect usage (such
as recursive calls, or violation of locking order, and so forth) can lead to
deadlocks, or worse, data inconsistencies.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp
.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE ATTRIBUTE VALUE
_
MT-Level MT-Safe
.TE
.SH SEE ALSO
.sp
.LP
\fBmmap\fR(2), \fBshmop\fR(2), \fBmutex_destroy\fR(3C), \fBmutex_init\fR(3C),
\fBmutex_lock\fR(3C), \fBmutex_trylock\fR(3C), \fBmutex_unlock\fR(3C),
\fBpthread_create\fR(3C), \fBpthread_mutex_destroy\fR(3C),
\fBpthread_mutex_init\fR(3C), \fBpthread_mutex_lock\fR(3C),
\fBpthread_mutex_trylock\fR(3C), \fBpthread_mutex_unlock\fR(3C),
\fBpthread_mutexattr_init\fR(3C), \fBattributes\fR(5), \fBstandards\fR(5)
.SH NOTES
.sp
.LP
In the current implementation of threads, \fBpthread_mutex_lock()\fR,
\fBpthread_mutex_unlock()\fR, \fBmutex_lock()\fR \fBmutex_unlock()\fR,
\fBpthread_mutex_trylock()\fR, and \fBmutex_trylock()\fR do not validate the
mutex type. Therefore, an uninitialized mutex or a mutex with an invalid type
does not return \fBEINVAL\fR. Interfaces for mutexes with an invalid type have
unspecified behavior.
.sp
.LP
By default, if multiple threads are waiting for a mutex, the order of
acquisition is undefined.
.sp
.LP
The system does not support multiple mappings to the same logical synch object
if it is initialized as process-private (\fBUSYNC_THREAD\fR for Solaris,
\fBPTHREAD_PROCESS_PRIVATE\fR for POSIX). If you need to \fBmmap\fR(2)a synch
object to different locations within the same address space, then the synch
object should be initialized as a shared object (\fBUSYNC_PROCESS\fR for
Solaris, \fBPTHREAD_PROCESS_SHARED\fR for POSIX).
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