mutex(5)						Standards, Environments, and Macros						  mutex(5)

NAME

       mutex - concepts relating to mutual exclusion locks

DESCRIPTION

       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.

       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 mmap(2)), and  have  been  initialized  for
       this task.

       The following table lists mutex functions and the actions they perform.

       +-----------------------+-----------------------------------+
       |   FUNCTION	       |	      ACTION		   |
       |mutex_init	       | Initialize a mutex.		   |
       |mutex_destroy	       | Destroy a mutex.		   |
       |mutex_lock	       | Lock a mutex.			   |
       |mutex_trylock	       | Attempt to lock a mutex.	   |
       |mutex_unlock	       | Unlock a mutex.		   |
       |pthread_mutex_init     | Initialize a mutex.		   |
       |pthread_mutex_destroy  | Destroy a mutex.		   |
       |pthread_mutex_lock     | Lock a mutex.			   |
       |pthread_mutex_trylock  | Attempt to lock a mutex.	   |
       |pthread_mutex_unlock   | Unlock a mutex.		   |
       +-----------------------+-----------------------------------+

   Initialization
       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  initial-
       ized with all zeros and its scope is set to be within the calling process.

       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.

   Locking and Unlocking
       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.

       Mutexes have ownership, unlike semaphores. Only the thread that locked a mutex, (that is, the owner of the mutex), should unlock it.

       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.

   Caveats
       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.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       mmap(2),   shmop(2),   mutex_destroy(3C),   mutex_init(3C),   mutex_lock(3C),   mutex_trylock(3C),   mutex_unlock(3C),  pthread_create(3C),
       pthread_mutex_destroy(3C),    pthread_mutex_init(3C),	pthread_mutex_lock(3C),    pthread_mutex_trylock(3C),	 pthread_mutex_unlock(3C),
       pthread_mutexattr_init(3C), attributes(5), standards(5)

NOTES

       In  the	current  implementation  of threads, pthread_mutex_lock(), pthread_mutex_unlock(), mutex_lock() mutex_unlock(), pthread_mutex_try-
       lock(), and mutex_trylock() do not validate the mutex type. Therefore, an uninitialized mutex or a mutex with  an  invalid  type  does  not
       return EINVAL. Interfaces for mutexes with an invalid type have unspecified behavior.

       By default, if multiple threads are waiting for a mutex, the order of acquisition is undefined.

       USYNC_THREAD  does  not support multiple mappings to the same logical synch object. If you need to mmap() a synch object to different loca-
       tions within the same address space, then the synch object should be  initialized  as  a  shared  object  USYNC_PROCESS	for  Solaris,  and
       PTHREAD_PROCESS_PRIVATE for POSIX.

SunOS 5.10							    20 Jul 1998 							  mutex(5)