mutex(5) Standards, Environments, and Macros mutex(5)NAMEmutex - concepts relating to mutual exclusion locks
DESCRIPTION
Mutual exclusion locks (mutexes) prevent multiple threads from simulta‐
neously 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 ini‐
tialized; by default, a statically allocated mutex is initialized 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 exclu‐
sive 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 suc‐
cessful 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 perfor‐
mance 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 ALSOmmap(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.
The system does not support multiple mappings to the same logical synch
object if it is initialized as process-private (USYNC_THREAD for
Solaris, PTHREAD_PROCESS_PRIVATE for POSIX). If you need to mmap(2)a
synch object to different locations within the same address space, then
the synch object should be initialized as a shared object
(USYNC_PROCESS for Solaris, PTHREAD_PROCESS_SHARED for POSIX).
SunOS 5.10 5 Jun 2007 mutex(5)
