pthread_mutex_lock, pthread_mutex_trylock, or pthread_mutex_unlock Subroutine

Purpose

Locks and unlocks a mutex.

Library

Threads Library (libpthreads.a)

Syntax

#include 

int pthread_mutex_lock (mutex)

pthread_mutex_t *mutex;

int pthread_mutex_trylock (mutex)

pthread_mutex_t *mutex;

int pthread_mutex_unlock (mutex)

pthread_mutex_t *mutex;

Description

The mutex object referenced by mutex is locked by calling pthread_mutex_lock. If
the mutex is already locked, the calling thread blocks until the mutex becomes
available. This operation returns 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 is not 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 behaviour
results.

If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error checking is provided.
If a thread attempts to relock a mutex that it has already locked, an error will
be returned. If a thread attempts to unlock a mutex that it has not locked or a
mutex which is unlocked, an error will be returned.

If the mutex type is PTHREAD_MUTEX_RECURSIVE, then the mutex maintains the
concept of a lock count. When a thread successfully acquires a mutex for the
first time, the lock count is set to one. Every time a thread relocks this mutex,
the lock count is incremented by one. Each time the thread unlocks the mutex, the
lock count is decremented by one. When the lock count reaches zero, the mutex
becomes 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 will be
returned.

If the mutex type is PTHREAD_MUTEX_DEFAULT, attempting to recursively lock the
mutex results in undefined behaviour. Attempting to unlock the mutex if it was
not locked by the calling thread results in undefined behaviour. Attempting to
unlock the mutex if it is not locked results in undefined behaviour.

The function pthread_mutex_trylock is identical 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 returns immediately.

The pthread_mutex_unlock function releases 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 is used to determine which thread shall acquire the mutex.
(In the case of PTHREAD_MUTEX_RECURSIVE mutexes, the mutex becomes 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 resumes waiting for the mutex as if it was not
interrupted.

Parameter

mutex Specifies the mutex to lock.

Return Values

If successful, the pthread_mutex_lock and pthread_mutex_unlock functions return
zero. Otherwise, an error number is returned to indicate the error.

The function pthread_mutex_trylock returns zero if a lock on the mutex object
referenced by mutex is acquired. Otherwise, an error number is returned to
indicate the error.

Error Codes

The pthread_mutex_trylock function will fail if:

EBUSY The mutex could not be acquired because it was already locked.

The pthread_mutex_lock, pthread_mutex_trylock and pthread_mutex_unlock functions
will fail if:

EINVAL The value specified by mutex does not refer to an initialised mutex
object.

The pthread_mutex_lock function will fail if:

EDEADLK The current thread already owns the mutex and the mutex type is
pthread_mutex_errorcheck.

The pthread_mutex_unlock function will fail if:

EPERM The current thread does not own the mutex and the mutex type is not
pthread_mutex_normal.

These functions will not return an error code of EINTR.

Implementation Specifics

These subroutines are part of the Base Operating System (BOS) Runtime.

Related Information

The pthread_mutex_init and pthread_mutex_destroy subroutines, pthread.h file.

Using Mutexes and Threads Library Quick Reference in AIX Version 4.3 General
Programming Concepts: Writing and Debugging Programs.
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