Sistemi operativi - Syscall pthread_mutex_lock

Pthread mutex lock

The Single UNIX ® Specification, Version 2 Copyright © 1997 The Open Group

Name

pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock - lock and unlock a mutex

SYNOPSIS

#include <pthread.h>
int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(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 acquires the mutex.