pthread mutex 基本用法

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锁是程序中经常需要用到的机制，尤其是多线程的程序中，如果没有锁的帮助，线程间的同
步就会非常麻烦甚至不可能。pthread中提供了mutex互斥量这种锁，在 linux 下经常
用到，以下是pthread_mutex_t的相关函数介绍及简单用法。

相关函数

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#include <pthread.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int pthread_mutex_init(pthread_mutex_t *restrict mutex,
                       const pthread_mutexattr_t *restrict attr);
int pthread_mutex_destroy(pthread_mutex_t *mutex);

int pthread_mutex_lock(pthread_mutex_t *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex);
int pthread_mutex_unlock(pthread_mutex_t *mutex);

使用方法

使用mutex的基本步骤就是：

定义muutex -> 初始化mutex -> 使用mutex(lock, unlock, trylock) -> 销毁mutex。

函数名也已经把它自己的功能描述的非常清楚了。只是有一些细节需要注意：

pthread_mutex_t的初始化有两种方法，一种是使用函数pthread_mutex_init，使用结
束需要调用函数pthread_mutex_destroy进行销毁，调用时mutex必须未上锁。

It shall be safe to destroy an initialized mutex that is unlocked. Attempting
to destroy a locked mutex or a mutex that is referenced (for example, while
being used in a pthread_cond_timedwait() or pthread_cond_wait()) by another
thread results in undefined behavior.

– man pthread_thread_destroy

大意是如果mutex是上锁状态，或者被pthread_cond_timedwait()或pthread_cond_wait()
函数引用，此时对其调用pthread_mutex_destroy()结果未定义。

第二种方法是使用PTHREAD_MUTEX_INITIALIZER。根据[1]中的描述，似乎对使用这种方法
初始化的mutex调用pthread_mutex_destroy()会产生错误，对未上锁的mutex调用
pthread_mutex_unlock也会产生错误。

Mutex initialization using the PTHREAD_MUTEX_INITIALIZER does not immediately
initialize the mutex. Instead, on first use, pthread_mutex_lock() or
pthread_mutex_trylock() branches into a slow path and causes the
initialization of the mutex. Because a mutex is not just a simple memory
object and requires that some resources be allocated by the system, an attempt
to call pthread_mutex_destroy() or pthread_mutex_unlock() on a mutex that has
statically initialized using PTHREAD_MUTEX_INITIALER and was not yet locked
causes an EINVAL error.

但并没有找到太多佐证，也不知道这个系统与 Linux 上的实现是否相同。

例子程序

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#include <stdio.h>
#include <pthread.h>

/* pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; */
pthread_mutex_t mutex;

int count;

void * thread_run(void *arg)
{
    int i;
    pthread_mutex_lock(&mutex);
    for (i = 0; i < 3; i++) {
        printf("[%#lx]value of count: %d\n", pthread_self(), ++count);
    }
    pthread_mutex_unlock(&mutex);
    return 0;
}

int main(int argc, char *argv[])
{
    pthread_t thread1, thread2;
    pthread_mutex_init(&mutex, 0);
    pthread_create(&thread1, NULL, thread_run, 0);
    pthread_create(&thread2, NULL, thread_run, 0);
    pthread_join(thread1, 0);
    pthread_join(thread2, 0);
    pthread_mutex_destroy(&mutex);
    return 0;
}

如果使用了第 4 行的初始化方法，可以删除 23 和 28 行。

Reference

[1] pthread_mutex_destroy()–Destroy Mutex - IBM Knowledge Center

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