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前面我們說(shuō)了線程的創(chuàng)建和撤銷，這里我們說(shuō)一下線程間的同步的問(wèn)題。

當(dāng)同一個(gè)進(jìn)程中存在多個(gè)線程的時(shí)候，多個(gè)線程共享相同的內(nèi)存，確保每個(gè)線程能夠看到一致的數(shù)據(jù)視圖，如果每個(gè)線程中都不會(huì)讀取或修改共同享有的變量，就不會(huì)存在一致性的問(wèn)題，同樣如果共享變量時(shí)只讀的也就不會(huì)存在這個(gè)問(wèn)題。但是，當(dāng)某個(gè)線程可一個(gè)修改變量，而其他的線程去讀取或修改這個(gè)變量的時(shí)候，就需要進(jìn)行線程間的同步，確保他們?cè)L問(wèn)變量的內(nèi)容時(shí)不會(huì)訪問(wèn)到無(wú)效的數(shù)據(jù)。

這里介紹一種實(shí)現(xiàn)同步的方法：互斥量

互斥鎖本質(zhì)上是一把鎖，在訪問(wèn)共享資源的時(shí)候?qū)�互斥量進(jìn)行加鎖，訪問(wèn)結(jié)束后解鎖。在這里我們說(shuō)一下如何去操作互斥鎖。

1、 創(chuàng)建和撤銷

互斥量用pthread_mutex_t數(shù)據(jù)類型來(lái)表示，在使用之前必須對(duì)其進(jìn)行初始化，用完之后釋放內(nèi)存。互斥量初始化可以用PTHREAD_MUTEX_INITIALIZER來(lái)初始化(靜態(tài)初始化)，亦可以使用pthread_mutex_init函數(shù)來(lái)實(shí)現(xiàn)，這種方法動(dòng)態(tài)的為互斥量分配內(nèi)存，使用后必須使用pthread_mutex_destroy來(lái)釋放內(nèi)存單元。下面是這些函數(shù)的原型：
        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);

2、 鎖操作

鎖操作主要包括加鎖pthread_mutex_lock()、解鎖pthread_mutex_unlock()和測(cè)試鎖pthread_mutex_trylock()三個(gè)。通過(guò)pthread_mutex_lock對(duì)互斥量加鎖，這里需要獲得鎖，如果無(wú)法獲得鎖則調(diào)用線程將阻塞到其他線程調(diào)用pthread_mutex_unlock對(duì)互斥量解鎖。
        int pthread_mutex_lock(pthread_mutex_t *mutex);
        int pthread_mutex_trylock(pthread_mutex_t *mutex);
        int pthread_mutex_unlock(pthread_mutex_t *mutex);

如果線程不希望被阻塞，則可以調(diào)用pthread_mutex_trylock嘗試對(duì)互斥量進(jìn)行加鎖，當(dāng)互斥量沒(méi)有被加鎖，則函數(shù)返回0，并鎖住互斥量，否則會(huì)失敗，返回EBUSY.

3、 示例

下面我們用一個(gè)例程說(shuō)明一下這些函數(shù)的使用。

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

pthread_mutex_t mutex;

void *thread_a(void *arg)
        {
                printf("thread a enter\n");
                pthread_mutex_lock(&mutex);
                printf("mutex lock\n");
                sleep(10);
                pthread_mutex_unlock(&mutex);
                printf("mutex unlock\n");
        }

void *thread_b(void *arg)
        {
                printf("thread b enter\n");
                while(pthread_mutex_trylock(&mutex))
                {
                        printf("pthread trylock\n");
                        sleep(1);
                }
                printf("mutex lock\n");
                pthread_mutex_unlock(&mutex);
                printf("mutex unlock\n");
        }

int main(int argc, char **argv)
        {
                pthread_t tid_a,tid_b;
                int err;

        if(pthread_mutex_init(&mutex, NULL) != 0)
                {
                        perror("pthread_mutex_init");
                }

        err = pthread_create(&tid_a,NULL,thread_a,NULL);
                if(err < 0)
                {
                        perror("pthread_create thread_a");
                }
                sleep(1);
                err = pthread_create(&tid_b,NULL,thread_b,NULL);
                if(err < 0)
                {
                        perror("pthread_create thread_a");
                }

        sleep(20);
                printf("the main close\n");
                return 0;
        }

結(jié)果：
        thread a enter
        mutex lock
        thread b enter
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        pthread trylock
        mutex unlock
        mutex lock
        mutex unlock
        the main close

由這里的結(jié)果可以看出mutex的用處及幾個(gè)相關(guān)函數(shù)的使用方法。