pthread 条件变量(condition variable) 的基本用法

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用法在我翻译的一篇问答里已经说得比较清楚了，这里只是给出我写的一个示例程序。文章
地址在这里。

例子中有两个线程为消费者，一个线程为生产者。在那篇问答中回答者给出了一个模型，但
人尝试的时候发现将获取和释放互斥量放在循环外会有问题，参考网上其他一些程序经过思
考后将其放在了循环内，程序如下：

例子程序

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

int loop = 5;
int count = 3;
int has_work = 1;
pthread_mutex_t lock;
pthread_cond_t  cond;
void *increase(void *arg);
void *decrease(void *arg);

int main(int argc, char *argv[])
{
    pthread_t thread_dec1, thread_dec2, thread_inc;
    pthread_mutex_init(&lock, 0);
    pthread_cond_init(&cond, 0);
    int has_work = 1;

    pthread_create(&thread_inc,  0, increase, 0);
    pthread_create(&thread_dec1, 0, decrease, 0);
    pthread_create(&thread_dec2, 0, decrease, 0);

    pthread_join(thread_inc,  0);
    pthread_join(thread_dec1, 0);
    pthread_join(thread_dec2, 0);

    pthread_cond_destroy(&cond);
    pthread_mutex_destroy(&lock);
    return 0;
}

void *increase(void *arg)
{
    while (1) {
        sleep(1);
        pthread_mutex_lock(&lock);

        count += 3;
        printf("thread [%#lx] increased count[%d]\n", pthread_self(), count);
        if (--loop <= 0) {
            has_work = 0;
            pthread_cond_broadcast(&cond);
            pthread_mutex_unlock(&lock);
            break;
        }

        pthread_cond_broadcast(&cond);
        pthread_mutex_unlock(&lock);
    }
    /* printf("increase exited\n"); */
    return 0;
}

void *decrease(void *arg)
{
    while (has_work) {
        /* printf("decrease into loop\n"); */
        pthread_mutex_lock(&lock);
        pthread_cond_wait(&cond, &lock);
        if (count > 0) {
            --count;
            printf("thread [%ld] decreased count[%d]\n", pthread_self(), count);
        }
        pthread_mutex_unlock(&lock);
    }
    /* printf("decrease exited\n"); */
    return 0;
}

References

条件变量pthread_cond_t怎么用
Linux多线程编程详细解析—-条件变量 pthread_cond_t

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