[Linux]多线程同步之sem_wait()学习笔记

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1、semaphore 的这种信号量不仅可用于同一进程的线程同步，也可以用于不同进程间同步。

一个生产者-消费者例子：生产者不停的向一个固定大小的环形队列中添加数据，消费者从环形队列中清零数据，如果生产者积累的数据大于环形队列长度，则等待消费者清除数据有空位后再生产。

#include <stdlib.h>#include <pthread.h>#include <stdio.h>#include <semaphore.h>#include <unistd.h>#define NUM 5int queue[NUM];sem_t blank_number, product_number;void *producer(void *arg){    int p = 0;    while(1){        sem_wait(&blank_number);        queue[p] = rand() % 1000 + 1;        printf("produce %d\n", queue[p]);        p = (p + 1) % NUM;        sem_post(&product_number);        sleep(1);        //sleep(rand()%5);    }    }void *consumer(void *arg){    int c = 0, i;    while(1) {        sem_wait(&product_number);        for(i=0; i < NUM; i++) {            printf("%d ", queue[i]);        }        putchar('\n');        printf("consume %d\n", queue[c]);        queue[c] = 0;        sem_post(&blank_number);        c = (c+1)%NUM;           sleep(3);        //sleep(rand()%5);    }}int main(){    pthread_t pid, cid;        // 可用资源数为 NUM = 5    sem_init(&blank_number, 0, NUM);    sem_init(&product_number, 0, 0);    pthread_create(&pid, NULL, producer, NULL);    pthread_create(&cid, NULL, consumer, NULL);    pthread_join(pid, NULL);    pthread_join(cid, NULL);    sem_destroy(&blank_number);    sem_destroy(&product_number);    return 0;}

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2、用Condition Variable实现Semaphore ，即用pthread_cond_wait()等方式实现sem_wait()等方式。

#include <stdlib.h>#include <pthread.h>#include <stdio.h>#include <unistd.h>#define NUM 5int queue[NUM];int current_num; pthread_cond_t has_product = PTHREAD_COND_INITIALIZER;pthread_cond_t blank_product = PTHREAD_COND_INITIALIZER;pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;void *producer(void *arg){    int p = 0, i;    while(1){                pthread_mutex_lock(&lock);        while(current_num >= NUM) {            pthread_cond_wait(&blank_product, &lock);        }        queue[p] = rand() % 1000 + 1;        printf("produce %d\n", queue[p]);        p = (p + 1) % NUM;        current_num ++;        pthread_mutex_unlock(&lock);                printf("after produce --- current_num: %d\n", current_num);        for(i=0; i < NUM; i++) {            printf("%d ", queue[i]);        }        putchar('\n');        sleep(1);       //sleep(rand()%5);               pthread_cond_signal(&has_product);           }    }void *consumer(void *arg){    int c = 0, i;    while(1) {                       pthread_mutex_lock(&lock);        pthread_cond_wait(&has_product, &lock);        printf("consume %d\n", queue[c]);                queue[c] = 0;        c = (c+1)%NUM;           current_num--;        printf("after consume --- current_num: %d\n", current_num);        for(i=0; i < NUM; i++) {            printf("%d ", queue[i]);        }        putchar('\n');            pthread_mutex_unlock(&lock);               sleep(3);        //sleep(rand()%5);                pthread_cond_signal(&blank_product);    }}int main(){    pthread_t pid, cid;    pthread_create(&pid, NULL, producer, NULL);    pthread_create(&cid, NULL, consumer, NULL);    pthread_join(pid, NULL);    pthread_join(cid, NULL);    printf("----\n");    return 0;}

运行结果：

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