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[Linux]多线程同步之sem_wait()学习笔记

2016-11-13 20:03 543 查看
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 5
int 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;
}


运行结果：



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 5
int 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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标签：  多线程 semaphore sem_wait 线程同步 linux
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