线程同步-信号量(semaphore)
生产者与消费者问题再思考
在实际生活中，只要有商品，消费者就可以消费，这没问题。但生产者的生产并不是无限的，例如，仓库是有限的，原材料是有限的，生产指标受消费指标限制等等。为了进一步，解决好生产者与消费者问题，引入信号量进机制。
信号量
信号量(semaphore)是互斥量的升级版：互斥量的状态为0或1，而信号量可以为n。也就是说，使用互斥量时，最多允许一个线程进入关键区，而信号量允许多个，具体值是信号量当前的内部值。
相关函数
sem_t //信号量类型sem_init(sem_t *sem, int pshared, unsigned int value);重要的是理解：sem_wait和sem_post两个函数。
sem_wait(sem_t *sem)
sem_trywait
sem_timedwait
sem_post(sem_t *sem)
sem_destroy
sem_wait(sem);当sem为零时，线程阻塞；否则，sem减一，线程不阻塞。
sem_post(sem);sem加一。
此外，使用sem_init方法，对信号量类型初始化，第二个参数，默认是0，标明用于线程之间。第三个参数指定了初始值。
单生产者与单消费者
#include <stdio.h>#include <unistd.h>#include <pthread.h>#include <semaphore.h>#define NUM 5sem_t blank_num, product_num;int i, j, k;int goods[NUM];void *producer(void *argv){while (1){sem_wait(&blank_num);goods[i] = rand() % 100 + 1;printf("produce %d\n", goods[i]);sem_post(&product_num);i = (i + 1) % NUM;sleep(rand() % 2);}}void *comsumer(void *argv){while (1){sem_wait(&product_num);printf("comsume %d\n", goods[j]);goods[j] = 0;sem_post(&blank_num);j = (j + 1) % NUM;sleep(rand() % 2);}}int main(void){i = j = k = 0;//初始化信号量sem_init(&blank_num, 0, NUM);sem_init(&product_num, 0, 0);pthread_t pro, com;pthread_create(&com, NULL, producer, NULL);pthread_create(&pro, NULL, comsumer, NULL);pthread_join(com, NULL);pthread_join(pro, NULL);sem_destroy(&blank_num);sem_destroy(&product_num);return 0;}
多生产者与多消费者
#include <stdio.h>#include <unistd.h>#include <pthread.h>#include <semaphore.h>#define NUM 5pthread_mutex_t m1, m2;sem_t blank_num, product_num;int goods[NUM];int i, j, k;void *producer(void *argv){while (1){sem_wait(&blank_num);pthread_mutex_lock(&m1);goods[i] = rand() % 100 + 1;printf("produce %d\n", goods[i]);i = (i + 1) % NUM;pthread_mutex_unlock(&m1);sem_post(&product_num);sleep(rand() % 2);}}void *comsumer(void *argv){while (1){sem_wait(&product_num);pthread_mutex_lock(&m2);printf("comsume %d\n", goods[j]);goods[j] = 0; //置零j = (j + 1) % NUM;pthread_mutex_unlock(&m2);sem_post(&blank_num);sleep(rand() % 2);}}int main(void){i = j = k = 0;//初始化信号量及互斥量sem_init(&blank_num, 0, NUM);sem_init(&product_num, 0, 0);pthread_mutex_init(&m1, NULL);pthread_mutex_init(&m2, NULL);pthread_t pro[2], com[3];for (k = 0; k < 3; k++)pthread_create(&com[k], NULL, producer, NULL);for (k = 0; k < 2; k++)pthread_create(&pro[k], NULL, comsumer, NULL);for (k = 0; k < 3; k++)pthread_join(com[k], NULL);for (k = 0; k < 2; k++)pthread_join(pro[k], NULL);pthread_mutex_destroy(&m1);pthread_mutex_destroy(&m2);sem_destroy(&blank_num);sem_destroy(&product_num);return 0;}
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