信號量和互斥鎖(mutex)的區別:互斥鎖只允許一個線程進入臨界區,而信號量允許多個線程同時進入臨界區。
不多做解釋,要使用信號量同步,需要包含頭文件semaphore.h。
主要用到的函數:
下列的代碼演示了如何用信號量同步,模擬一個窗口服務系統。
/* @purpose: 基於信號量的多線程同步,操作系統原理中的P,V操作
* @author: [email protected]
* @create: 2015-03-20 Fri
* */
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
/* @Scene: 某行業營業廳同時只能服務兩個顧客。
* 有多個顧客到來,每個顧客如果發現服務窗口已滿,就等待,
* 如果有可用的服務窗口,就接受服務。 */
/* 將信號量定義為全局變量,方便多個線程共享 */
sem_t sem;
/* 每個線程要運行的例程 */
void * get_service(void *thread_id)
{
/* 注意:立即保存thread_id的值,因為thread_id是對主線程中循環變量i的引用,它可能馬上被修改 */
int customer_id = *((int *)thread_id);
if(sem_wait(&sem) == 0) {
usleep(100); /* service time: 100ms */
printf("customer %d receive service ...\n", customer_id);
sem_post(&sem);
}
}
#define CUSTOMER_NUM 10
int main(int argc, char *argv[])
{
/* 初始化信號量,初始值為2,表示有兩個顧客可以同時接收服務 */
/* @prototype: int sem_init(sem_t *sem, int pshared, unsigned int value); */
/* pshared: if pshared == 0, the semaphore is shared among threads of a process
* otherwise the semaphore is shared between processes. */
sem_init(&sem, 0, 2);
/* 為每個顧客定義一個線程id, pthread_t 其實是unsigned long int */
pthread_t customers[CUSTOMER_NUM];
int i, ret;
/* 為每個顧客生成一個線程 */
for(i = 0; i < CUSTOMER_NUM; i++){
int customer_id = i;
ret = pthread_create(&customers[i], NULL, get_service, &customer_id);
if(ret != 0){
perror("pthread_create");
exit(1);
}
else {
printf("Customer %d arrived.\n", i);
}
usleep(10);
}
/* 等待所有顧客的線程結束 */
/* 注意:這地方不能再用i做循環變量,因為可能線程中正在訪問i的值 */
int j;
for(j = 0; j < CUSTOMER_NUM; j++) {
pthread_join(customers[j], NULL);
}
/* Only a semaphore that has been initialized by sem_init(3)
* should be destroyed using sem_destroy().*/
sem_destroy(&sem);
return 0;
}
編譯:gcc main.c -lpthread。
運行結果(注意,每次運行都不相同):
Customer 0 arrived.
Customer 1 arrived.
customer 0 receive service ...
Customer 2 arrived.
customer 1 receive service ...
Customer 3 arrived.
customer 2 receive service ...
Customer 4 arrived.
customer 3 receive service ...
Customer 5 arrived.
customer 4 receive service ...
Customer 6 arrived.
customer 5 receive service ...
Customer 7 arrived.
customer 6 receive service ...
Customer 8 arrived.
customer 7 receive service ...
Customer 9 arrived.
customer 8 receive service ...
customer 9 receive service ...