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linux 下用posix semaphore 解决资源竞争问题实例

news 2025/8/26 21:14:01

/* author: hjjdebug
date: 2023年 09月 20日星期三 09:33:58 CST
description: 10辆汽车通过承重5辆汽车的桥,处理一个资源争用问题
* 10个线程代表10辆汽车
* 桥上只能承载5辆汽车, 代表最大只能同时有5辆汽车通过
概要:
让10个线程竞争5个资源,用posix 接口, semaphore 来实现
1. sem_init(信号量,num) 初始化资源数
2. 每个线程要 sem_wait(信号量) 申请资源,申请不到会等待
3. 用完要sem_post(信号量), 归还资源

*/
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>

sem_t semPass;
#define MAX_THREAD 10
pthread_t gThread[MAX_THREAD];
#define MAX_LOAD 5

void *func(void *arg)
{
   int id = *(int *)arg; //保留下参数
    //等待信号量的值
    sem_wait(&semPass); //让10个线程竞争5个资源
    printf(">>>>>>>>>>>>>> car %d passing >>>>>>>>>>>>>> \n", id);
    sleep(2); //这里可以用sleep(rand()%5+1)模拟随即等待1-5秒的时间
    printf("============== car %d finished ============== \n", id);
   sem_post(&semPass); //资源释放
   return NULL;
}

int main()
{
   sem_init(&semPass, 0, MAX_LOAD); //初始化资源数(整数)
   srand(time(NULL));
   for(int i=0; i<MAX_THREAD; i++)
   {
       //创建线程
       pthread_create(&gThread[i], NULL, func, &i); //创建10个线程
       //控制一下节奏,否则i变化太快,func拿不到正确参数i,i又变了.
       //当然也可以用其它同步手段,例如互斥量等,但usleep 是最简单的了
       usleep(1000);
   }

   for(int i=0; i<MAX_THREAD; i++)
   {
       //等待线程结束
       pthread_join(gThread[i],NULL);
   }

    return 0;
}
/*
* 运行结果
./semaphore
>>>>>>>>>>>>>> car 0 passing >>>>>>>>>>>>>>
>>>>>>>>>>>>>> car 1 passing >>>>>>>>>>>>>>
>>>>>>>>>>>>>> car 2 passing >>>>>>>>>>>>>>
>>>>>>>>>>>>>> car 3 passing >>>>>>>>>>>>>>
>>>>>>>>>>>>>> car 4 passing >>>>>>>>>>>>>>
============== car 0 finished ==============
>>>>>>>>>>>>>> car 5 passing >>>>>>>>>>>>>>
============== car 1 finished ==============
>>>>>>>>>>>>>> car 6 passing >>>>>>>>>>>>>>
============== car 2 finished ==============
>>>>>>>>>>>>>> car 7 passing >>>>>>>>>>>>>>
============== car 3 finished ==============
>>>>>>>>>>>>>> car 8 passing >>>>>>>>>>>>>>
============== car 4 finished ==============
>>>>>>>>>>>>>> car 9 passing >>>>>>>>>>>>>>
============== car 5 finished ==============
============== car 6 finished ==============
============== car 7 finished ==============
============== car 8 finished ==============
============== car 9 finished ==============
*/