linux线程-使用信号量同步

Posted 2022-02-20 为了维护世界和平_

线程中访问临界区的方法

1、信号量：控制一组对象访问时，比如从5条可用的电话线中分配一个给某个线程的情况
2、互斥锁：想控制任一时刻只能有一个线程可以访问一些共享内存

本文使用二值信号量保护临界资源

include<semaphore.h>

//创建信号量
int sem_init(sem_t *sem,int pshared,unsigned int value);

//控制信号量的值
sem_wait(sem_t * sem);//信号量的值 -1
sem_post(sem_t * sem);//给信号量的值 +1

sem_destroy(sem_t *sem)；//清理信号量所有资源

临界区资源 work_area

主线程获取输入，子线程统计字符串大小

#include<stdio.h>
#include<unistd.h>
#include<string.h>
#include<stdlib.h>
#include<semaphore.h>
#include<pthread.h>

void *thread_function(void *arg);
sem_t bin_sem;


#define WORK_SIZE 1024

char work_area[WORK_SIZE];

int main()

	int res;
	pthread_t a_thread;

	void *thread_result;

	res = sem_init(&bin_sem,0,0);
	if(res != 0)
	
		perror("Semaphore initialization failed\\n");
		exit(EXIT_FAILURE);
	

	res = pthread_create(&a_thread,NULL,thread_function,NULL);
	if(res !=0)
	
		perror("Thread creation failed");
		exit(EXIT_FAILURE);
	
	printf("Input some txt,Enter 'end' to finish\\n");
	while(strncmp("end",work_area,3) !=0)
	
		fgets(work_area,WORK_SIZE,stdin);
		sem_post(&bin_sem);
	

	printf("\\n Waiting for thread to finish...\\n");
	res = pthread_join(a_thread,&thread_result);
	if(res !=0)
	
		perror("Thread join failed");
		exit(EXIT_FAILURE);
	
	printf("Thread joined\\n");
	sem_destroy(&bin_sem);
	exit(EXIT_SUCCESS);





void *thread_function(void *arg)

	sem_wait(&bin_sem);
	while(strncmp("end",work_area,3) != 0)
	
		printf("You input %d characters\\n",strlen(work_area) -1);
		sem_wait(&bin_sem);
	
	pthread_exit(NULL);

打印输出

(base) wy@ubuntu:~/process$ ./thread3 
Input some txt,Enter 'end' to finish
hello
You input 5 characters
end

 Waiting for thread to finish...
Thread joined
(base) wy@ubun

程序修改,在主线程中 收到FAST输入后，立刻将work_area的数据替换掉，信号量增加了不止一次，则子线程没有时间去执行第一次输入的字符串，并且获得到信号量后多次统计work_area被系统修改掉的字符串。

        while(strncmp("end",work_area,3) !=0)
        
                if(strncmp(work_area,"FAST",4) == 0)
                
                        sem_post(&bin_sem);
                        strcpy(work_area,"Wheeee");
                
                else
                
                        fgets(work_area,WORK_SIZE,stdin);
                
                sem_post(&bin_sem);
        

(base) wy@ubuntu:~/process$ ./thread3 
Input some txt,Enter 'end' to finish
hello
You input 5 characters
FAST
You input 5 characters
You input 5 characters
You input 5 characters
end

这种情况再增加一个信号量，让主线程等到统计完字符后再继续赋值。

//主
        while(strncmp("end",work_area,3) !=0)
        
                if(strncmp(work_area,"FAST",4) == 0)
                
                        sem_wait(&bin_sem2);
                        strcpy(work_area,"Wheeee");
                
                else
                
                        fgets(work_area,WORK_SIZE,stdin);
                        sem_post(&bin_sem);
                
        
//子
        while(strncmp("end",work_area,3) != 0)
        
                printf("You input %s  %dcharacters\\n",work_area,strlen(work_area) + 1);
                sem_wait(&bin_sem);
                if(strncmp(work_area,"FAST",4) == 0)
                        sem_post(&bin_sem2);
        

更简单的方法使用互斥量。