信号量的使用

使用两个信号量的实例

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

//线程函数
void *thread_func(void *msg);
sem_t sem;//信号量
sem_t sem_add;//增加的信号量

#define MSG_SIZE 512

int main()
{
int res = -1;
pthread_t thread;
void *thread_result = NULL;
char msg[MSG_SIZE];
//初始化信号量,初始值为0
res = sem_init(&sem, 0, 0);
if(res == -1)
{
perror(“semaphore intitialization failed\n”);
exit(EXIT_FAILURE);
}
//初始化信号量,初始值为1
res = sem_init(&sem_add, 0, 1);
if(res == -1)
{
perror(“semaphore intitialization failed\n”);
exit(EXIT_FAILURE);
}
//创建线程，并把msg作为线程函数的参数
res = pthread_create(&thread, NULL, thread_func, msg);
if(res != 0)
{
perror(“pthread_create failed\n”);
exit(EXIT_FAILURE);
}
//输入信息，以输入end结束，由于fgets会把回车（\n）也读入，所以判断时就变成了“end\n”
printf(“Input some text. Enter 'end’to finish…\n”);

sem_wait(&sem_add);
while(strcmp("end\n", msg) != 0)
{
	if(strncmp("TEST", msg, 4) == 0)
	{
		strcpy(msg, "copy_data\n");
		sem_post(&sem);
		//把sem_add的值减1，即等待子线程处理完成
		sem_wait(&sem_add);
	}
	fgets(msg, MSG_SIZE, stdin);
	//把信号量加1
	sem_post(&sem);
	//把sem_add的值减1，即等待子线程处理完成
	sem_wait(&sem_add);
}


printf("Waiting for thread to finish...\n");
//等待子线程结束
res = pthread_join(thread, &thread_result);
if(res != 0)
{
	perror("pthread_join failed\n");
	exit(EXIT_FAILURE);
}
printf("Thread joined\n");
//清理信号量
sem_destroy(&sem);
sem_destroy(&sem_add);
exit(EXIT_SUCCESS);

}

void* thread_func(void *msg)
{
char *ptr = msg;
//把信号量减1
sem_wait(&sem);
while(strcmp(“end\n”, msg) != 0)
{
int i = 0;
//把小写字母变成大写
for(; ptr[i] != ‘\0’; ++i)
{
if(ptr[i] >= ‘a’ && ptr[i] <= ‘z’)
{
ptr[i] -= ‘a’ - ‘A’;
}
}
printf(“You input %d characters\n”, i-1);
printf(“To Uppercase: %s\n”, ptr);
//把信号量加1，表明子线程处理完成
sem_post(&sem_add);
//把信号量减1
sem_wait(&sem);
}
sem_post(&sem_add);
//退出线程
pthread_exit(NULL);
}

作者：ljianhui
来源：优快云
原文：https://blog.youkuaiyun.com/ljianhui/article/details/10813469
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