Linux无名信号量(semaphore)

头文件：

编译：添加 -lpthread 选项
示例：gcc -lpthread code.c -o a.out

#include 
#include 
#include 
#include 
#include 
#include 
#include 

unsigned int number = 0;

void* thread1(void* para);	//线程1服务函数
void* thread2(void* para);	//线程2服务函数

int main(int argc, char** argv)
{
	pthread_t threadID1, threadID2;
	int ret = 0;

	ret = pthread_create(&threadID1, NULL, thread1, NULL);	//创建线程1
	if(ret)
	{
		printf("Thread1 create failed!n");
		exit(EXIT_FAILURE);
	}

	ret = pthread_create(&threadID2, NULL, thread2, NULL);	//创建线程2
	if(ret)
	{
		printf("Thread2 create failed!n");
		exit(EXIT_FAILURE);
	}

	pthread_join(threadID1, NULL);	//等待线程thread1结束
	pthread_join(threadID2, NULL);	//等待线程thread2结束
	ptherad_exit((void*)0);			//退出现场main

	return 0;
}

void* thread1(void* para)
{
	printf("Function thread1 running...n");
	
	while(1)
	{
		number = 100;
		sleep(1);
		printf("number: %un", number);
	}

	pthread_exit((void*)0);
}

void* thread2(void* para)
{
	unsigned int count = 0;
	printf("Function thread2 running...n");
	
	while(1)
	{
		count++;
		number = count;
	}

	pthread_exit((void*)0);
}

结果分析：没有二元信号量的约束，线程thread1对number的赋值几乎不会等于100，原因是线程thread2对number的访问导致的。

#include 
#include 
#include 
#include 
#include 
#include 
#include 

unsigned int number = 0;
sem_t semID;

void* thread1(void* para);
void* thread2(void* para);

int main(int argc, char** argv)
{
	pthread_t threadID1, threadID2;
	int ret = 0;

	ret = pthread_create(&threadID1, NULL, thread1, NULL);
	if(ret)
	{
		printf("Thread1 create failed!n");
		exit(EXIT_FAILURE);
	}

	ret = pthread_create(&threadID2, NULL, thread2, NULL);
	if(ret)
	{
		printf("Thread2 create failed!n");
		exit(EXIT_FAILURE);
	}

	ret = sem_init(&semID, 0, 1);
	if(ret == -1)
	{
		printf("Sem create failed!n");
		exit(EXIT_FAILURE);
	}

	pthread_join(threadID1, NULL);
	pthread_join(threadID2, NULL);
	ptherad_exit((void*)0);

	return 0;
}

void* thread1(void* para)
{
	printf("Function thread1 running...n");
	
	while(1)
	{
		sem_wait(&semID);	//等待信号量
		number = 100;
		sleep(1);
		printf("number: %un", number);
		sem_post(&semID);	//释放信号量
	}

	pthread_exit((void*)0);
}

void* thread2(void* para)
{
	unsigned int count = 0;
	printf("Function thread2 running...n");
	
	while(1)
	{
		sem_wait(&semID);
		count++;
		number = count;
		sem_post(&semID);
	}

	pthread_exit((void*)0);
}

结果分析：加入信号量后，不管是线程thread1还是线程thread2对number的访问都不能同时进行，对全局变量number进行了很好的保护。