线程3——信号量

创建信号量，并赋予初值

int sem_init(sem_t *sem, int pshared, unsigned int value);

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销毁信号量

int sem_destroy(sem_t *sem);

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阻塞，信号量数值-1

int sem_wait(sem_t *sem);

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非阻塞申请
int sem_trywait(sem_t *sem);

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计时申请一个信号量
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);

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发送信号量，数值 +1
int sem_post(sem_t *sem);

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获取信号量值

int sem_getvalue(sem_t *sem, int *sval);

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 示例：和互斥锁案例相同

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


void* task1(void *arg);
void* task2(void *arg);
void* task3(void *arg);

static int i = 10 ;
static sem_t sem;            //信号量指针


int main(void)
{
    int ret;
    sem_init(&sem,0,1);    //初始化信号量 初值为1
    
    //声明三个指针，指向线程的id
    pthread_t tid1,tid2,tid3;
    //创建三个新线程，用刚才的指针，
    pthread_create(&tid1,NULL,task1,(void*)5);
    pthread_create(&tid2,NULL,task2,(void*)6);
    pthread_create(&tid3,NULL,task3,(void*)7);
    
    printf("Main thread
");
    //等待线程结束运行
    pthread_join(tid1,(void **)&ret);
    pthread_join(tid2,(void **)&ret);
    pthread_join(tid3,(void **)&ret);
    
    sleep(1);
    printf("New thread1 id:%ld
",tid1);
    printf("New thread2 id:%ld
",tid2);
    printf("New thread3 id:%ld
",tid3);
    //销毁信号量
    sem_destroy(&sem);
    

}

void* task1(void *arg)
{
    while(i<30){
    //信号量-1
    sem_wait(&sem);    
    i++;
    printf("%d
",i);
    //信号量+1
    sem_post(&sem);
    usleep(1);
    }
    pthread_exit(NULL);
    
}

void* task2(void *arg)
{
    while(i<30){
    //信号量-1
    sem_wait(&sem);    
    i++;
    printf("%d
",i);
    //信号量+1
    sem_post(&sem);
    usleep(1);
    }
    pthread_exit(NULL);
    
}

void* task3(void *arg)
{
    while(i<30){
    //信号量-1
    sem_wait(&sem);    
    i++;
    printf("%d
",i);
    //信号量+1
    sem_post(&sem);
    usleep(1);
    }
    pthread_exit(NULL);
    
}

计时申请信号量：（结构体见上一篇条件变量）

struct timeval now;
struct timespec outtime;
gettimeofday(&now, NULL);//获取当前时间
outtime.tv_sec = now.tv_sec + 5; //设定唤醒的绝对时间是当前时间+5s
outtime.tv_nsec = now.tv_usec * 1000;
sem_timedwait(&sem, &outtime);