1信号量
信号量可以有n把鎖。
依賴的頭檔案
#include <semaphore.h>
函數聲明
sem_t表示信号量
int sem_init(sem_t *sem, int pshared,unsigned int value);
名稱：
sem_init
功能：
頭檔案：
函數原形：
int sem_init(sem_t *sem, int pshared, unsigned int value);
參數：
傳回值：
sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
sem_wait
sem_trywait
lock a semaphore 一直阻塞等待直到信号量
> 0.
all of these functions return 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate
the error.
int sem_timedwait(sem_t *sem, const structtimespec *abs_timeout);
sem_timedwait
lock a semaphore，阻塞等待若幹時間直到信号量 > 0
int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
int sem_post(sem_t *sem);
sem_post
unlock a semaphore，使信号量加1。
sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.
int sem_destroy(sem_t *sem);
sem_destroy
destroy an unnamed semaphore釋放信号量。和sem_init對應
sem_destroy() return 0 on success;on error,-1 is returned,an errno is set to indicate the error.
案例說明：
#include <stdlib.h>
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#define num 5
int queue[num];
sem_t blank_number,product_number;
void *producer(void *arg)
{
int p = 0;
while(1) {
//blank_num = 5生産者最多生産5個
//一直阻塞等待信号量大于0
sem_wait(&blank_number);
queue[p] = rand() % 1000 + 1;
printf("produce %d\n",queue[p]);
//product_number = 0 ->1
//使信号量加1
sem_post(&product_number);
p = (p + 1) % num;
sleep(rand() % 5);
}
}
void *consumer(void *arg) {
int c = 0;
while (1) {
//等待信号量大于0
sem_wait(&product_number);
printf("consume %d\n",queue[c]);
queue[c] = 0;
sem_post(&blank_number);
c = (c + 1) % num;
int main(int argc ,char *argv[])
pthread_t pid,cid;
//将blank_num信号量初始化的值為5
sem_init(&blank_number,0,num);
//将product_number信号量初始化的值變為0
sem_init(&product_number,0,0);
pthread_create(&pid,null,producer,null);
pthread_create(&cid,null,consumer,null);
pthread_join(pid,null);
pthread_join(cid,null);
sem_destroy(&blank_number);
sem_destroy(&product_number);
return 0;
運作結果：
