Linux笔记--信号量

介绍

信号量是Linux内部的一种进程同步方式，在多个进程同时执行时，使用信号量可保护临界区代码执行时不会乱序。

函数

//创建并获取信号量
int semget(key_t key, int num_sems, int sem_flags);
//改变信号量的值
int semop(int sem_id, struct sembuf *sem_opa, size_t num_sem_ops);
//控制信号量
int semctl(int sem_id, int sem_num, int command, ...);

举例

sem1.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/sem.h>
#include <string.h>

#define SEMKEY 1234

union semun{

	int val;
	struct semid *buf;
	unsigned short *array;
	struct seminfo *__buf;
};

void sem_p(int semid){

	int ret;
	struct sembuf sbuf;

	sbuf.sem_num = 0;
	sbuf.sem_op = -1;
	sbuf.sem_flg = SEM_UNDO;

	ret = semop(semid,&sbuf,1);
	if(ret  == -1){
	
		perror("semop");
		return;
	}
}

void sem_v(int semid){
	
	int ret;
	struct sembuf sbuf;

	sbuf.sem_num = 0;
	sbuf.sem_op = 1;
	sbuf.sem_flg = SEM_UNDO;

	ret = semop(semid,&sbuf,1);
	if(ret  == -1){
	
		perror("semop");
		return;
	}
}

int main(){
	
		int semid;
		union semun unsem;

		semid = semget(SEMKEY,1,0);
		if(semid == -1){
		
			perror("semget");
			exit(1);
		}

		unsem.val = 1;
		semctl(semid,0,SETVAL,unsem);
		
		while(1){
			
			sem_p(semid);

			printf("This is the father process!\n");

			sleep(10);

			sem_v(semid);
		}
			semctl(semid,0,IPC_RMID);
	
	return 0;
}

sem2.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/sem.h>
#include <string.h>

#define SEMKEY 1234

union semun{

	int val;
	struct semid *buf;
	unsigned short *array;
	struct seminfo *__buf;
};

void sem_p(int semid){

	int ret;
	struct sembuf sbuf;

	sbuf.sem_num = 0;
	sbuf.sem_op = -1;
	sbuf.sem_flg = SEM_UNDO;

	ret = semop(semid,&sbuf,1);
	if(ret  == -1){
	
		perror("semop");
		return;
	}
}

void sem_v(int semid){
	
	int ret;
	struct sembuf sbuf;

	sbuf.sem_num = 0;
	sbuf.sem_op = 1;
	sbuf.sem_flg = SEM_UNDO;

	ret = semop(semid,&sbuf,1);
	if(ret  == -1){
	
		perror("semop");
		return;
	}
}

int main(){
		int semid;
		union semun unsem;
			
		semid = semget(SEMKEY,1,0);
		if(semid == -1){
		
			perror("semget");
			exit(1);
		}

		//unsem.val = -1;
		//semctl(semid,0,SETVAL,unsem);

		while(1){

			sem_p(semid);
		
			printf("This is the child process!\n");

			sleep(1);

			sem_v(semid);
		}
			semctl(semid,0,IPC_RMID);
	
	return 0;
}

执行效果

sem1.c 每隔10s输出一次信息，sem2.c虽然设置的是1s输出一次信息，但是由于信号被sem1.阻塞，所以也变成10s输出一次。