前言：共享内存指 (shared memory)在多处理器的计算机系统中，可以被不同中央处理器（CPU）访问的大容量内存。由于多个CPU需要快速访问存储器，这样就要对存储器进行缓存（Cache）。

writer：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#define SEM_KEY 4001
#define SHM_KEY 5678
union semun {
    int val;
};
int main(void){
    /*create a shm*/
    int semid,shmid;
    shmid = shmget(SHM_KEY,sizeof(int),IPC_CREAT|0666);
    if(shmid<0){
      printf("create shm error\n");
      return -1;
    }
    void * shmptr;
    shmptr =shmat(shmid,NULL,0);
    if(shmptr == (void *)-1){
      printf("shmat error:%s\n",strerror(errno));
      return -1;
    }
    int * data = (int *)shmptr;
    semid = semget(SEM_KEY,2,0666);
    struct sembuf sembuf1;
    union semun semun1;
    while(1){
      sembuf1.sem_num=1;
      sembuf1.sem_op=-1;
      sembuf1.sem_flg=SEM_UNDO;
      semop(semid,&sembuf1,1);
      scanf("%d",data);  
      sembuf1.sem_num=0;
      sembuf1.sem_op=1;
      sembuf1.sem_flg=SEM_UNDO;
      semop(semid,&sembuf1,1);
    }
     return 0;
}

reader：

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#define SEM_KEY 4001
#define SHM_KEY 5678
union semun {
    int val;
};
int main(void){
    /*create a shm*/
    int semid,shmid;
    shmid = shmget(SHM_KEY,sizeof(int),IPC_CREAT|0666);
    if(shmid<0){
        printf("create shm error\n");
        return -1;
    }
    void * shmptr;
    shmptr =shmat(shmid,NULL,0);
    if(shmptr == (void *)-1){
        printf("shmat error:%s\n",strerror(errno));
        return -1;
    }
    int * data = (int *)shmptr;    
    semid = semget(SEM_KEY,2,IPC_CREAT|0666);
    union semun semun1;
    semun1.val=0;
    semctl(semid,0,SETVAL,semun1);
    semun1.val=1;
    semctl(semid,1,SETVAL,semun1);
    struct sembuf sembuf1;
    while(1){
      sembuf1.sem_num=0;
      sembuf1.sem_op=-1;
      sembuf1.sem_flg=SEM_UNDO;
      semop(semid,&sembuf1,1);
      printf("the NUM:%d\n",*data);
      sembuf1.sem_num=1;
      sembuf1.sem_op=1;
      sembuf1.sem_flg=SEM_UNDO;
      semop(semid,&sembuf1,1);
    }
    return 0;
}

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