Shared memory for communication between processes

                                               Shared memory for communication between processes

1 Description

       Take a couple as an analogy. For example, a man and a woman are going on a date, and the two parties use a piece of paper to exchange information. Assuming that there is a table between the two and a piece of paper on the table, the man can write his thoughts on the paper in front of him. Next, I will show it directly to the woman. The same reason, in terms of processes, there are two processes A and B. Process A has its own independent space, and process B has its own independent space. The independent process spaces will not communicate with each other. , But there is a public memory space in the memory, and this public memory address A and B can be obtained, they can face the same physical space, if A has a pointer to the public address space, although the pointer The address of is not in the space of A, but A can read the contents of the pointer address, and the same is true for B.

       So what is the programming idea to realize shared memory?

1. First, create a shared memory space, if any, open it directly
2. Mapping, mapping the shared space to A, B space
3. Then you can exchange data
4. Free shared memory after data exchange

 

 

2  The code prototypes of shared memory are:

1 #include <sys/shm.h>

 

2 int shmget(key_t key, size_t size, int shmflag);

// Create or obtain a shared memory: return the shared memory ID if successful , and return -1 if it fails

   Size: The size of the shared memory must be aligned in megabytes, so it opens up a mega-level space

  

例：shmid = shmget(key,1024*4,IPC_CREAT|0666);

 

3 void *shmat(int shmid, const void *shmaddr, int shmflag);

// Connect the shared memory to the address space of the current process: return a pointer to the shared memory on success, return -1 on failure

Shmid: We get the ID of the shmid above shmid = shmget(key_t key, size_t size, int shmflag);

Shmaddr: We generally take 0 and automatically arrange shared memory for us.

Shmflag: We generally also take 0 , 0 means the mapped shared memory is readable and writable. Of course you can arrange it yourself

例：shmat(shmid,0,0);

4 int shmdt(void *addr);

// Disconnect from the shared memory: return 0 on success , return -1 on failure

 

5 int shmctl(int shmid, int cmd, struct shmid_ds *buf);

// Remove information about shared memory to prevent space occupation: return 0 on success , return -1 on failure

Smd: is the instruction

3 example

                         Practice with simple examples

shmw.c

#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdlib.h>
#include<stdio.h>
#include<string.h>



int main()
{

        int shmid;
        char *shmaddr;


        key_t key;
        key = ftok(".",1);

        shmid = shmget(key,1024*4,IPC_CREAT|0666);  // 可读可写

if(shmid == -1){	//判断是否成功，成功返回//指向共享内存的指针，失//败返回-1


                printf("shmget error\n");
                exit(-1);

        }
        shmaddr = shmat(shmid,0,0);             //映射共享内存

        printf("shmat ok\n");
        strcpy(shmaddr,"Hello, welcome");        //映射成功后直接，把内容复制到该地址中

        sleep(5);                              //睡眠5秒，让其他程序读
        shmdt(shmaddr);                       //卸载共享内存

        shmctl(shmid,IPC_RMID,0);               //关掉共享内存
        printf("quit\n");




        return 0;
}

shmr.c

#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdlib.h>
#include<stdio.h>
#include<string.h>



int main()
{

        int shmid;
        char *shmaddr;


        key_t key;
        key = ftok(".",1);

        shmid = shmget(key,1024*4,0);//获取不创建设为0

        if(shmid == -1){

                printf("shmget error\n");
                exit(-1);

        }
        shmaddr = shmat(shmid,0,0);

        printf("shmat ok\n");
        printf("data: %s\n",shmaddr);  //直接读取
        

        shmdt(shmaddr); 
    
         
        printf("quit\n");
        
    
    
    
        return 0;
}