Shared memory communication between processes

Shared memory

Shared memory is mapping a piece of memory that can be accessed by other processes , This shared memory is created by a process , But multiple processes can access . Shared memory is the fastest IPC The way , It is specially designed for the low efficiency of other inter process communication modes .

But it does not provide a synchronization mechanism , That is, before a process finishes writing to shared memory , It does not prevent the second process from starting to read it , Therefore, we need to artificially prevent synchronization through other mechanisms .

shmget() function

Used to get a shared memory identifier or create a shared memory .

The function prototype ：

int shmget(key_t key, size_t size, int shmflg);

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Parameters	explain
key	Key value identifying shared memory ,shmget() When the function succeeds, it returns an and key Associated shared memory identifier （ Non-negative integer ）, For subsequent shared memory functions . Call failure returns -1.
size	Indicates the size of the shared memory , In bytes
shmflg	Authority sign IPC_CREAT If shared memory does not exist , Create a shared memory , Otherwise, open the operation . IPC_EXCL Only when shared memory does not exist , New shared memory is created , Otherwise, there will be an error .

shmat() function

After the shared memory is created for the first time , It can't be accessed by any process yet ,shmat() The function is used to start access to the shared memory , And connect the shared memory to the address space of the current process .

Prototype ：

void *shmat(int shmid, const void *shmaddr, int shmflg)

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Parameters	explain
shmid	Shared storage id, Such as shmget() The return value of the function
shmaddr	Specifies the location of the shared memory connection to the current process , It's usually 0, Means to let the kernel decide an appropriate address location
shmflg	SHM_RDONLY： Read only mode , Others are in read-write mode , Such as 0

When the call succeeds, it returns a pointer to the first byte of shared memory , If the call fails, it returns -1.

shmdt() function

And shmat The function is the opposite , Is the address used to disconnect from the shared memory attachment point , Prohibit this process from accessing this shared memory , But it is not to delete the shared memory .

Prototype ：

int shmdt(const void *shmaddr);

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shmaddr: The starting address of the connected shared memory , Such as shmat() The return value of the function . Return when the call is successful 0, Return... On failure -1.

shmctl() function

Control shared memory .

Prototype ：

int shmctl(int shm_id, int command, struct shmid_ds *buf);

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Parameters	explain
shm_id	Shared memory identifier , Such as shmget() The return value of
command	Some orders , Such as IPC_STAT： Get the state of shared memory , Put the shared memory shmid_ds The structure is copied to buf in IPC_SET： If the process has sufficient permissions , Set the current association value of shared memory to shmid_ds The value given in the structure IPC_RMID： Delete this shared memory
buf	Shared memory management structure .

shmid_ds structure Include at least the following members ：

struct shmid_ds
{
    uid_t shm_perm.uid;
    uid_t shm_perm.gid;
    mode_t shm_perm.mode;
};

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For example, a simple example , Parent and child processes communicate by sharing memory ：

/*
 * @Author: YaleXin
 * @Date: 2020-04-22 14:22:16
 * @LastEditTime: 2020-04-22 21:38:34
 * @LastEditors: YaleXin
 * @Description:
 * @FilePath: \my_c_workspace\others\shareMemory.c
 * @ Prayer does not appear BUG
 */
#include <stdio.h>
#include <string.h>
#include <sys/shm.h>
#include <unistd.h>
struct shareMemoryDate {
    int mod;
    char msg[100];
};
int main() {
    int *shm = NULL;
    struct shareMemoryDate *share;
    //  Create a shared memory 
    // IPC_CREAT Indicates if the shared memory does not exist , Create a shared memory , Otherwise, open the operation .
    //  Need and IPC Object access rights （ Such as 0600） Conduct | Operation to determine the access rights of the semaphore set 
    int shm_id =
        shmget((key_t)0, sizeof(struct shareMemoryDate), 0600 | IPC_CREAT);
    if (shm_id == -1) {
        printf(" Failed to create shared memory , I don't know why \n");
        return 0;
    }
    int pId = fork();
    if (pId == -1) {
        printf(" Failed to create child process , I don't know why \n");
    } else if (pId == 0) {
        // Subprocesses 
        //  Connect shared memory to the address space of the current process 
        shm = shmat(shm_id, 0, 0);
        if (shm == NULL) {
            printf(" link failure , I don't know why \n");
            return 0;
        }
        share = (struct shareMemoryDate *)shm;
        char buffer[100];
        printf(" I'm a subprocess , Please enter what you want to write to the shared memory :\n");
        fgets(buffer, sizeof(buffer), stdin);
        strcpy(share->msg, buffer);
        //  Indicates that the data has been written 
        share->mod = 1;
        //  Detach shared memory from the current process 
        if (shmdt(shm) == -1) {
            printf(" Detach failed , I don't know why \n");
        }
    } else if (pId > 0) {
        // The parent process 
        //  Connect shared memory to the address space of the current process 
        shm = shmat(shm_id, 0, 0);
        if (shm == NULL) {
            printf(" link failure , I don't know why \n");
            //  Delete shared memory 
            if (shmctl(shm_id, IPC_RMID, 0) == -1) {
                printf("（ Can't link ） Failed to delete shared memory , I don't know why \n");
                return 0;
            }
        }
        share = (struct shareMemoryDate *)shm;
        // Wait for the child process to finish writing 
        while (share->mod != 1) sleep(1);
        printf(" I'm the parent process , Let me see what my child processes have written in shared memory ：\n");
        printf("%s\n", share->msg);
        printf(" Oh , It's these ghosts \n");
        //  Delete shared memory 
        if (shmctl(shm_id, IPC_RMID, 0) == -1) {
            printf("( Finished reading ) Failed to delete shared memory , I don't know why \n");
        }
    }
    return 0;
}

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debugging ：

[email protected]:/media/yalexin/ Software /my_c_workspace/others$ gcc -o share shareMemory.c
[email protected]:/media/yalexin/ Software /my_c_workspace/others$ ./share
 I'm a subprocess , Please enter what you want to write to the shared memory :
 How do you do 
 I'm the parent process , Let me see what my child processes have written in shared memory ：
 How do you do 

 Oh , It's these ghosts 
[email protected]:/media/yalexin/ Software /my_c_workspace/others$ 

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