I would like to send an int from one process to another through shared memory.

I tried simply placing the value of the int into the shared memory (&number) - didnt work.

I tired casting the string to bytes into a char array (memcpy) and reading a sizeof(int) from the other process - didn't work.

i tired memcpy the value of the int into a char array, sending it to the other process, copying back it with memcpy : from the char array into an int - didn't work

My last attempt is this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/shm.h>
#include <sys/stat.h> // S_IRUSR

int main(int argc, char *argv[])
{
    pid_t gyerek;
    key_t kulcs;
    int oszt_mem_id;
    char *s;

    kulcs = ftok(argv[0], 1);
    oszt_mem_id = shmget(kulcs, 500, IPC_CREAT | S_IRUSR | S_IWUSR);

    s = shmat(oszt_mem_id, NULL, 0);

    gyerek = fork();

    if (gyerek == 0)
    {

        //printf("Child read this: %s\n", s);

        char szamarr[10];
        int szam = 12; 
        memcpy(&szamarr, &szam, sizeof(int)); 
        strcpy(s, szamarr);

        sleep(1);

        strcpy(s, &szam);
        
        sleep(3);
        shmdt(s);
    }
    else
    {
        sleep(2);
        int szam;
        char szamarr[10];
        memcpy(&szamarr, &s, sizeof(int));

        printf("Parent read this: %s\n", szamarr);

        sleep(1);

        int szam2 = (int) s;

        printf("Parent read this: %s\n", s);

        shmdt(s);

        wait(NULL);
        shmctl(oszt_mem_id, IPC_RMID, NULL);
    }
}

The result is either a random number and nothing

CodePudding user response：

You don't need to involve strings if you only want to pass an int. However generally, it's easier to use structures for this kind of communication:

typedef struct {
    int szam;
    // ...
} mystruct_t;


int main(int argc, char *argv[])
{
    pid_t gyerek;
    key_t kulcs;
    int oszt_mem_id;
    char *s;

    kulcs = ftok(argv[0], 1);
    oszt_mem_id = shmget(kulcs, sizeof(mystruct_t), IPC_CREAT | S_IRUSR | S_IWUSR);

    s = shmat(oszt_mem_id, NULL, 0);

    gyerek = fork();

    if (gyerek == 0)    // child
    {
        mystruct_t ms={0};
        ms.szam = 12;
        memcpy(s, &ms, sizeof(mystruct_t));
        sleep(3);
        shmdt(s);
    }
    else    // parent
    {
        sleep(1);
        mystruct_t ms={0};
        memcpy(&ms, s, sizeof(mystruct_t));
        printf("Parent read this: %d\n", ms.szam);
        shmdt(s);
        wait(NULL);
        shmctl(oszt_mem_id, IPC_RMID, NULL);
    }
}