How can i use shared memory on a fork with shmget?

Question

I'm study the shared memory on C but i don't understand how to use the share memory when i want to change the value on the child process. I worte this simple code but it doesn't work as i expexted, did i miss something?

CODE:

    #include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <sys/sem.h>
#include <stdbool.h>
int create_shared_memory(){
    return shmget(IPC_PRIVATE,sizeof(int),0644 | IPC_CREAT);
}
int get_shared_memory(int key){
    return shmget(key,sizeof(int),0644 | IPC_CREAT);
}
int* attachd_shared_memory(int key){
    return shmat(key,NULL,0);
}

int main(int argc, char const *argv[])
{
    printf("\nSono il PID: %d di PID: %d\n",getpid(),getppid());
    int sh_id = create_shared_memory();
    int *result = attachd_shared_memory(sh_id);
    
    int x = 5;
    result = &x;
    printf("Puntatore: %i Numero(x): %d\n",*result,x);
    *result = 9;
    printf("Puntatore: %i Numero(x): %d\n",*result,x);
    if (!fork())
    {
        int sh2_id = get_shared_memory(sh_id);
        int* result2 = attachd_shared_memory(sh2_id);
        int y = 4;
        result2 = &y;
        printf("Sono il PID: %d di PID: %d \n",getpid(),getppid());
        printf("ID: %d %d\n",sh_id,sh2_id);
        printf("Puntatore: %i Numero(x): %d\n",*result2,x);
        exit(0);
    }
  
      for (int i = 0; i < 2; i++)
         {
            printf("\nHo finito di aspettare:%d  sono il padre: %d\n",wait(NULL),getppid());
         }
           shmctl(sh_id, IPC_RMID, NULL);
    printf("Puntatore: %i Numero(x): %d \n",*result,x);
    return 0;
}

Thank for the advice, i have to read better the basic of c but this satisfy my request. SOLUTION:

int create_shared_memory(){
    return shmget(IPC_PRIVATE,sizeof(int),0644 | IPC_CREAT);
}
int get_shared_memory(int key){
    return shmget(key,sizeof(int),0644);
}
int* attachd_shared_memory(int key){
    return shmat(key,NULL,0);
}

int main(int argc, char const *argv[])
{
    printf("\nSono il PID: %d di PID: %d\n",getpid(),getppid());
    int sh_id = create_shared_memory();
    int *result = attachd_shared_memory(sh_id);
    
    int x = 5;
    *result = x;
    printf("Puntatore: %i Numero(x): %d\n",*result,x);
    *result = 9;
    printf("Puntatore: %i Numero(x): %d\n",*result,x);
    if (!fork())
    {
       *result = 4;
        printf("Sono il PID: %d di PID: %d \n",getpid(),getppid());
       
        exit(0);
    }
  
      for (int i = 0; i < 2; i++)
         {
            printf("\nHo finito di aspettare:%d  sono il padre: %d\n",wait(NULL),getppid());
         }
           shmctl(sh_id, IPC_RMID, NULL);
    printf("Puntatore: %i Numero(x): %d \n",*result,x);
    return 0;
}

Answer 1

You're overwriting the pointer to the shared memory when you do

result = &x;

Now result points to the private memory of x instead of the shared memory segment.

The child has the same problem when it does

result = &y;

You can't turn private memory into shared memory by assigning its address to result. So don't do those assignments.

Just use *result as the shared memory. Anything you assign to that in one process will be visible in the other.

So use

*result = x;

to store the value of x in the shared memory, and

y = *result;

to retrieve it.

Answer 2

The endpoint is returning an unauthorized error so you're definitely hitting the endpoint. I see in the documentation that the API key can be restricted to specific IP addresses so check if the API key is correct and has no restrictions. If it does, make sure your IP configured to use the API key.