Sending int through shared memory between two processes - c

Question

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

Answer 1

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);
}
}