Shared memory linux - c

Question

I'm trying to work with shared memory at the first time. I created one child process and I write to the shared memory from Parent and change it from Child, before program ends I print shared memory from Parent and shared memory hasn't change, here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <semaphore.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <signal.h>
sem_t *semaphore;
int main(){
int i = 0, status;
pid_t pid=fork(), w;
int id;
if ((id = shmget(IPC_PRIVATE, sizeof(int), IPC_CREAT | 0666)) == -1){
perror("shmget");
exit(1);
}
int *sh;
if ((sh =(int *) shmat(id, NULL, 0)) == NULL){
perror("shmat");
exit(1);
}
*sh = 10;
if ((semaphore = sem_open("/semaphore", O_CREAT, 0666, 0)) == SEM_FAILED){
perror("semaphore");
exit(1);
}
if (pid==0) {
while(1){
sleep(1);
*sh = 50;
printf("child %d, %d\n", i, *sh);
i++;
if(i == 5){
sem_post(semaphore);
exit(0);
}
}
} else if (pid==-1) {
perror("process error\n");
} else {
printf("Parent, %d\n", *sh);
sem_wait(semaphore);
printf("child end => parent end\n");
printf("Parent, %d\n", *sh);
}
shmctl(id, IPC_RMID, NULL);
sem_close(semaphore);
sem_unlink("/semaphore");
return 0;
}
If I understand shared memory little bit than I can change it from everywhere if I have a pointer in my case is a "sh".
Output of program is:
Parent, 10
child 0, 50
child 1, 50
child 2, 50
child 3, 50
child 4, 50
child end => parent end
Parent, 10
Why is the number in shared memory different in Parent and in Child?

Answer 1

You fork() before you create the shared memory with the key IPC_PRIVATE, so both processes create their own shared memory and don't actually share it.
If you remove the =fork() from
pid_t pid=fork(), w;
and insert
pid = fork();
somewhere after the shmget call, it works the way you expect, since the child process will inherit the shared memory identifier from the parent and not create a different one.