System call how to make parent wait for child - c

Question

This is my code system call in C.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int n;
int i;
pid_t pid;
int time = 1000;
int sum = 0;
int main(void) {
printf("n: ");
scanf("%d", &n);
pid = fork();
if (pid < 0) {
printf("Fork Failed");
exit(-1);
} else if (pid == 0) {
//child
for (i = 1; i <= n; i++) {
sum += i;
}
printf("Sum of 1 to %d: %d\n", n, sum); // this is ok
} else {
// parent
wait(&time);
printf("Sum of 1 to %d: %d\n", n, sum); // this always return 0;
}
return 0;
}
I don't know why in parent's code block, the sum is always equal to 0.
How to make parent wait for child or am I doing something wrong ?

Answer 1

Waiting for the child works. However, your expectations are wrong.
Apparently you think that computations in the child process after the fork are visible in the parent process. They are not. The child is a new copy of the parent program at the time of fork. At that time, the parent's sum is 0 and stays that way.
There are several mechanisms to pass data from child to parent (the search term is interprocess communication, IPC).
exit() status
files
shared memory
pipes
signals
message queues
anything else I have missed

Answer 2

The issue here is the variable sum is not shared by the parent & child process, after fork() call the child will have its own copy of the variable sum.
Use shmget(),shmat() from POSIX api. Or use pthread which will share the same memory space for the newly created thread.
Update---
Added the shared memory to your code hopes this helps.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/shm.h>
int n;
int i;
pid_t pid;
int time = 1000;
int main(void) {
int shmid;
int *sum;
printf("n: ");
scanf("%d", &n);
/*request the shared memory from the OS using the shmget()*/
shmid = shmget(IPC_PRIVATE, sizeof(int), 0777|IPC_CREAT);
pid = fork();
if (pid < 0) {
printf("Fork Failed");
exit(-1);
} else if (pid == 0) {
//child
/* shmat() returns a char pointer which is typecast here
to int and the address is stored in the int pointer. */
sum = (int *) shmat(shmid, 0, 0);
for (i = 1; i <= n; i++) {
*sum += i;
}
printf("Sum of 1 to %d: %d\n", n, *sum); // this is ok
/* each process should "detach" itself from the
shared memory after it is used */
shmdt(sum);
} else {
// parent
wait(&time);
sum = (int *) shmat(shmid, 0, 0);
printf("Sum of 1 to %d: %d\n", n, *sum); // this always return 0;
shmdt(sum);
/*delete the cretaed shared memory*/
shmctl(shmid, IPC_RMID, 0);
}
return 0;
}
Refer for more info- https://man7.org/linux/man-pages/man2/shmget.2.html