fork() and pipe() in C goes wrong - c

Question

I am learning about fork and pipe but have a problem with the following: My aim was to build a program with 3 processes and I did that but my question is: Why does printf("Sum of first half: %d\n", sum); get executed twice?
I checked the code for any logical errors that I made but couldn't find anything.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int size = sizeof(arr) / sizeof(int);
int sum;
int fd[2];
pipe(fd);
int id = fork();
if (id == 0) // Child process
{
for (int i = 0; i < size / 2; i++)
sum = sum + arr[i];
int j = fork();
if (j == 0)
{
printf("Hello I'm the grandchild\n");
}
}
else // Parent process
{
for (int i = size / 2; i < size; i++)
sum = sum + arr[i];
}
if (id == 0) // Child process writing to the pipe
{
close(fd[0]);
write(fd[1], &sum, sizeof(sum));
close(fd[0]);
printf("Sum of first half: %d\n", sum);
}
else // Parent process reading from the pipe
{
int x;
close(fd[1]);
read(fd[0], &x, sizeof(sum));
close(fd[1]);
printf("Sum of second half: %d\n", sum);
printf("Total sum: %d\n", x + sum);
}
}

Answer 1

Your code simplified:
int main()
{
int id = fork();
if (id == 0)
fork();
if (id == 0)
printf("Sum of first half\n");
else
printf("Sum of second half\n");
}
And the explanation:
code
Parent
Child
Granchild
fork()
fork
N/A
N/A
id value
id != 0
id==0
N/A
if (id == 0) fork()
then not executed
fork
N/A
id value
id != 0
id == 0
id == 0
if (id == 0) printf("sum first")
then not executed
printf
printf
else printf("sum second half")
printf
else not executed
else not executed