Using Pipes in C - c

Question

I wrote the following code to help me understand how pipes work in C.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
struct sum_ {
int a;
int b;
};
int main (void) {
int pipe1[2];
int pid;
struct sum_ sum;
if ( (pipe(pipe1) != 0)){
printf("pipe(): %d %s\n", errno, strerror(errno));
exit(1);
}
pid = fork();
if (pid == -1) {
printf("fork(): %d %s\n", errno, strerror(errno));
exit(1);
}
else if (pid == 0) { // Child
close(pipe1[0]);
sleep(5);
sum.a = read(pipe1[0], &sum.a, sizeof(sum.a));
printf("Your number was: %d", sum.a);
}
else { // Father
close(pipe1[1]);
printf("\nWrite a number: \n");
char a[4];
sum.a = atoi(fgets(a, 4, stdin));
write(pipe1[1], &sum.a, sizeof(sum.a));
}
return 0;
}
The code has a father and a son process. It is quite simple, the father uses a pipe to send a number to the son and the son displays the number for the user.
I always get -1 as result. What have I done wrong?

Answer 1

close(pipe1[0]);
sleep(5);
sum.a = read(pipe1[0], &sum.a, sizeof(sum.a));
You close the file descriptor pipe1[0], then read from it (and so get -1 returned). You make the equivalent error in the father, too. I think you mean to close pipep1[0] here and pipe1[1] in the father
Also, when you fix that, lthough you're reading into sum.a by passing the address, you're also setting it from the return value, which will overwrite what you read.