Using Pipes in 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.