"Bad file descriptor" error, when using file descriptors of the socketpair function more than once

Question

The following code sends messages from child processes to their parents using socket pairs.

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MESSAGE "Hello"

int main()
{ 
    char message[100];
    int i, pidFork, sockets[2];

    /*
     *Trying to use a single socketpair function call
     *
     *if(socketpair(AF_UNIX, SOCK_STREAM, 0, sockets) == -1) {
     *   perror("Creating socketpair");
     *   exit(-1);
     *}
     */

    for(i=0; i<2; i++) { 

        if(socketpair(AF_UNIX, SOCK_STREAM, 0, sockets) == -1) {
            perror("Creating socketpair");
            exit(-1);
        }

        printf("i: %d\n", i);
        pidFork = fork();
        if (pidFork == -1) {
            perror("Creating child process");
            exit(-1);
        }

        else if (pidFork != 0) { //Parent
            close(sockets[0]);
            if(read(sockets[1], message, strlen(MESSAGE)) == -1) {
                perror("Reading data");
                exit(-1);
            }
            printf("Data: %s\n\n", message);
            close(sockets[1]);
        }

        else { //Child
            close(sockets[1]);
            if(write(sockets[0], MESSAGE, strlen(MESSAGE)) == -1) {
                perror("Writing data");
                exit(-1);
            }
            close(sockets[0]);
            exit(0);
        }
    }
    return 0;
}

First I tried to get the file descriptors for a socket pair with a single call to the socketpair function before entering the for loop, such as the commented lines shows, but this only works for the first iteration, from the second on iteration I get a "Bad file descriptor error", the way I made it work was moving the socketpair function call inside the for loop, however the first approach worked for me when using pipes.

So I would like to know why this happens, Am I making a mistake? Or Is this a difference between both IPC mechanisms?

Thanks

Update: There's no difference between file descriptors of pipe and socketpair. I thought that it was working with pipes because an error in another part of my full program. The accepted answer solved my issue.

Answer 1

You're closing both ends of the socketpair in both parent and child.

In general, use one socket pair or pipe per child process.