How to send and receive messages using ordinary pipes in C between two different c files

Using ordinary pipes only, I want to create a full-duplex communication between two programs named A and B. The A program (A.c) will open a file, read its text and write the text to the write end of a pipe. The B program (B.c) written in a different C file will read the pipe contents written by A and count the number of characters. Then the B program will write a number of characters to a second pipe, and the A program will read the number of characters written by B and display it.

Here is Some Code: I have tried:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>


int main(int argc, char *argv[]) {
    int fd[2];
    char buf[] = "HELLO WORLD!", red[100];
    if (pipe(fd))
    {
        perror("pipe");
        return -1;
    }
    switch(fork()){
        case -1:
            perror("fork");
            return -1;
        case 0:
            // child
            close(fd[1]); // close write end
            dup2(fd[0], STDIN_FILENO); // redirect stdin to read end
            close(fd[0]);
            execl("./child", NULL);
            exit(0);

        default:
           
            close(fd[0]); // close read end
            write(fd[1], buf, sizeof(buf)); // write to write end
            close(fd[1]); // close write end
            wait(NULL);
    }
    printf("\nEND~\n");
    return 0;
}