Demonstration of using a pipe to communicate between processes

pipe_demo.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

void fork_and_pipe(int *process, int *writing_fd, int *reading_fd0, int *reading_fd1, int *reading_fd2, int pipe_fd[4][2]);

void pipe_demo() {
    int pipe_fd[4][2];
    int process;
    int writing_fd, reading_fd0, reading_fd1, reading_fd2;
    int const size_of_buffer = 256;
    char buffer[size_of_buffer];
    int integer_buffer[1];
    fork_and_pipe(&process, &writing_fd, &reading_fd0, &reading_fd1, &reading_fd2, pipe_fd);
    switch (process) {
        case 0:
            printf("Process 0 is going to sleep...\n");
            sleep(1);
            printf("Process 0 sending a message to process 1...\n");
            write(writing_fd, "go!", 4);
            printf("Process 0 is waiting for process 1...\n");
            read(reading_fd0, buffer, size_of_buffer);
            printf("Process 0 received a message from process1: %s\n", buffer);
            break;
        case 1:
            printf("Process 1 is waiting for process 0...\n");
            read(reading_fd0, NULL, size_of_buffer);    // we don't care what was written
            printf("Process 1 received a message from process 0.\n");
            read(reading_fd1, integer_buffer, sizeof(int));
            int value1 = integer_buffer[0];
            printf("Process 1 received %d from process 2.\n", value1);
            read(reading_fd2, integer_buffer, sizeof(int));
            int value2 = integer_buffer[0];
            printf("Process 1 received %d from process 3.\n", value2);
            sprintf(buffer, "%d + %d = %d", value1, value2, value1 + value2);
            printf("Process 1 sending a message to process 0...\n");
            write(writing_fd, buffer, strlen(buffer) + 1);
            break;
        case 2:
            integer_buffer[0] = 42;
            write(writing_fd, integer_buffer, sizeof(int));
            break;
        case 3:
            integer_buffer[0] = 73;
            write(writing_fd, integer_buffer, sizeof(int));
            break;
        default:
            printf("Process %d reached unreachable code; claims to be the %dth process.\n", getpid(), process);
            exit(-1);
    }
    // files should close when the process terminates, but we'll practice good hygiene anyway
    for (int i = 0; i < 4; i++)
        for (int j = 0; j < 2; j++)
            close(pipe_fd[i][j]);
}

void fork_and_pipe(int *process, int *writing_fd, int *reading_fd0, int *reading_fd1, int *reading_fd2, int pipe_fd[4][2]) {
    for (int i = 0; i < 4; i++) {
        pipe(pipe_fd[i]);
    }
    *process = 0;
    if (fork() == 0) {
        *process += 1;
    }
    if (fork() == 0) {
        *process += 2;
    }
    switch (*process) {
        case 0:
            *reading_fd0 = pipe_fd[1][0];
            break;
        case 1:
            *reading_fd0 = pipe_fd[0][0];
            *reading_fd1 = pipe_fd[2][0];
            *reading_fd2 = pipe_fd[3][0];
            break;
        case 2:
        case 3:
            // processes 2 & 3 won't read in this example
            break;
        default:
            printf("Process %d reached unreachable code; claims to be the %dth process.\n", getpid(), *process);
            exit(-1);
    }
    *writing_fd = pipe_fd[*process][1];
}