fork_test.c

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

#define NUM_JOBS 3
#define BUFFER_SIZE 1024

const char *jobs[NUM_JOBS] = {
    "date",
    "uptime",
    "echo 'Hello from child!'"
};

void child_process(int write_fd, const char *command) {
    char buffer[BUFFER_SIZE];
    FILE *fp;

    // Continuous execution loop
    while (1) {
        // Execute command and open pipe to read its output
        fp = popen(command, "r");
        if (fp == NULL) {
            snprintf(buffer, BUFFER_SIZE, "Error executing command: %s\n", strerror(errno));
            write(write_fd, buffer, strlen(buffer));
            exit(EXIT_FAILURE);
        }

        // Read command output and send to parent
        while (fgets(buffer, BUFFER_SIZE, fp) != NULL) {
            write(write_fd, buffer, strlen(buffer));
        }

        pclose(fp);
        // sleep(1); // Delay for demonstration; adjust as needed
    }
}

int main() {
    int pipes[NUM_JOBS][2];
    pid_t pids[NUM_JOBS];

    for (int i = 0; i < NUM_JOBS; i++) {
        if (pipe(pipes[i]) == -1) {
            perror("pipe failed");
            exit(EXIT_FAILURE);
        }

        pids[i] = fork();
        if (pids[i] < 0) {
            perror("fork failed");
            exit(EXIT_FAILURE);
        }

        if (pids[i] == 0) {  // Child process
            close(pipes[i][0]); // Close read end in child
            child_process(pipes[i][1], jobs[i]);
            close(pipes[i][1]);
            exit(EXIT_SUCCESS);
        } else {  // Parent process
            close(pipes[i][1]); // Close write end in parent
        }
    }

    // Parent collects output
    fd_set read_fds;
    char buffer[BUFFER_SIZE];

    while (1) {
        FD_ZERO(&read_fds);
        for (int i = 0; i < NUM_JOBS; i++) {
            FD_SET(pipes[i][0], &read_fds);
        }

        // Wait for any child output
        if (select(FD_SETSIZE, &read_fds, NULL, NULL, NULL) == -1) {
            perror("select failed");
            exit(EXIT_FAILURE);
        }

        // Read from ready pipes
        for (int i = 0; i < NUM_JOBS; i++) {
            if (FD_ISSET(pipes[i][0], &read_fds)) {
                ssize_t bytes_read = read(pipes[i][0], buffer, BUFFER_SIZE - 1);
                if (bytes_read > 0) {
                    buffer[bytes_read] = '\0';
                    printf("Output from job %d (%s):\n%s", i, jobs[i], buffer);
                }
            }
        }
    }

    // Should not reach here as children run infinitely
    return 0;
}