Example showing how to

process-child-output-example.c

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

const char *handle_child_process_output(const char *buffer, ssize_t count) {
    // we just print it, but we could do any other processing here...
    write(STDOUT_FILENO, buffer, (size_t)count);
    return buffer;
}

int main() {
    /*
     * 1. Create a pipe
     * 2. Within the child, connect the write end (entrance) of the pipe to STDOUT_FILENO
     * 3. Within the child, close the original ends of the pipe
     * 4. Within the parent, close the write end (exit) of the pipe
     * 5. Read the child ouput from the read end
     */

    int filedes[2];
    if (pipe(filedes) < 0) {
        perror("pipe");
        exit(1);
    }

    pid_t pid = fork();
    if ((int)pid < 0) {
        /* ERROR */

        perror("fork");
        exit(1);
    } else if (pid == 0) {
        /* CHILD */

        // connect the read end to stdout
        while ((dup2(filedes[1], STDOUT_FILENO) == -1) && (errno == EINTR)) {}

        // close the write end
        close(filedes[1]);

        // close the read end
        close(filedes[0]);

        execlp("ls", "ls", "-l", (char*)0);

        perror("execl");
        exit(1);
    } else {
        /* PARENT */
        
        // close the write end
        close(filedes[1]);
        
        // read child ouput
        char buffer[4096];
        while (1) {
            ssize_t count = read(filedes[0], buffer, sizeof(buffer));
            if (count == -1) {
                if (errno == EINTR) {
                    continue;
                } else {
                    perror("read");
                    exit(1);
                }
            } else if (count == 0) {
                break;
            } else {
                // process child output
                handle_child_process_output(buffer, count);
            }
        }
        close(filedes[0]);
        wait(0);
    }

    return 0;
}