Is there a more efficient way to do this?

Question

First of all, yes this is a laboratory activity in my class, but I have already submitted and defended this exercise. What I would like to know is if there is another way, a more efficient way to write this code.

What we were tasked to do was to write a code that creates a process, which creates a child, which in turn creates another child and finally creates a another child.

*edit: I separated and numbered the requirements for better readability :)

1. The last child would show the current processes running in the system.

2. Its parent would then ask for a word then create a file using the input by the user.

3. Its parent would then ask for a word or a phrase, then find it within the libraries in your machine (Let's say I typed hi, it should find and list the files containing hi & its directory. The position of the word hi should not matter)

4. Lastly, the main parent would just print its parent id.

Here is my complete code for that matter:

int main(void){ 
    char fileName[30];
    char phrase[30];
    int pid = fork();
    int fd[2];
    pipe(fd);
    if(pid==0){
        printf ("CHILD1: I am the 1st child\n");
        printf ("CHILD1: ID is %d \n", getpid());
        printf ("CHILD1: Parent ID is %d \n", getppid());
        int pid2 = fork();
        if(pid2==0){
            printf ("\t CHILD2: I am the 2nd child\n");
            printf ("\t CHILD2: ID is %d \n", getpid());
            printf ("\t CHILD2: Parent ID is %d \n", getppid());
            int pid3 = fork();      
                if(pid3==0){
                    printf ("\t\t CHILD3: I am the 3rd child\n");
                    printf ("\t\t CHILD3: ID is %d \n", getpid());
                    printf ("\t\t CHILD3: Parent ID is %d \n", getppid());
                    execlp ("/usr/bin/top", "top", NULL);
                }else if(pid3==-1){
                    printf ("ID is %d", getpid());
                    printf ("error");
                    exit(1);
                }else{
                    wait(NULL);
                    printf ("\t CHILD2: Enter a filename: ");
                    scanf ("%s", fileName);
                    printf ("\t CHILD2: %s was succcessfully created!\n", fileName);
                    execlp ("/bin/touch", "touch", fileName, NULL); 
                }
        }else if(pid2==-1){
            printf ("ID is %d", getpid());
            printf ("error");
            exit(1);
        }else{
            wait(NULL);
            int pid4 = fork();
                if(pid4 > 0) {
                    printf ("CHILD1: Enter a pharse: ");
                    scanf ("%s", phrase);
                    close(fd[1]);
                    close(STDIN_FILENO);
                    dup2(fd[0],STDIN_FILENO);
                    execlp ("/bin/grep", "grep", phrase, NULL);
                }else if (pid4 == 0) {
                    close(fd[0]);
                    close(STDOUT_FILENO);
                    dup2(fd[1],STDOUT_FILENO);
                    execlp ("/usr/bin/find", "find", NULL);
                }else {
                    printf ("error");
                }
        }       
    }else if(pid==-1){
        printf ("ID is %d", getpid());
        printf ("error");
        exit(1);
    }else{
        wait(NULL);
        printf ("PARENT: I am the parent\n");
        printf ("PARENT: ID is %d \n", getpid());
    }
}

Answer 1

Your code seems reasonably efficient in terms of run-time and number of lines, but a single main function, deeply nested, does few favors to the humans who would try to read and understand what you've done.

Consider another idiom: one that's more modular, where the ancestry of processes is (IMHO) easier to follow by a reviewer. (I also invoke ps instead of top so that there's no need for keyboard interaction in that step.)

Is my approach more "efficient"? Arguably no, although I prefer the more direct coding of this child-per-function approach. The chaining of parent and child process is artificial, but of course so is your assignment.

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

/*
 * run the passed function in a child process, and return
 * from this function only if the child process runs and
 * exits with status of 0.
 */
static void
run_func_in_child(void (*f)())
{
    int status;
    pid_t pid = fork();

    switch (pid) {
    case -1:
        perror("fork");
        exit(1);
    case 0: /* child */
        (*f)();
        break;
    default: /* parent */
        if (waitpid(pid, &status, 0) == -1) {
             perror("waitpid");
             exit(1);
        }
        if (WIFEXITED(status) && WEXITSTATUS(status) == 0) {
             return;
        }
        fprintf(stderr, "child did not exit cleanly\n");
        exit(1);
    }
}

/*
 * scanf would be simpler, but let's protect against buffer overruns
 */
static void
get_rsp(const char *prompt, char *buf, size_t blen)
{
    int bl;

    printf("%s: ", prompt);
    fflush(stdout);

    if (fgets(buf, blen, stdin) == NULL) {
        if (ferror(stdin)) {
            perror("read");
        }
        exit(1);
    }
    bl = strlen(buf);
    if (bl > 0 && buf[bl - 1] == '\n') {
        buf[bl - 1] = '\0';
    }
}

static void
child_4()
{
    execlp("/usr/bin/ps", "ps", "-www", "-e", "f", NULL);

    perror("exec /usr/bin/ps");
    exit(1);
}

static void
child_3()
{
    char buf[256];
    int fd;

    run_func_in_child(child_4);

    get_rsp("File name", buf, sizeof buf);

    if (access(buf, F_OK) == 0) {
        fprintf(stderr, "%s already exists\n", buf);
        exit(1);
    }

    if ((fd = creat(buf, 0644)) == -1) {
        perror("creat");
        exit(1);
    }
    close(fd);

    printf("Created empty file %s\n", buf);

    exit(0);
}

static void
child_2()
{
    char buf[80];
    int fd[2];
    pid_t pid;

    run_func_in_child(child_3);

    get_rsp("Phrase", buf, sizeof buf);

    if (pipe(fd) == -1) {
        perror("pipe");
        exit(1);
    }

    pid = fork();

    switch (pid) {
    case -1:
        perror("fork");
        exit(1);
    case 0:
        /* no explicit wait for this child-of-child
         * process, but when its parent (the grep) exits,
         * init becomes the parent, and does the wait
         */
        dup2(fd[1], 1);
        close(fd[0]);
        close(fd[1]);
        execlp("/usr/bin/find", "find", NULL);
        perror("exec of find");
        exit(1);
    default:
        dup2(fd[0], 0);
        close(fd[0]);
        close(fd[1]);
        execlp("/usr/bin/grep", "grep", buf, NULL);
        perror("exec of grep");
        exit(1);
    }
}

static void
child_1()
{
    run_func_in_child(child_2);

    printf("Child 1: pid is %d; ppid is %d\n", getpid(), getppid());

    exit(0);
}

int
main(int ac, char *av[])
{
    run_func_in_child(child_1);

    return 0;
}