Fork() 代码未按预期工作 - 层次结构制作

Question

Good afternoon.

I am currently working on a C program that takes one and only one parameter which designates the number of "child generation"s to be created (the own father counts as 1 already). "wait()" system calls are not to be used for this exercise (the version with "wait" calls happens to work exactly as expected).

For instance, the call $program 4 should generate a hierarchy like this: Process A creates B Process B creates C Process C creates D

The printed messages are not important, as they are merely orientative for the task. With the following code (which happens to work exactly how I want with a "wait()" call) states that all the child processes derive from the same father, which I don't understand why it's happening.

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

    int main(int argc, char *argv[]) {
    int counter; pid_t result; int i;

    /*
    We are going to create as many processes as indicated in argv[1] taking into account that the main father already counts as 1!
    */
    if (argc > 2 || argc == 1) {puts("IMPOSSIBLE EXECUTION\n"); exit(-1);}
    int lim = atoi(argv[1]);


    //We eliminate the impossible cases
    if (lim < 1) {puts("IMPOSSIBLE EXECUTION\n"); exit(-1);}


    if (lim == 1) {puts("The father himself constitutes a process all by his own, therefore:\n");
    printf("Process%d, I'm %d and my father: %d\n", counter, getpid(), getppid());
     }
    else {
        for (i = 0; i < lim; i++) {
            result = fork();
            if (result < 0) {
                printf("Call%d \n", counter); perror("Has failed!");
                exit(-1);
            }
            else if (result) {
                break; //Father process
            }
            else {
                counter++;    //Child processes increment the counter
                printf("Process%d, I am %d and my father: %d\n", counter, getpid(), getppid());
            }
        }
    }

The hierarchy generated by the code above is not the one I expected...

All help is greatly appreciated. Thank you

Answer 1

With the following code (which happens to work exactly how I want with a "wait()" call) states that all the child processes derive from the same father, which I don't understand why it's happening.

I don't see that in my tests, nor do I have any reason to expect that it's actually the case for you. HOWEVER, it might appear to be the case for you if what you see is some or all of the child processes reporting process 1 as their parent. That would happen if their original parent terminates before the child's getppid() call is handled. Processes that are orphaned in that way inherit process 1 as their parent. If the parent wait() s for the child to terminate first then that cannot happen, but if instead the parent terminates very soon after forking the child then that result is entirely plausible.

Here's a variation on your loop that will report the original parent process ID in every case:

    pid_t my_pid = getpid();

    for (i = 0; i < lim; i++) {
        result = fork();
        if (result < 0) {
            printf("Call%d \n", counter); perror("Has failed!");
            exit(-1);
        } else if (result) {
            break; //Father process
        } else {
            pid_t ppid = my_pid;  // inherited from the parent
            my_pid = getpid();
            counter++;    //Child processes increment the counter
            printf("Process%d, I am %d and my father: %d\n", counter, (int) my_pid, (int) ppid);
        }
    }

Answer 2

You are missing a crucial function call.

    for (i = 0; i < lim; i++) {
        fflush(stdout);             // <============== here
        result = fork();

Without it, your fork duplicates parent's stdout buffer into the child process. This is why you are seeing parent process output repeated several times --- its children and grandchildren inherit the output buffer.

Live demo (with fixed formatting for your reading convenience).