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c++forkflush

Forking a process prints differently each time

I have a piece of code to print out PID's once Forked, and just a basic user input thing, but each time I run the script, sometimes it runs as expected and it all prints out within one command, then the next time the child prints it's PID, it does so on a next line and leaves the terminal in a state in which you need to press enter for it to continue. I've researched this website about people flushing at what not, which I've added literally everywhere but I still can't get it to work.

My code:

#include <unistd.h>
#include <stdio.h>
#include "sys/types.h"
int main(){
int pid;
int userInput;

    printf("Please enter a number: ");
    userInput = getchar();

    printf("\nYou entered: ");
    putchar(userInput);
    printf ("\n");
    fflush(0);

    pid = fork();
    if (0 == pid)
    {
        printf ("I am the child process, my PID is %d \n", getpid());
        fflush(0);
    }
    else
    {
        printf ("I am the parent process, my PID is %d \n", getpid());
        fflush(0);
    }
return 0;
}

And this is what happens when I run it a few times:

Could someone point out where I've gone wrong and how to correct it?

Thanks!

Solution

  • Forking a process prints differently each time ? when you do fork() 1st parent will run and it runs in foreground and then child will run and its run in background. Now reason you are getting differently because if foreground process completed background process comes to foreground & displayss its output & vice versa.

    If command prompt is free i.e if parent completed then child will come to foreground & displays its output, shell won't stop child process being executed on foreground.

    Note that sequence of execution of child & parent process may defer as its OS dependent.