Odd behavior of a c program with fork()

Question

In my homework I should explain what is happening in the following code:

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

int main(){
    int x = 1; 
    if(fork() == 0){// child
        printf("printf1: x=%d\n", ++x);// add then print
    }
    printf("printf2: x=%d\n", --x);
    exit(0);  
}

It's pretty straightforward and easy to understand. Most of the time I get the following output:

printf2: x=0
printf1: x=2
printf2: x=1

This means that the parent process was completed before the child and the child became a zombie process. But sometimes I get the following output:

printf1: x=2
printf2: x=1

After printing that the program freezes (It does not print anything and does not stop). I am running the program on Ubuntu. Any explanation will be appreciated.

after the program 'freezes' what happens if you push the 'enter' button? Are you getting the prompt or not? — Serge
– Serge, Commented May 18, 2020 at 2:09
It appears that the child process prints its output after the prompt is displayed. I didn't notice that. — muhammed-shihebi
– muhammed-shihebi, Commented May 18, 2020 at 11:07

HAL9000 · Accepted Answer · 2020-05-17 23:47:41Z

3

You have 3 processes writing to your terminal: parent, child and the shell interpreter. The parent process and the shell have "syncronized" output, but the child process may interleave its output with either of those. What you may perceive as a hanged process, may actually only be mangled output.

When you think it has hanged, you may try to enter a command and press enter...

answered May 17, 2020 at 23:47

HAL9000

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3 Comments

muhammed-shihebi Over a year ago

Not sure I understand what you mean with the synchronized output. Child and parent processes prints normally on the terminal even when they are working simultaneously. I don't understand why the output is mangled.

Some programmer dude Over a year ago

@Nour0700 Think about what happens if the parent process finishes before the child process. Then the parent process exits, and the shell takes over and print its prompt. Then the child process prints its output and exits. But there's no "synchronization" between the child process and the shell, so no new prompt will be printed. That could make it seem like the program just hangs. You need to look at the full output in the window. And press the Enter key in the shell to get a new prompt.

muhammed-shihebi Over a year ago

@Someprogrammerdude Now I understand. It's not hanging it just printing the output after the prompt. Thank you.

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