Question

int main(){

    char ch;

    fork();

    cin >> c;
}

After calling fork() I should have 2 exact processes running the same code. Why after running this simple example, I am either asked to enter a character only once, either twice? Shouldn’t the system expect 2 inputs every single time I run this program?

>./a.out 
a
>./a.out
a
b
>

Answer 1

You have two processes reading from the terminal at the same time. It is anybody’s guess which process gets the input.

• If the parent process gets the input first, it exits and returns control to the shell. (Note that this actually causes a repeat of the same situation, with shell and the child process fighting for input.)
• If the child process gets the input first, it exits but control does not return to the shell untill the parent process exits.

You should not expect consistent behavior if you have two processes reading from the same terminal.