UNIX for Dummies Questions & Answers

When a process fork(), the child share the same file descriptors as his father. Thus, they share the same stdin. Quick and dirty exemple below (sorry for the ugly gets() call) :

Code:

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

int main()
{
        char buf[512];

        if (fork()) { /*parent */
                while(1) {
                        gets(buf);
                        printf("[%d] parent: %s\n", getpid(), buf);
                }
        } else { /* child */
                while(1) {
                        gets(buf);
                        printf("[%d] children: %s\n", getpid(), buf);
                }
        }
}

Question 1 :
If I'm right, after the fork(), the parent and the child belong to the same group process which is a foreground group and which share the same console. As there is a concurrent read on stdin, I was thinking that the process that read the lines in input is pick up randomly. However, on my Linux system, the child always read first. Is it normal ?

Question 2:
If I launch the command like that :

Code:

$ ./a.out > /tmp/foobar

I was thinking that after the redirection, the parent and the child would both write on /tmp/foobar. But the /tmp/foobar file remains totaly empty !
Why ?

Thanks for your help,

Arnauld

Q1: Right. On some systems and in some circumstances, both parent and child indeed get the lines randomly. I've seen instances of this when background process and foreground process were both opening the tty for input. Crazy.

Q2: Perform a flush() after each printf. That might change things.