Question

When I try to explain some simple code to my friends,something strange happens:

#include <stdio.h>
int main() 
{
    int x;
    printf("%x\n",x);
}

I have tryed millions of times ,and the last 12 bits of x always turns out to be 0xff0.
I’ve disass the code,but still can’t figure out what’s going on here

My operating system is ubuntu10.10 ,compiler is gcc4.7.2

Answer 1

First of all, if the value always matches 0xFF0 then the “lower 12 bits” are that value, not the lower 3.

As for why this happens, you’re reading a variable that you haven’t initialized. That’s undefined behavior, and anything could have happened when you tried to do this, from a crash to a pizza getting delivered.

But what actually happens is this: the location that x references has some data that it held previously. Chances are that on your setup, that variable happens to contain data whose lower twelve bits are 0xFF0; in which case it’s most likely data associated a previous system call, during processing work done by the system before your main is called.

To make a long story short: initialize your variables before using them and don’t ask why the uninitialized ones have the value they do.