A C program compiled under 32-bit Debian Squeeze causes a segfault on my friend's 64-bit one

Question

Not so long ago I've installed Debian and configured it with my friend's help.
Yesterday I have downloaded GCC 4.4 and I created a simple program to test it out.
This is the code:

#include <stdio.h>

int main () {
    int result;
    printf ("Hello Wor... Linux! This is my %dst program compiled in Debian.\nHow many is 2+2?\n", 1);
    scanf ("%d", &result);
    while (result!=4) {
        printf ("Oh no! You're not going anywhere until you type the correct result! 2+2 is?\n");
        scanf ("%d", &result);
    }
    printf ("Congrats!\n");
    return 0;
}

I've compiled it by typing gcc-4.4 myfile.c in bash. Then I've tried to run the resulting binary file and it worked just as I wanted it to. Then I've sent the binary file to my friend to test it on his PC also. When he tried to run it, he received a segmentation fault message and the program didn't work.
He also uses Debian and his kernel's version is very similar to mine (2.6.32-5-686). The only difference is that his kernel is an amd-64 one (he owns a 64-bit processor, while mine is 32-bit).
Why is this happening? Does it mean that 64-bit Linux users will be unable to run my 32-bit programs? If so, can I compile it in a way which will let them to run it?
Please note that I'm not really experienced with Linux.

Answer 1

1. Never use scanf() for user input. It has no delimiters, so somone can break your program.
2. Do not use the int type, because its size is not machine independent defined.

Answer 2

he may need a chroot for it.

apt-get install ia32-libs 

should work for most cases.

see "Using an IA32 chroot to run 32bit applications" http://alioth.debian.org/docman/view.php/30192/21/debian-amd64-howto.html#id292205

Answer 3

chroot is one option. But remember it requires a lot of disk space as it installs 32-bit libraries.

Alternatively you can compile your file for a 64-bit environment by using the -m64 compiler flag of gcc which sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.

Answer 4

Alternatively, set up your compiler to target 64-bit binaries by following the instructions at the OSDev wiki: In brief:
Set up the new repos in /etc/apt/sources.list

    deb http://www.tucs.org.au/~jscott4/debian/ stable main    #Primary Mirror. Hosted by University of Tasmania.

Add the signing key:

    gpg --recv-keys 0x2F90DE4A
    gpg -a --export 0x2F90DE4A | sudo apt-key add -

Update your repo indices and get the appropriate cross-compilation package:

    apt-get update
    apt-get install osdev-crosscompiler-x86-64-elf

Then use the x86_64-elf variant of gcc to target x64. For instance

    x86_64-elf-gcc --pedantic -Wall -o foo foo.c

(In fact all the GCC tools and Binutils will have an x86_64-elf- variant now.)

EDIT -- Vastly improved instructions by pulling from a reference instead of from memory. EDIT -- removed stale mirror