Using "%d" to print double variable outputs differently in IA32 and IA32-64

Question

Why does The following code work totally different on IA-32 and x86-64?

#include <stdio.h>

int main() {
    double a = 10;
    printf("a = %d\n", a);
    return 0;
}

On IA-32, the result is always 0. However, on x86-64 the result can be anything between MAX_INT and MIN_INT. What is the reason behind this?

Answer 1

%d actually is used for printing int. Historically the d stood for "decimal", to contrast with o for octal and x for hexadecimal.

For printing double you should use %e, %f or %g.

Using the wrong format specifier causes undefined behaviour which means anything may happen, including unexpected results.