Question

GCC compiles (using gcc --omit-frame-pointer -s):

    int the_answer() { return 42; } 

into

            .Text     .globl _the_answer     _the_answer:         subl    $12, %esp         movl    $42, %eax         addl    $12, %esp         ret        .subsections_via_symbols 

What is the ‘$12’ constant doing here, and what is the ‘%esp’ register?

Answer 1

Short answer: stack frames.

Long answer: when you call a function, compilers will manipulate the stack pointer to allow for local data such as function variables. Since your code is changing esp, the stack pointer, that’s what I assume is happening here. I would have thought GCC smart enough to optimize this away where it’s not actually required, but you may not be using optimization.