GCC's assembly output of an empty program on x86, win32

Question

I write empty programs to annoy the hell out of stackoverflow coders, NOT. I am just exploring the gnu toolchain.

Now the following might be too deep for me, but to cont

Answer 1

There's a very similar exercise outlined here: http://en.wikibooks.org/wiki/X86_Assembly/GAS_Syntax

You've figured out most of it -- I'll just make additional notes for emphasis and additions.

__main is a subroutine in the GNU standard library that takes care of various start-up initialization. It is not strictly necessary for C programs but is required just in case the C code is linking with C++.

_main is your main subroutine. As both _main and __main are code locations they have the same storage class and type. I've not yet dug up the definitions for .scl and .type yet. You may get some illumination by defining a few global variables.

The first three instructions are setting up a stack frame which is a technical term for the working storage of a subroutine -- local and temporary variables for the most part. Pushing ebp saves the base of the caller's stack frame. Putting esp into ebp sets the base of our stack frame. The andl aligns the stack frame to a 16 byte boundary just in case any local variables on the stack require 16 byte alignment (for the x86 SIMD instructions require that alignment, but alignment does speed up ordinary types such as ints and floats.

At this point you'd normally expect esp to get moved down in memory to allocate stack space for local variables. Your main has none so gcc doesn't bother.

The call to __main is special to the main entry point and won't typically appear in subroutines.

The rest goes as you surmised. Register eax is the place to put integer return codes in the binary spec. leave undoes the stack frame and ret goes back to the caller. In this case, the caller is the low-level C runtime which will do additional magic (like calling atexit() functions, set the exit code for the process and ask the operating system to terminate the process.