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 continuie the empty program saga I have started to examine the output of the C compiler, the stuff GNU as consumes.

gcc version 4.4.0 (TDM-1 mingw32)

test.c:

int main()
{
    return 0;
}

gcc -S test.c

    .file   "test.c"
    .def    ___main;    .scl    2;  .type   32; .endef
    .text
.globl _main
    .def    _main;  .scl    2;  .type   32; .endef
_main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    call    ___main
    movl    $0, %eax
    leave
    ret

Can you explain what happens here? Here is my effort to understand it. I have used the as manual and my minimal x86 ASM knowledge:

.file "test.c" is the directive for the logical filename.
.def: according to the docs "Begin defining debugging information for a symbol name". What is a symbol (a function name/variable?) and what kind of debugging information?
.scl: docs say "Storage class may flag whether a symbol is static or external". Is this the same static and external I know from C? And what is that '2'?
.type: stores the parameter "as the type attribute of a symbol table entry", I have no clue.
.endef: no problem.
.text: Now this is problematic, it seems to be something called section and I have read that its the place for code, but the docs didn't tell me too much.
.globl "makes the symbol visible to ld.", the manual is quite clear on this.
_main: This might be the starting address (?) for my main function
pushl_: A long (32bit) push, which places EBP on the stack
movl: 32-bit move. Pseudo-C: EBP = ESP;
andl: Logical AND. Pseudo-C: ESP = -16 & ESP, I don't really see whats the point of this.
call: Pushes the IP to the stack (so the called procedure can find its way back) and continues where __main is. (what is __main?)
movl: this zero must be the constant I return at the end of my code. The MOV places this zero into EAX.
leave: restores stack after an ENTER instruction (?). Why?
ret: goes back to the instruction address that is saved on the stack

Thank you for your help!

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

5 Answers