Thread: assembly and C

Its was hard to determine where this post belonged, so I put it here...

I have been playing with two versions of a 'hello, world' program, in order to see the differences produced by declaring main as returning an int, and declaring main as not returning anything. When I declare main to return an int, and return 0, everything goes as expected, and when I check the exit status code it is indeed 0. But when I do a void main version of the program, I get an exit status of 13. So I have done a fair bit of searching, and the best I can come up with is that the 13 I get on my machine is something left over in the register eax. (When I ran the program in gdb, and checked eax, sure enough it was 13 stored there) Anyway, I am not strong with assembly, and so I am not sure of how eax ends up holding the value 13, and I was wondering if someone could shed some light on the matter for me. Here is the code I am using:

C version

Code:

test $ cat hello1.c
#include <stdio.h>

void main(void)
{
     printf("hello, world\n");
}

And the assembly output from gcc

Code:

test $ cat hello1.s
        .file   "hello1.c"
        .section        .rodata
.LC0:
        .string "hello, world\n"
        .text
        .align 2
.globl main
        .type   main,@function
main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        subl    %eax, %esp
        subl    $12, %esp
        pushl   $.LC0
        call    printf
        addl    $16, %esp
        leave
        ret

I am not interested in using void main, but simply am trying to discover some of the inner workings of things.