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.

In this case, it's simply the return result of printf()
Which for keen manual readers is the number of characters printed

Originally Posted by Salem

In this case, it's simply the return result of printf()
Which for keen manual readers is the number of characters printed

Ah ok - thanks Salem - and yes I will go read the man page now..

edit::

If anyone who reads this has trouble finding info about the return value of printf by using 'man printf', try using 'man 3 printf'

Another thing... I was wondering if anyone can tell me if the startup routine would interpret that number 13 as if I had explicitly returned 13 from main()?

~/

> I was wondering if anyone can tell me if the startup routine would interpret that number 13 as if I had explicitly returned 13 from main()?
Yes it would

Originally Posted by Salem

> I was wondering if anyone can tell me if the startup routine would interpret that number 13 as if I had explicitly returned 13 from main()?
Yes it would

So if you were checking against WEXITSTATUS in a parent program, having an unknown value returned from the child process could have some funny results? Or am I missing something?