Date: Tue, 2 Mar 2004 22:19:19 +0100 From: jan.muenther@nruns.com To: Daniela <dgw@liwest.at> Cc: questions@freebsd.org Subject: Re: Strange behaviour in assembly language program Message-ID: <20040302211919.GA10074@ergo.nruns.com> In-Reply-To: <200403022110.50014.dgw@liwest.at> References: <200403022110.50014.dgw@liwest.at>
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Howdy,
> Here it is:
>
> .text
> .global _start
> _start:
> pushl $0
> movl $1, %eax
> int $0x80
>
> I looked everywhere (Developer's handbook, Google, ...) to find the solution,
> but all resources I consulted tell me this is the right way to do it.
> This program, however, always exits with 1 regardless of the value I push.
>
> Please, can someone tell me that I made a really stupid error? I'm already
> pulling my hair out.
I sympathize. This has actually cost me quite some nerves as well, before
through some debugging and experimentation I found the answer:
The kernel expects the first argument 4 bytes below of the current stack
pointer, which means you have to put the int 80h call on its own label to
get it right.
I usually use nasm (hate AT&T syntax, sorry),
should translate easily, something like:
_start:
push 0
mov eax, 1
call syscall
syscall:
int 80h
ret
should do the job.
Greetings, J.
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