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Date: Mon, 18 Oct 2010 18:32:51 +0700
From: Eugene Grosbein <eugen@grosbein.pp.ru>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: hackers@freebsd.org
Subject: Re: syscall
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On 18.10.2010 18:11, Kostik Belousov wrote:
> On Mon, Oct 18, 2010 at 05:27:21PM +0700, Eugene Grosbein wrote:
>> Hi!
>>
>> I've written an utility in C that does not link libc normally,
>> instead it includes <sys/syscall.h> and calls syscall().
>> It works nice for FreeBSD8/i386.
>>
>> Now I'm porting it to FreeBSD8/amd64 and just cannot find
>> how to call syscall() directly from C code.
> Show what you tried to do. Syscall() at the C-level works the same
> (well, almost, but the differences are too subtle for this discussion)
> for all architectures.

I'm prepearing a binary that would start before /sbin/init
to make just a couple of ioctl(MDIOCATTACH)/nmount system calls
then execve(/sbin/init). It has to be small in size for NanoBSD build.
Detailed explanation (in russian) and source code are available here:
http://dadv.livejournal.com/105161.html

In short:

#include <sys/syscall.h>

#define MESG    "Hello, world!\n"
#define MESG_SZ sizeof(MESG)-1

int syscall(const int n, ...);

#define _exit(a)       syscall(SYS_exit, a)
#define write(a, b, c) syscall(SYS_write, a, b, c)

int errno;

int main() {
  write(1,MESG,MESG_SZ);
  _exit(0);
  return 0; /* make compiler happy */
}

>> For arm, i386 and mips there are:
>>
>> lib/libc/arm/sys/syscall.S
>> lib/libc/i386/sys/syscall.S
>> lib/libc/mips/sys/syscall.S
> amd64 syscall() wrapper code is autogenerated.
> 
>>
>> What about amd64?
> Indeed, what is your issue with amd64 ?

I cannot find a module to link with
to resolve syscall() symbol when I do not want to link with libc.

Eugene Grosbein