Where is the source to the system calls?

[Dan Strick]

On Sun, 7 Nov 2004 16:29:42 -0800 (PST), I wrote:
>
> Does anyone know where the system calls are really defined?
> I followed open() to _open() to __sys_open() which seems
> to be part of something called libc_r before I ran into a
> blank wall.  I grepped all of the regular files in /usr/src
> and /usr/include and turned up nothing.  I even tried
> grepping for open in the output of "nm -g /usr/lib/libc.a".
> There is no __sys_open() in libc.  Am I dealing with
> C-compiler magic?  Secret macro instructions invoking
> undocumented gnu C-compiler asm() features?  A CIA plot?
>

Perhaps I did not make it clear that I was looking for the definitions
of the subroutines in libc that converted C-language function calls,
such as open(), into assembler language system calls.  For example, in
version 7 pdp11 unix, the open() system call was defined in the very
straight forward assembler language program /usr/src/libc/sys/open.s:

	/ C library -- open

	/ file = open(string, mode)
	/ file == -1 means error

	.globl  _open,
	.globl  cerror
	.open = 5.

	_open:
	        mov     r5,-(sp)
	        mov     sp,r5
	        mov     4(r5),0f
	        mov     6(r5),0f+2
	        sys     0; 9f
	        bec     1f
	        jmp     cerror
	1:
	        mov     (sp)+,r5
	        rts     pc
	.data
	9:
	        sys     .open; 0:..; ..

Being unable to find any analogous code in FreeBSD (release 4.10),
I surmised it was done with multiple layers of highly complex cpp
macro instructions constructing a highly complex asm() statement
that was compiled in-line and never visible in a source file because
it existed only momentarily during compilation in the byte stream
passed from the c-preprocessor to the first phase of the c-compiler.


On Mon, 8 Nov 2004 13:14:08 +0200, Andrey Simonenko responded:
>
> You didn't say the version of FreeBSD you use (I guess
> that you use 4.x).
>
> __sys_open is an entry name for open() syscall and it (and most
> of other ones) is "constructed" in the /usr/src/lib/libc/i386/SYS.h
> file for i386, check it.
>

Thanks for the pointer to /usr/src/lib/libc/i386/SYS.h.  It contains
precisely the "secret macro instructions invoking undocumented gnu
C-compiler asm() features" that I suspected but could not find.

I still don't understand all the details but I do understand enough
to realize that I don't want to understand any more.

Thanks to all of you who responded to my question.

Dan Strick
strick at covad.net