Re: basic system call question

[John Levon <moz@compsoc.man.ac.uk>]

On Tue, 15 May 2001, Joseph A. Knapka wrote:

> Which reminds me of a question I had some time ago, but wasn't
> able to investigate at the time. The kernel_thread() function,
> called to spawn kernel threads, seems to invoke int0x80 with
> the contents of EAX undefined. Am I insane, or what? I suppose
> something about the calling convention used in the kernel
> must gaurantee the EAX value is something useful, but is that
> documented anywhere? I have not been able to figure out why
> this code works.
> 
> <http://lxr.linux.no/source/arch/i386/kernel/process.c#L439>

I imagine it is relying on gcc to always allocate the "anonymous" IN
parameter for the inline asm to eax, namely this :

458                 :"" (__NR_clone), "i" (__NR_exit),

will make sure eax will contain the __NR_clone value.

I suppose you can look at process.s (cd /usr/src/linux && make kernel/process.s)
to confirm this.

Can anyone quote the relevant part from the gcc inline asm manual ?

john

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