Re: 2nd help about handling HW interrupt and apologizes.

[Andi Kleen <ak@muc.de>]

Geert Bosch <geert@gonzo.sun3.iaf.nl> writes:

> Andi Kleen <ak@muc.de> wrote:
>    To just get the job done it is probably easier to just write a small
>    stub kernel driver in C that communicates with the Ada program running
>    in userspace over a device or a netlink socket.
> 
> Of course you'll have exactly the same problems when you write these
> drivers in C as when you write them in Ada! As you mentioned it is
> not safe to use the C run-time library as well. So, yes, you should
> write your code using a very limited subset of the language and you'll
> have to know what kind of code your compiler generates.

Because the complete kernel is written in C linux has already most required
runtime library support for C. Of course the in-kernel environment is not
full ISO-C compliant. The user callable library is very different, and there
are even a few language differences:
- Initialisation of local arrays does not work as expected, because gcc
calls bzero() for that and the kernel runtime library only has memset.
- Unitialised global or static variables are not guranteed to be zero
at kernel start, because the kernel doesn't clear the BSS segment.
- More complex long long arithmetic [which is not ISO C anyways] like the 
diviosn probably won't work on 32bit machines because the kernel doesn't 
include the necessary code from libgcc.

Overall I would say it is safer to write kernel code in C than in Ada,
simply because it is a much better understood topic. Also the kernel
includes rely heavily on include functions, C macros and inline assembler
so you might have a hard time to write a ada binding for that.

> For example when using GNAT it is possible to get almost identical
> code as when using the GNU C compiler if you just limit yourself to
> what would be possible in C. It is easy to do this experiment yourself
> and look at the generated code; this is something you should do in
> any case when writing code like system kernel modules. 

This is true, you'll probably need to invest much more exploration work
for a Ada module - I think it is not worth it when a small stub is 
sufficient.

-Andi