Extending syscalls

H. Peter Anvin hpa at zytor.com
Thu Jan 17 11:26:23 PST 2008

Jonathan Corbet wrote:
> Heh, indeed.  But we do seem to have a recurring problem of people
> wanting to extend sys_foo() beyond the confines of its original API.
> I've observed a few ways of doing that:
>  - create sys_foo2() (or sys_foo64(), or sys_fooat(), or sys_pfoo(),
>    or...) and add the new stuff there.
> The first approach has traditionally been the most popular.  If we have
> a consensus that this is the way to extend system calls in the future,
> it would be nice to set that down somewhere.  We could avoid a lot of
> API blind alleys that way.

I would argue it is the right approach.  It lets the kernel system call 
entry dispatch directly to the system call for the "new" case, and to a 
compatibility thunk for the "old" case.  It has the following desirable 

- No overhead for the "new" case.
- Minimal overhead for the "old" case.
- Easily dealt with by tools like strace that examine system calls.


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