# Re: \[PATCHv3 1/1\] Documentation: describe how to add a system call

- From: Pavel Machek \<hidden\>
- Date: 2015-08-05 16:21:37 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/08/4551600
- Thread: https://ratatoskr.run/linux-api/2015/07/4547734/t.md

```text
Hi!

> Add a document describing the process of adding a new system call,
> including the need for a flags argument for future compatibility, and
> covering 32-bit/64-bit concerns (albeit in an x86-centric way).
> 
> Signed-off-by: David Drysdale <redacted>
> Reviewed-by: Michael Kerrisk <redacted>
> Reviewed-by: Eric B Munson <redacted>
> Reviewed-by: Kees Cook <redacted>
> Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
> ---
>  Documentation/adding-syscalls.txt | 531 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 531 insertions(+)
>  create mode 100644 Documentation/adding-syscalls.txt

We usually align documentation to less than 80 columns, afaict.

> +call.  To make sure that userspace programs can safely use flags between kernel
> +versions, check whether the flags value holds any unknown flags, and reject the
> +sycall (with EINVAL) if it does:

syscall?

> +New system call proposals, like any change to the kernel's API, should always
> +be cc'ed to linux-api@vger.kernel.org

. at and of sentence?

> +System Calls Returning Elsewhere
> +--------------------------------
> +
> +For most system calls, once the system call is complete the user program
> +continues exactly where it left off -- at the next instruction, with the same
> +stack and registers as before the system call, and with the same virtual
> +memory space.

Umm. Normally we place return value in register. And I'm not sure.. do
syscalls preserve registers that are normally caller-clobbered in the ABI?

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
```