Re: Representing errno in SPARK

[roderick.chapman@googlemail.com]

On Jun 9, 3:06 pm, xorquew...@googlemail.com wrote:
> What is the correct way to model a pair of subprograms
> that affect state completely outside of the SPARK program?

Create an abstraction of that state as a SPARK abstract own
variable, thus:

package POSIX
--# own Errno;

  procedure Errno_Set (Code : Integer);
  --# global out Errno;
  --# derives Errno from Code;
  pragma Import (C, Errno_Set, "posix_errno_set");

  function Errno_Get return Integer;
  --# global in Errno;
  pragma Import (C, Errno_Get, "posix_errno_get");

end POSIX;

Notes

1) POSIX.Errno is _never_ actually declared in SPARK - it's
just an abstraction for something that is outside the
SPARK boundary.

2) do NOT use an external own variable for this - these
are for _volatile_ states, which Errno is not...

3) You should read the "Informed Design Method" manual
(Informed.pdf) that comes with the SPARK distibution.

 - Rod