Re: valid_pointer_p

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: storm@cua.dk (Kim F. Storm)
> Date: Sun, 30 Jul 2006 02:05:25 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Can someone ``in the know'' please explain what clever idea is behind
> > the function valid_pointer_p, and whether that idea is supposed to be
> > portable?  
> 
> If you have some better way to do this on some platforms, please tell me.

Well, I really don't understand what are the assumptions of the code.
Are you assuming that accessing an invalid pointer inside a system
call (such as `read') will never segfault?  Does Posix really mandate
that?  Should we ask people to try that on different platforms?

It goes without saying that on MS-Windows, the code does segfault if
the argument is an invalid pointer.

As for other ways, we could, for example, set up a temporary signal
handler for SIGSEGV around the call to valid_pointer_p.  That should
work on most, if not all, supported platforms.

Then there's the procfs API, which probably lets you actually read
from the process memory on those platforms where procfs is available.

On Windows, we could try reading from the address using the
ReadProcessMemory API, which is used by debuggers.  (If ptrace allows
reading from the calling process, we could do the same on Posix
platforms.)