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SubjectRe: mmap bug?


On Fri, 29 May 1998, Matthew Kirkwood wrote:

> On Fri, 29 May 1998, Natarajan Krishnaswami wrote:
>
> > [someone] wrote:
> > >> It seems that attempts to mmap a region of size zero causes
> > >> mmap to return zero. Is this a bug?
> > >>
> > >> Since it doesn't return -1, it looks like success, but of
> > >> course dereferencing this NULL pointer causes a crash.
> >
> > Zero is a multiple of the page size, so there is no partial page mapped:
> > the map is empty. Returning a pointer which may not be dereferenced
> > seems reasonable. However, mmap is explicitly forbidden from returning
> > 0, and SIGBUS would probably be the correct signal to deliver if an
> > address in a 0-length map were dereferenced.
>
> Thank you.
>
> I was surprised at the number of people prepared to tell me not to
> mmap zero length areas. Here's a trivial patch to fix this, although
> I don't know if this will affect many programs.

I'm surprised that you never answered the important question: If you're
mapping 0 bytes then how the hell do you get a SEGV? You shouldn't be
referencing anything at the address. Your program is at fault as much or
more than linux. (I can't find a reference which claims that 0 is not a
valid response.)

Dean


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