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SubjectRe: mmap bug?
On 29 May 1998, Ben Pfaff 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.
>
> Why would you dereference it if you didn't specify any memory? It
> seems to me that this is valid kernel behavior, and that the program
> has the bug.

The program, in this case, is glibc.

It is mmapping a file which it has been told by stat() is zero length.

I was just curious as to whether Unix98 or similar specifies any
behaviour; nowhere in the man page is the possibility of a zero return
from mmap mentioned. This is a free segfault out of many programs; of
course, as a NULL pointer dereference, it's not exactly dangerous.

FWIW, Solaris (x86 and Sparc) returns -1 and sets errno to EINVAL.
Digital Unix returns -1 and ENXIO.

Matthew.


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