PyMem_MALLOC (was [Python-Dev] Snake farm)
Guido van Rossum
Fri, 22 Nov 2002 13:26:16 -0500
> Guido van Rossum <email@example.com> writes:
> > This can be solved (as MAL suggested) by fixing configure so that
> > malloc(0) returning 0x800 is treated the same as malloc(0) returning
> > NULL. That way, pymalloc's free code doesn't have to special-case
> > this.
> This is nearly as bad as hard-coding the system on which it
> happens. If system developers come to like this trick, they may decide
> to return 0xFFFF0000 for malloc(0) (system developers, when confronted
> with a non-conformity in their implementation, always love to find a
> conforming but surprising implementation).
> Given that this is quite hard to debug if it happens, I'd rather like
> to see a better test. It's not easy to find one, though.
> One would be to do MALLOC_ZERO_RETURNS_ALWAYS_THE_SAME_THING, which
> would cover this and similar implementations (i.e. you test malloc(0)
> == malloc(0)).
> Another test would be MALLOC_ZERO_RETURNS_NO_MEMORY: malloc(0), round
> down to the page beginning, read a word there, expect a crash. This
> tests precisely the functionality that pymalloc needs.
Yes, the test I proposed was naive. But I'd like to see this fixed in
configure, not in pymalloc. (Tim seems to favor always ensuring that
we never call malloc(0), but I can't see how that can be done without
an extra test+jump. :-( )
--Guido van Rossum (home page: http://www.python.org/~guido/)