Subject: Re: bug#11519: "Wrong type argument: characterp" building custom-deps while boostrapping

From: Stefan Monnier <[email-masked]>
Date: Wed, 23 May 2012 10:16:17 -0400

> Which other places use C pointers to buffer text and call functions
> that can allocate memory?

IIUC any place that uses STRING_CHAR_AND_LENGTH on buffer text is
vulnerable to the problem.

> Anyway, are you against committing this to the release branch?  I'd be
> very sad if you were, having invested so much time in hunting this
> bug, but I guess I'll survive.

I'm not dead set against it, and I'm glad we found the culprit so we can
fix it: fixing it on the release branch is not that important, since this
bug has been with us since Emacs-23.1, AFAICT.

If you really want to install your workaround on the emacs-24 branch, go
for it but let's try to find a real fix for the trunk.

>> >> I wonder: why do we use REL_ALLOC?
>> > AFAIK, we do that only on platforms that don't support mmap for
>> > allocating buffer text.
>> So, IIUC the only reason to use it is so that we can more often return
>> memory to the OS even for the non-mmap case?  Is that because returning
>> memory can only be done via sbrk style memory management?
> I don't think this is only about _returning_ memory.  It is first and
> foremost about not _asking_ for more memory when we can come up with
> it by reshuffling buffer text.

So you're saying it's use for fragmentation reasons?
But on other platforms where we use mmap, we do suffer from this
fragmentation, and yet it doesn't seem to be a real source of problem.
That's why I think the only real reason is because memory can only be
returned via sbrk-style memory management (i.e. only free memory at the
end of the heap can be returned).  Is that right?

I guess my question turns into "why do we use gmalloc.c instead of
a malloc library that uses mmap (or some other mechanism that lets it
return large free chunks to the OS)"?

AFAIK, Windows is pretty much the only system where we use gmalloc.c and
ralloc.c nowadays.  Does anyone remember why we don't use the system
malloc under Windows (and Cygwin)?

        Stefan