cvs commit: src/lib/libc/stdlib malloc.c

[Jason Evans]

On Jan 14, 2006, at 1:32 PM, Giorgos Keramidas wrote:
> On 2006-01-13 11:07, Jason Evans <jasone at FreeBSD.org> wrote:
>> On Jan 12, 2006, at 5:29 PM, Giorgos Keramidas wrote:
>>>
>>> [...]
>>>
>>> Does this look like an off-by-one error to you too Jason?
>>> Apparently, the allocated size of s->data is s->size, which is 873
>>> bytes, but then Emacs tries to access s->data[873].
>>>
>>> Does it look like I'm right in thinking that this is a bug in Emacs?
>>
>> This looks like a bug in emacs, as you say, but I don't know if it
>> has any particular relation to the posix_memalign() changes.
>
> Apparently it does seem related to posix_memalign() changes.
> When I bootstrap Emacs without posix_memalign(), by manually
> tweaking src/config.h after configure runs, and #undef
> POSIX_MEMALIGN, then it passes the bootstrap stage normally.

I looked into this some, and it appears that it is a configuration  
problem in emacs.  When I ran 'configure', both GNU_MALLOC and  
HAVE_POSIX_MEMALIGN were defined in config.h.  This appears to be a  
recipe for disaster, since the allocator included with emacs does not  
provide posix_memalign().  This probably means that emacs is calling  
into both its own allocator and libc's, then trying to free memory  
that was allocated by libc, using its own allocator.

Additionally, I see dlmalloc-specific code that tells dlmalloc not to  
mmap() during allocation, since emacs's undump procedure doesn't  
preserve mmap'ed regions.

It looks like emacs is by default configured to use emacs's  
allocator, which is good (getting emacs to work with FreeBSD's malloc  
probably isn't worth the trouble).  However, the emacs code makes the  
mistake of deciding whether to use posix_memalign() based on  
HAVE_POSIX_MEMALIGN, rather than something like USE_MEMALIGN -- emacs  
is mixing and matching allocators and their features, which is not  
good.  I'm not familiar enough with emacs's configuration system to  
know the correct way of fixing this, but one way or another, emacs  
needs to not use posix_memalign().

Jason