Re: standard memory allocator alignment issue...

Spiros Bousbouras <spibou@[EMAIL PROTECTED]
> writes:
> On 12 May, 23:41, "Chris Thomasson" <cris...@[EMAIL PROTECTED]
> wrote:
>> "Eric Sosman" <Eric.Sos...@[EMAIL PROTECTED]
> wrote in message
>>
>> news:1210630927.374478@[EMAIL PROTECTED]
>>
>> > Chris Thomasson wrote:
>> >> [... determining alignment with a struct and offsetof ...]
>>
>> >> I have been successfully using the previous method in several
>> >> general-purpose memory allocators. However, I wanted to see if there
is a
>> >> better way; offsetof works fine. BTW, does anybody know why there is
not
>> >> something like ALIGN_MAX in <limits.h> already?
>>
>> >     The Rationale doesn't say why not.  My guess (and it's only
>> > a guess) is that the Committee didn't want to get too involved
>> > in specifying exactly how pointers convert to and from integers.
>> > Without knowledge of which integer bits have what significance,
>> > you can't make effective use of things like ALIGN_MAX.
>>
>> Humm, a compiler vendor already has to supply a malloc implementation
which
>> returns an address that is aligned on a sufficient boundary for all
integral
>> types; right? Well, IMVHO, the standard could mention that a vendor
shall
>> set the value of ALIGN_MAX to a sufficient boundary analogous to the
>> non-NULL return value of malloc which can accompany the alignment of
any
>> integral type. The rational is that a vendor can likely extract
ALIGN_MAX
>> from their existing malloc implementation...
>
> If you had ALIGN_MAX what would you do with it ?

One thing I might do is write my own memory allocator.  Call malloc()
once to get a big chunk of memory (guaranteed to be aligned properly),
then dole out properly-aligned subchunks of it in response to
my_malloc() calls.  Free everything at once by free()ing the big
chunk.  Without ALIGN_MAX, I can't think of a ****table way to
guarantee that the pointers returned by my_malloc are properly
aligned.

-- 
Keith Thompson (The_Other_Keith) kst-u@[EMAIL PROTECTED]
 
<http://www.ghoti.net/~kst>
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