Re: standard memory allocator alignment issue...

Ben Bacarisse <ben.usenet@[EMAIL PROTECTED]
> writes:
> Keith Thompson <kst-u@[EMAIL PROTECTED]
> writes:
>> Spiros Bousbouras <spibou@[EMAIL PROTECTED]
> writes:
[...]
>>> 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.
>
> The suggestion seems to be that ALIGN_MAX is some arithmetic value.  I
> can't see a way to adjust a pointer using it in any potable way.  I
> think this is the point behind Eric's "Without knowledge of which
> integer bits have what significance, you can't make effective use of
> things like ALIGN_MAX".  The standard declines to say anything about
> what the arithmetic value of a well-aligned pointer is.

Agreed.  An ALIGN_MAX constant (which might typically be 8 on modern
systems) is useful only if you start with an address that you already
know is adequately aligned -- which is why I started with malloc().
It's not enough to let you write a ****table malloc()-like function
from scratch.

> If you are assuming that the suggestion is for a function-line macro:
>
> #define ALIGN_MAX(p)
>
> that behaves like a function declared void *ALIGN_MAX(void *) which
> adjusts the pointer it is given, then I agree it would be possible to
> write an allocator.  (But I would suggest the name be lower-cased to
> match things like offsetof and changed to sound less like a constant
> -- another parameter might also be required to indicate "direction").

Or you could have, say an alignment_offset(p) macro which, given a
char* value, gives you a small integer value you can add to it to
yield a pointer that's guaranteed to be adequately aligned.

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