Re: [Dev-C++] Re: malloc , extra bytes ! Somewhat urgent !

[Per W. <pw...@ia...>]

The main reasons for allicating larger memory blocks than requested are:
- Make sure the hidden header before any allocated data are always
aligned. Aligned data is faster to access. Also, most processors
doesn't support unaligned memory. The x86 series processors are just
very-very nice.
- Make sure that the returned pointer get a very high alignment -
normally at least align 8, to allow perfectly aligned 64-bit integers
and doubles. Remember that the runtime library doesn't know if you expect
to store 1024 single bytes or 128 doubles in a 1kB request.
- Reduce memory fragmentation. By limiting number of allowed block sizes,
new requests have a higher probability of being able to reuse a previously
released memory block.

The buddy system or a binary subdivision are the extremes, where you might
get +50 - 100% extra memory even for large allocations. The advantages are
that they are lightning-fast at locating available blocks or subdividing a
too large block into a returnable chunk. When you return a block of
memory, they know exactly where the neighbours are, and can check if the
neighbour has also been released thereby directly combining multiple
released blocks into the next larger block size.

/Per W

On Mon, 24 Feb 2003, OROSZI Bal=E1zs wrote:

> Hello!
>
> veenurs wrote:
> > if( (buffer =3D ( PBYTE )calloc( 1000 ,sizeof(BYTE) )) =3D=3D NULL ){
> > }
> > size =3D _msize( buffer );
> > The size I get it is not 1000 , but 1008  !Same for malloc.
>
>  From the C Runtime Library Reference:
>
> ----
> Syntax
>
> #include <malloc.h>
> size_t _msize(void *block);
>
> Description
>
> Returns the size of a heap block.
>
> _msize returns the size of the allocated heap block whose address is
> block. The block must have been allocated with malloc, calloc, or
> realloc. The returned size can be larger than the number of bytes
> originally requested when the block was allocated.
>
> Return Value
>
> _msize returns the size of the block in bytes.
> ----
>
> Especially take care of this sentence:
> The returned size can be larger than the number of bytes originally
> requested when the block was allocated.
>
> I suppose the reason for this is because it would take longer to give
> you the EXACT amount of memory, so it gives the amount that is nearly
> (and at least) what you need, but what can be given the fastest - only
> what I think.
>
> But you should always use the REQUESTED amount, not the actually
> RECIEVED amount. If calloc doesn't return NULL, then the requested
> amount is at your hand for sure.
>
> Anyway, why is it a problem if you have more memory? If I get more money
> for example, I would rather keep it, than complaining about it :)
> --
> Greetings,
>    Bal=E1zs
>
>
>
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