Re: I_to_LEbytes/LEbytes_to_I

[Sam S. <sd...@gn...>]

> * Bruno Haible <oe...@py...> [2005-05-30 14:47:28 +0200]:
>
> Sam wrote:
>> >> 1. I_to_LEbytes() interface is counterintuitive: the size argument is
>> >>    the bit size of the object (which can be computed by I_to_LEbytes() -
>> >>    actually, it must be 1+I_integer_length(obj) for positive obj!),
>> >
>> > The I_to_LEbytes() interface is made for the cases where you know in
>> > advance the maximum size of the integers
>> 
>> no, I have no idea about the maximum size of the integers.
>> ...
>> I want to be able to simply do
>> 
>>       byte buffer[64];
>>       I_to_LEbytes(fixnum(7),buffer,64);
>
> Where do you know that a buffer of size 64 will be sufficient, if
> you don't know about the maximum size of the integers?

I don't - I want I_to_LEbytes() to find that out and return an error
code if the buffer space is insufficient.

>> suppose I have fixed size records in the database, e.g., 64 bytes.
>> suppose I want to write number 7 (3 bits) there.
>> right now I have to do this:
>> 
>>       byte buffer[64];
>>       memset(buffer,0,64);
>>       I_to_LEbytes(fixnum(7),1+I_integer_length(fixnum(7)),buffer,64);
>> 
>> I want to be able to simply do
>> 
>>       byte buffer[64];
>>       I_to_LEbytes(fixnum(7),buffer,64);
>
> The current interface allows you to do this through
>
>         byte buffer[64];
>         I_to_LEbytes(fixnum(7),8*64,buffer);

if you remove memset() from modules/berkeley-db/bdb.c:fill_dbt(), the
tests will fail.

> Which is - except for the multiplication by 8 - exactly what you are
> asking for.

what is "8"?
how do I know it's not "3" or "17"?

>> the normal way to interface with a serializing facility is:
>> 
>> int serialize_foo   (foo_t object,  int buffer_size, byte* buffer);
>> int deserialize_foo (foo_t *object, int buffer_size, byte* buffer);
>> int foo_size (foo_t object);
>> 
>> and serialize_foo() should take care of padding.
>
> Yes. This is why I_to_LEbytes()'s implementation ends with a memset.

no I_to_LEbytes does not pad, see the bdb above.

>> the current interface is highly unusual:
>> 
>> 1. an extra argument to serialize_foo()
>
> No. It takes the usual 3 arguments: the object, the buffer, and the
> buffer's size.

the size is in the wrong units.

there is no function that returns the size, like foo_size() above.

how do I write

      int size = foo_size(x);
      byte buf = malloc(size);
      serialize_foo(x,size,buf);



>> 2. user has to do the padding, not serialize_foo()
> No.
see above.
if the I_to_LEbytes does padding, it is broken.



-- 
Sam Steingold (http://www.podval.org/~sds) running w2k
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