Re: How to byte swap an IEEE Float?

[Marius Amado Alves <amado.alves@netcabo.pt>]

On 28 Jun 2005, at 16:55, Simon Wright wrote:

> Marius Amado Alves <amado.alves@netcabo.pt> writes:
>
>> On 28 Jun 2005, at 14:53, Björn wrote:
>>
>>> I need to read some float values from file that have been written in
>>> big-endian byte order from a c-program. The simple swapping procedure
>>> that I have just interchanges the byte order of type IEEE_Float_32 to
>>> get little-endian. The problem is that for some values (eg. 
>>> 33.229000)
>>> it is a "NaN" when doing IEEE_Float_32'Read and I get a constraint
>>> error (invalid data) from stream_io when the value is read. How do I
>>> get around this?
>>
>> Simple. Setup an array of bytes with the same representation of the
>> float type (use representation clauses and pragma Pack). Read the
>> array of bytes. Swap the bytes. Convert to the float type (use
>> unchecked conversion).
>
> Marius,
>
> I think you mean 'of the same length as the float type'?
>
> OP doesn't say what processor he gets the BE value from, but assuming
> PowerPC you either get Float (4 bytes) for digits <= 6 or Long_Float
> (8 bytes) for digits 7 .. 15.
>
> On Intel hardware there's an 80-bit Long_Long_Float (digits 18).

Sure. You know, I meant representation = bits and bytes, implying same 
length. It seems the OP already has the float type (IEEE_Float_32), 
32-bit = 4-byte long, and the data are floats of this length, only 
different in byte order.

OP, if the data are of a different length, then you have to unchecked 
convert first to a float type of that length, and then normal convert 
to the final type (risking Constraint_Error).