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Date: Wed, 29 Jun 2005 22:10:57 +0200
From: Damien <damien.carbonne@free.fr>
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Subject: Re: How to byte swap an IEEE Float?
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Date: 2005-06-29T22:11:00+02:00
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Bj�rn a �crit :
> 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?
> 
I had to deal with byte swapping for floats and doubles between Intel 
and PPC. This was C++ and I discovered that swapping had to be done in a 
neutral form (byte array) before interpretation to float or double. 
Initialy I directly swapped floats and doubles, and for certain values 
some bits were changed.
So a function with such a signature should never be defined:
function Swap (X : Float) return Float;
It seems that all bit patterns can not be legal floats. I interpreted 
this as a normalisation of the float, but this is not sure at all.

So, to swap a float or a double X, do this :
1) convert X to a byte array Y (right size)
2) swap Y from host 1 format to network or archive format
3) send Y
4) read Y
5) swap Y from network or archive format to host 2 format
6) convert Y to X

I don't know if this is related to your problem, but it seems similar, 
and it may help.
It was C++, but those issues are language independent.

Damien Carbonne