Subj : Re: reading Java floats from C
To   : comp.lang.c,comp.programming
From : rem642b
Date : Tue Jun 28 2005 06:48 pm

(I've cross-posted this to comp.programming where it's more relevant.
Also I've blacked out the specific names of programming languages
because that's irrelevant to my general answer.)
> From: sbalko@gmail.com
> I am trying to read ###-floats (IEEE ??? encoding) stored in a binary
> file from %%% (??? on ???, more specifically). Unfortunately, %%% seems
> to expect floats to be stored somewhat differently than ### does. I
> suspected an endianess problem and tried out ntohl/htonl but it
> doesn't help. Any clues?

If you can't find such an answer from online documents, why didn't you
just do some experiments? For example, try this to see how ### writes
floats in binary mode: Write a test program that writes out exactly
five values of exactly 0.0, then write out these values in sequence:
9.0 0.0 10.0 0.0 11.0 0.0 12.0 0.0 13.0 0.0 14.0 0.0 15.0 0.0, and then
examine the resultant file to see if you can find:
- The same exact pattern repeating exactly five times before it's
broken by other patterns not the same, to show you what the 0.0 looks
like in binary file format.
- Alternating original pattern and other patterns the same length, to
make sure you haven't accidently used different precision for the
non-zero values generated from the index variable in your loop and the
zero values generated by literals.
- Among those non-zero groups of bytes, see if you can find a bit
pattern that goes somewhat like this:
1001
1010
1011
1100
1101
1110
1111
The '1' might be missing if it's in a notation where the 1 is assumed
rather than explicit, but the other bits should follow that pattern.

At that point you have a good idea where the mantissa is located. Now
to find where the exponent is located, generate this sequence:
0.0 1.0 0.0 2.0 0.0 4.0 0.0 8.0 0.0 16.0 0.0 32.0 0.0 64.0 0.0
You should see a similar pattern in the bits.

Finally you need to know how negative numbers are expressed.
I leave that as an exercise for the reader.

Once you know all that for ###, do the same for %%%.
Write that test data from the program that will be doing reading.
(Unless it's totally broken, it should write data in the same layout
that it expects to read it in.)

Now compare what you learned about ### and %%%, whether sequence of
bytes is the only difference, or there's a more complicated difference
in representation.

.