unpack('>f', b'\x00\x01\x00\x00')

[kuaile xu]

On Nov 30, 6:02 pm, Chris Rebert <c... at rebertia.com> wrote:
> On Wed, Nov 30, 2011 at 2:24 PM, kuaile xu <kuaile... at gmail.com> wrote:
> > Hi:
>
> > I am working on a python script that parses mp4 video header. Once of
> > the field is a 32-bit fixed-point number.
>
> > I know that the four bytes are: 00, 01, 00, 00. I have a third party
> > mp4 parsing program which displays this field's value is:1.0.
>
> > However, the struct.unpack gets a value of 0.0.
>
> > Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit
> > (AMD64)] on win32
> > Type "help", "copyright", "credits" or "license" for more information.
> >>>> from struct import *
> >>>> unpack('>f', b'\x00\x01\x00\x00')
> > (9.183549615799121e-41,)
>
> Floating-point and fixed-point are *separate* number formats with
> distinct representations. You cannot expect to correctly (un)pack one
> as if it was the other. Similarly, converting between the two formats
> can introduce range and/or imprecision error.
>
> C does not have a built-in fixed-point datatype, so the `struct`
> module doesn't handle fixed-point numbers directly. You're going to
> have to unpack it (or parts of it) as something more raw, and then do
> the necessary bit manipulation yourself.
>
> Cheers,
> Chris
> --http://rebertia.com
>
>

Thanks a lot.