Re: [gpsd-dev] [PATCH 3/3] Print offset in ntpshmmon instead of reference time

[Gary E. Miller]

Yo Miroslav!

> > -                    offset = (float(record[3]) - float(record[4]))
> > +                    offset = float(record[4])
> 
> Python floats are 53 bits of precision.  The offset needs 62 bits of 
> precision.  So this discards the imporant parts of the offset.
> 
> Take a look at this program to see how it fails:
>       test_timespec -v


My mistake, Python floats are only 52 bits of precision:

http://stackoverflow.com/questions/8216088/how-to-check-the-size-of-a-float-in-python

Run this program:

import numpy as np
for f in (np.float32, np.float64, float):
    finfo = np.finfo(f)
    print finfo.dtype, finfo.nexp, finfo.nmant

You get this result on Gentoo 64 bit:

float32 8 23
float64 11 52
float64 11 52

Run this program:

d = float("1431035819.123456789");
print("{0:.9f}".format(d))


And you get:

1431035819.123456717

Not acceptable.

Maybe you can try float128?

RGDS
GARY
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