ctypes' c_longdouble: underflow error (bug?)

[Thomas Jollans]

On 07/14/2010 03:24 PM, kj wrote:
> 
> 
> 
> I have a C library function hg that returns a long double, so when
> I import it using C types I specify this return type like this:
> 
> MYLIB.hg.restype = ctypes.c_longdouble
> 
> But certain non-zero values returned by hg appear as zero Python-side.
> If I modify hg so that it prints out its value right before returning
> it, I get stuff like the following:
> 
>>>> 0 == MYLIB.hg(100, 200, 100, 6000)
> from hg: 2.96517e-161 
> False
>>>> 0 == MYLIB.hg(200, 200, 200, 6000)
> from hg: 5.28791e-380
> True
> 
> So, although the value returned by hg in the second invocation
> above is 5.28791e-380, Python sees it as 0.
> 
> What am I doing wrong?

Nothing.

http://docs.python.org/library/ctypes.html#fundamental-data-types

c_longdouble maps to float

http://docs.python.org/library/stdtypes.html#numeric-types-int-float-long-complex

"floating point numbers are implemented using double in C"

ergo, the extra precision a long double gives you versus a normal double
is lost if you use Python (or, at least, ctypes)

If you really want to keep the precision, you need a new/different type.
ctypes won't help you. Cython and NumPy may or may not be useful here.

 - Thomas