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 -- http://mail.python.org/mailman/listinfo/python-list
