long double returns and ctypes

Question

i have a c function which returns a long double. i'd like to call this function from python using ctypes, and it mostly works. setting so.func.restype = c_longdouble does the trick -- except that python's float type is a c_double so if the returned value is larger than a double, but well within the bounds of a long double, python still gets inf as the return value. i'm on a 64 bit processor and sizeof(long double) is 16.

any ideas on getting around this (e.g. using the decimal class or numpy) without modifying the c code?

Answer 1

I'm not sure you can do it without modifying the C code. ctypes seems to have really bad support for long doubles - you can't manipulate them like numbers at all, all you can do is convert them back and forth between the native float Python type.

You can't even use a byte array as the return value instead of a c_longdouble, because of the ABI - floating-point values aren't returned in the %eax register or on the stack like normal return values, they're passed through the hardware-specific floating-point registers.

Answer 2

If you need high-precision floating point, have a look at GMPY.

GMPY is a C-coded Python extension module that wraps the GMP library to provide to Python code fast multiprecision arithmetic (integer, rational, and float), random number generation, advanced number-theoretical functions, and more.

GMP contains high-level floating-point arithmetic functions (mpf). This is the GMP function category to use if the C type `double' doesn't give enough precision for an application. There are about 65 functions in this category.