Calling a C function in Python and returning 2 values

Question

I am trying to figure out how to return 2 vales from a C function that I called in python. I have read through the material online and am using struct to output the two vari

Answer 1

Your C code is fine, the problem you are experiencing is in how you use python ctypes. You should tell that the function returns a struct re_val and not a double:

calling_function.c_func.restype =  ctypes.c_double

The above makes the function return a single double value in the eyes of ctypes. You should tell python that the function returns a structure:

import ctypes as ct

# Python representation of the C struct re_val
class ReVal(ct.Structure):
    _fields_ = [("predict_label", ct.c_double),("prob_estimates", ct.c_double)]

calling_function = ctypes.CDLL("/home/ruven/Documents/Sonar/C interface/Interface.so")
calling_function.c_func.argtypes = [ctypes.c_char_p, ctypes.c_double, ctypes.c_double, ctypes.c_double, ctypes.c_double]
# and instead of c_double use:
calling_function.c_func.restype = ReVal

This way you tell python's ctypes that the function returns a aggregate object that is a subclass of ctypes.Structure that matches the struct re_val from the c library.

NOTE Be very carefull with argtypes and restype, if you use these incorrectly it is easy to crash the python interpreter. Then you get a segfault instead of a nice traceback.