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 variables. I am able to output the variables when I call this function in the same C file. However, when I try to call it in python, it still only returns one value.

This is my C code:

struct re_val {
 double predict_label;
 double prob_estimates; 
};
struct re_val c_func(const char* dir, double a, double b, double c, double d )
{
 double x[] = {a,b,c,d};
 printf ("x[0].index: %d \n", 1);
 printf ("x[0].value: %f \n", x[0]);
 printf ("x[1].index: %d \n", 2);
 printf ("x[1].value: %f \n", x[1]);
 printf ("x[2].index: %d \n", 3);
 printf ("x[2].value: %f \n", x[2]);
 printf ("x[3].index: %d \n", 4);
 printf ("x[3].value: %f \n", x[3]);
 printf ("\nThis is the Directory: %s \n", dir); 
 struct re_val r;
 r.predict_label = 5.0;
 r.prob_estimates = 8.0; 
 return r;
}

This is my Python code:

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]
calling_function.c_func.restype = ctypes.c_double
q = calling_function.c_func("hello",1.3256, 2.45, 3.1248, 4.215440)
print q

Currently, when I run my python file in the terminal it outputs this:

x[0].index: 1 
x[0].value: 1.325600 
x[1].index: 2 
x[1].value: 2.450000 
x[2].index: 3 
x[2].value: 3.124800 
x[3].index: 4 
x[3].value: 4.215440 
This is the Directory: hello 
5.0

Instead,I would like it to output this:

x[0].index: 1 
x[0].value: 1.325600 
x[1].index: 2 
x[1].value: 2.450000 
x[2].index: 3 
x[2].value: 3.124800 
x[3].index: 4 
x[3].value: 4.215440 
This is the Directory: hello 
5.0
8.0

• Junhee Shin: I don't know this code will work or not but. check this link. it may be helpful to you. gist.github.com/Overdrivr/cdd58cea15d7e28c50ea
• hetepeperfan: The problem is incalling_function.c_func.restype = ctypes.c_double You are essentially telling to ctypes the the function result type is a c_double, whereas the function from you c library doesn't return a double, it returns a struct re_val.
• Ruven Guna: @hetepeperfan I thought about that but i couldn't find a way to change calling_function.c_func.restype to return a struct as ctypes.c_struct does not exist.

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.

• Ruven Guna: Hi, unfortunately when I run the program it gives me the following message: <__main__.ReVal object at 0x7fb3bd1e0560>
• hetepeperfan: Yes, that is right, but that is the struct as returned by ctypes, it is still an aggregate. So the returned object has two fields "predict_label" and "prob_estimates". So you can access them via q.predict_label and q.prob_estimates You generally need to take some extra steps to extract the values from the struct.