I am trying to interact with a dll from python using ctypes, reading the documentation the C++ method signature is of the following:
my_c_pp_function(user *param1[],const int_8 param2,const int_8 values3[],const int_8 length_param1)
Essentially the c++ function requires a list of users, an integer,a list of values and the number of users, also an integer
Note: the users is a structure containing name,age, and id. Something like:
typedef struct
{
char name[255];
int_16 age;
int_32 uid;
}user;
When I try calling this function from python code using ctypes I do:
def call_my_c_pp_function(list_of_users,int_param2,list_of_values,int_lenght_of_list):
myparam1=(ctypes.c_char_p * len(list_of_users))(*list_of_users)
myparam2=ctypes.c_int8(int_param2)
myparam3=(ctypes.c_int8 * len(list_of_values))(*list_of_values)
myparam4=ctypes.c_int8(int_lenght_of_list)
self.dll_object.my_c_pp_function.argtypes(ctypes.POINTER(ctypes.c_char_p),ctypes.c_int8,ctypes.POINTER(ctypes.c_int8),ctypes.c_int8)
ret_value=self.dll_object.my_c_pp_function(myparam1,myparam2,myparam3,myparam4)
Now every time I call the python function I get an error basically if the function succeeds the return value should be 0, any non zero number indicates some kind of problem.
I keep getting a large non-zero number as the return value. What could I possibly be doing wrong? is the way I'm creating the array of users, and the array of values wrong?
I am not sure how to populate/make use of the c++ user structure in my python code so that my list is not just a list of strings but a list of users
I'm using Python2.7 on Windows
Regards
Assuming that you cannot change the signature of your library function.
I would recommend you to create a simple adapter for your c++ function, which just acts as an interface to your actual function. This function would just take all the values you need and then convert them to your needed types.
Here is a working example (tested with python27). Note that I added an additional parameter for the number of elements in the int array.
py_caller.py
import ctypes
import os
class user(ctypes.Structure):
_fields_ = [("name", ctypes.c_char_p),
("age", ctypes.c_int),
("uid", ctypes.c_int)]
def create_users(count):
users = (user * count)()
for i in range(count):
users[i].name = ctypes.c_char_p("user" + str(i))
users[i].age = i
users[i].uid = i * i
return users
def create_values(count):
values = (ctypes.c_int * count)()
for i in range(count):
values[i] = i ** 2
return values
def main():
os.environ['PATH'] += ";" + os.path.dirname(os.path.abspath(__file__))
dll = ctypes.cdll.LoadLibrary('cppdll')
count = 4
users = create_users(count)
n_users = ctypes.c_int(count)
values = create_values(count)
n_values = ctypes.c_int(count)
some_int = ctypes.c_int(42)
res = dll.func(users, n_users, some_int, values, n_values)
print (res)
if __name__ == "__main__":
main()
export.h
#pragma once
typedef struct t_user
{
char *name;
int age;
int uid;
};
extern "C" __declspec(dllexport) int func(t_user *users, int n_users, int val, int *values, int n_values);
export.cpp
#include "export.h"
#include <iostream>
int func(t_user *users, int n_users, int val, int *values, int n_values)
{
std::cout << "users:\n";
for (auto i = 0; i < n_users; ++i)
{
std::cout << users[i].name
<< ": " << users[i].age
<< " - " << users[i].uid << "\n";
}
std::cout << "values:\n";
for (auto i = 0; i < n_values; ++i)
std::cout << values[i] << "\n";
std::cout << "some int: " << val << "\n";
// do something else with the values
// such as calling your actual library function
return 1;
}
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