How to pass an array of structs to a DLL function using cpython?

Question

#include <stdio.h>

struct struct_data {
    int price;
    char list[255];
    int count;
};

__declspec(dllexport) void __stdcall testcall(struct struct_data *array_data) {

    int i = 0;
    while(array_data[i].price != 0) {           // array_data is guaranteed to have a zero somewhere inside memory allocated for the array
        printf("%d\n", array_data[i].price);
        i++;
    }

}

I try to call it from Python using cpython as follows:

# C Backend imports
import os
import sys
import ctypes

# load DLL
dll = ctypes.WinDLL('C:\\Users\\user\Documents\\Pelles C Projects\\MarketlibDLL\\MarketlibDLL.dll')

class struct_data(ctypes.Structure):
    _fields_ = [
        ('price', ctypes.c_int),
        ('list', ctypes.c_char * 255),
        ('count', ctypes.c_int),
    ]

d = struct_data() * 100    # I try to do:   struct struct_data d[100]; 
d[0].price = 100
d[0].list = "ABC"
d[0].count = 3
d[1].price = 0

dll.testcall.argtypes = [ctypes.POINTER(struct_data)]
dll.testcall(d)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-36161e840b5f> in <module>
----> 1 d = struct_data() * 100
      2 d[0].price = 100
      3 d[0].list = "ABC"
      4 d[0].count = 3
      5 d[1].price = 0

TypeError: unsupported operand type(s) for *: 'struct_data' and 'int'

@golobich see edited answer, I am trying to create a 100 items array of struct_data's that I can pass to the C function — M.E.
– M.E., Commented Aug 25, 2019 at 11:37

CristiFati · Accepted Answer · 2019-08-27 08:27:33Z

2

[Python 3.Docs]: ctypes - A foreign function library for Python has all the required info (the Arrays section contains an example that maps perfectly over the current scenario).

Modify your code to (split in 2 lines for readability):

struct_data_array_100 = struct_data * 100  # !!! This is a type (array consisting of 100 struct_data elements) !!!
d = struct_data_array_100()  # Instantiate the array type
#d[0].price ...
# ...

edited Aug 27, 2019 at 8:27

answered Aug 26, 2019 at 14:37

CristiFati

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Comments

hyyking · Accepted Answer · 2019-08-26 12:31:01Z

0

if you want a list of 100 struct_data you need to multiply the list not the data:

d = [struct_data()] * 100

This is the same as writing:

d = [struct_data() for _ in range(100)]

answered Aug 26, 2019 at 12:31

hyyking

392 bronze badges

1 Comment

M.E. Over a year ago

I want a C array of 100 struct_data objects, which is different from a list. I finally learned I have to do this: d = (struct_data * 100)()

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