Passing nested ctypes structure with array to C library function not as expected

Question

I have the following C code which I would use from a python script.

This is just an excerpt of an auto-generated huge library which I'm not able to change, unfortunately. Here I just wanted to print the structure elements to a console to demonstrate what is going wrong.

// CFunc.h
#include <stdio.h>

typedef struct
{
    int npar;
    struct
    {
        int id;
        int value;
    } params[10];
} Data_t;

void Cfunc( const Data_t * d);


// CFunc.c

#include "CFunc.h"

void Cfunc( const Data_t * d)
{
    int inpar = 0;
    int maxnpar = 0;

    printf("%d:\n", d->npar);
    maxnpar = d->npar;

    inpar=0;
    while (maxnpar > inpar)
    {
        printf("  %d: %08x %08x\n", inpar, d->params[inpar].id, *(int*)&d->params[inpar].value);
        inpar++;
    }
}

It is compiled and linked to a share library with:

gcc -fPIC -c CFunc.c -o CFunc.o
gcc -shared -lrt -Wl,-soname,libCFunc.so.1 -o libCFunc.so CFunc.o

So I did the following implementation using ctypes:

from ctypes import *

lib = CDLL('./libCFunc.so')


class Data_2(Structure):
    pass
class Data_t(Structure):
    def __init__(self, list):
        self.npar = len(list)
        self.params = (Data_2 * self.npar)(*list)

Data_2._fields_ = [
    ('id', c_int),
    ('value', c_int),
]
Data_t._fields_ = [
    ('npar', c_int),
    ('params', POINTER(Data_2)),
]

def pyFunc(d):
    lib.Cfunc.argtypes = (POINTER(Data_t),)

    lib.Cfunc(byref(d))

    return

So I'm initializing the structure out of a list of given tuples, in this case just 2 and calling the C-function to see its output.

paramlist = ( 
    ( 0x050000000, 0x00000000 ),  
    ( 0x050000001, 0x447a0000 ) )
temp = Data_t(paramlist)

pyFunc(temp)

Unfortunately the output is not as expected:

2:
0: 00000000 79948ef0
1: 00007fe5 00000000

Any thoughts what I am missing ?

Answer 1

[Python 3]: ctypes - A foreign function library for Python.

• The structs from C and Python don't match
  • params is an array not a pointer
• Because of the above inconsistency, you overcomplicated things in Python:
  • You don't have incomplete types, so your structures can be statically defined

I restructured your code a bit.

dll.h:

#pragma once


typedef struct Data_ {
    int npar;
    struct
    {
        int id;
        int value;
    } params[10];
} Data;


void test(const Data *d);

dll.c:

#include "dll.h"
#include <stdio.h>


void test(const Data *d) {
    int inpar = 0;
    int maxnpar = 0;

    printf("%d:\n", d->npar);
    maxnpar = d->npar;

    inpar = 0;
    while (inpar < maxnpar)
    {
        printf("  %d: %08x %08x\n", inpar, d->params[inpar].id, *(int*)&d->params[inpar].value);
        inpar++;
    }
}

code.py:

#!/usr/bin/env python3

import sys
import ctypes


DLL = "./libdll.so"


class DataInner(ctypes.Structure):
    _fields_ = [
        ("id", ctypes.c_int),
        ("value", ctypes.c_int),
    ]


DataInnerArr10 = DataInner * 10


class Data(ctypes.Structure):
    _fields_ = [
        ("npar", ctypes.c_int),
        ("params", DataInnerArr10),
    ]

    def __init__(self, data):
        self.npar = len(data)
        self.params = DataInnerArr10(*data)


def main():
    dll_dll = ctypes.CDLL(DLL)
    test_func = dll_dll.test
    test_func.argtypes = [ctypes.POINTER(Data)]

    param_list = (
        (0x050000000, 0x00000000),
        (0x050000001, 0x447a0000),
    )

    d = Data(param_list)

    test_func(ctypes.byref(d))
    print("Done.")


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Output:

[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054888242]> ls
code.py  dll.c  dll.h
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054888242]> gcc -shared -fPIC -o libdll.so dll.c
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054888242]> ls
code.py  dll.c  dll.h  libdll.so
[cfati@cfati-5510-0:/cygdrive/e/Work/Dev/StackOverflow/q054888242]> python3 code.py
Python 3.6.4 (default, Jan  7 2018, 15:53:53)
[GCC 6.4.0] on cygwin

2:
  0: 50000000 00000000
  1: 50000001 447a0000
Done.