C to Python via Ctypes - Wrapping Struct of Function Pointers to Static Functions

Question

I have structs in a C library that are like this. The function pointers in DataFn point to static functions.

.h

struct Data {
    int i;
    int *array;
};

typedef struct {
    bool (* const fn1) (struct Data*, const char *source);
    ....
} DataFn;
extern DataFn const DATAFUNC

Using objdump, the table only contains DATAFUNC and a few other things from gcc.

This is fine in C where calling fn1 would go like DATAFUNC.fn1(..., ...), but how would something like this be wrapped around so fn1 can be called in python w/ ctypes?

Example python

libc = ctypes.cdll.LoadLibrary("./data.so")
print(libc.DATAFUNC)

results in <_FuncPtr object at 0x6ffffcd7430>

This is similar, but there isn't a factory function.

CristiFati · Accepted Answer · 2022-07-31 18:57:48Z

[Python.Docs]: ctypes - A foreign function library for Python contains everything required to solve this problem.

I believe that the main piece missing, was the in_dll method of a CTypes type (Accessing values exported from dll section).

Other than that, in order to work with C data, you need to let Python know of the data format. That applies to:

structs. Define Python counterparts by subclassing ctypes.Structure
Function pointers (applies to your case). Define them using ctypes.CFUNCTYPE

I prepared a simplified example that illustrates the above. Note that I didn't do any error handling (checking for NULLs (which you should)), to keep things simple.

dll00.h:

struct Data {
    int i;
};


typedef struct {
    int (* const Func00Ptr) (struct Data*, const char*);
} DataFunc;


extern DataFunc const dataFunc;

dll00.c:

#include <stdio.h>

#include "dll00.h"


static int func00(struct Data *pData, const char *source)
{
    printf("From C - Data.i: [%d], source: [%s]\n", pData->i, source);
    return -255;
}


DataFunc const dataFunc = { &func00 };

code00.py:

#!/usr/bin/env python

import ctypes as ct
import sys


DLL_NAME = "./dll00.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")


class Data(ct.Structure):
    _fields_ = (
        ("i", ct.c_int),
    )


Func00Type = ct.CFUNCTYPE(ct.c_int, ct.POINTER(Data), ct.c_char_p)


class DataFunc(ct.Structure):
    _fields_ = (
        ("func00", Func00Type),
    )


def main(*argv):
    data = Data(127)
    dll = ct.CDLL(DLL_NAME)
    data_func = DataFunc.in_dll(dll, "dataFunc")
    ret = data_func.func00(ct.byref(data), "abcd".encode())
    print("Function returned: {:d}".format(ret))


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.")
    sys.exit(rc)

Output:

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q049962265]> ~/sopr.sh
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[064bit prompt]> ls
dll00.c  dll00.h  code00.py
[064bit prompt]> gcc -shared -fPIC -o dll00.so dll00.c
[064bit prompt]> ls
dll00.c  dll00.h  code.py  dll00.so
[064bit prompt]> objdump -t dll00.so | grep dataFunc
0000000000200e10 g     O .data.rel.ro   0000000000000008              dataFunc
[064bit prompt]> python code00.py
Python 3.5.2 (default, Nov 23 2017, 16:37:01) [GCC 5.4.0 20160609] 064bit on linux

From C - Data.i: [127], source: [abcd]
Function returned: -255

Done.

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