How to make python binding to typedef pointer alias

Question

I am trying to use libfreefare in python and I decided to use ctypes. I understood how to reproduct structures in Python, however I don't know the way to reproduce a specific thing.

Consider this structure :

struct mifare_tag {
    nfc_device *device;
    nfc_iso14443a_info info;
    const struct supported_tag *tag_info;
    int active;
};

And the following typedef :

typedef struct mifare_tag *MifareTag;

How to make the equivalent of MifareTag in ctypes ?

deets · Accepted Answer · 2014-10-05 11:27:59Z

1

This is a run-of-the-mill struct declaration in ctypes. Something like this (without knowing the nfc_*-types, it can't be complete obviously.

from ctypes import Structure, POINTER, c_int


class mifare_tag(Structure):

    _fields_ = [
        ("device", POINTER(nfc_device)),
        ("info", nfc_iso14443a_info),
        ("tag_info", POINTER(supported_tag)),
        ("active", c_int),
        ]


MifareTag = POINTER(mifare_tag)

answered Oct 5, 2014 at 11:27

deets

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7 Comments

Luke Marlin Over a year ago

I'm only starting with ctypes, so just to be sure about your code : MifareTag would be a class that I can use as a normal one, right ?

Eryk Sun Over a year ago

@LukeMarlin, MifareTag is a pointer type.

deets Over a year ago

eryksun says it - MifareTag is a pointer. Don't be confused by the sprinkling of "struct" here and there, it's just idiosyncratic C.The important bit of the typedef is, that it defines MifareTag to be of type "struct mifare_tag ". So no, it is *not a normal object. Now it depends on your library/code. If you get values of MifareTag back from the lib, just use the contents-attribute to access the various items in it. If you have to create a new tag, use the mifare_tag-class, and the pointer or byref calls to pass it to the lib.

Luke Marlin Over a year ago

@deets Thank you, so I can create MifareTag in python, which would effectively be a wrapper of a mifare_tag value, right ?

Luke Marlin Over a year ago

I am currently trying to make other struct definitions, but each one leads to many others and I foresee it will be tedious. Should I switch to something like cython, or swig ?

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