using c structures from a shared library with ctypes in Python

Question

I've done a lot of research and nothing came up... I'm new to Python and Ctypes and I'm trying to call functions from a shared library. So far so good, but these functions take as parameter specifics datatypes from structures defined inside the .so

my question is, I've seen examples of how to declare the "class Structure" in Python, but this is what I have in the .so

typedef struct noPDDE
{
     void *x;
     struct noPDDE *y;
     struct noPDDE *z;
}NoPDDE,*pNoPDDE;

typedef struct PDDE
{
    int tam;
    pNoPDDE sup;
}PDDE;

I have no idea how to pass the PDDE pointer to the functions.

Any help is useful. thanks a lot.

PDDE is a struct, not a pointer. Do you mean "pass a pointer to a PDDE struct"? — cdarke
– cdarke, Commented Jul 19, 2015 at 15:01

deets · Accepted Answer · 2015-07-19 20:43:21Z

4

This is the way one declares recursive structures in ctypes:

 from ctypes import (
     Structure,
     c_void_p,
     POINTER,
     c_int,
     byref,
 )


 class noPDDE(Structure):
     pass

 noPDDE._fields_ = [
     ("x", c_void_p),
     ("y", POINTER(noPDDE)),
     ("z", POINTER(noPDDE)),
     ]


 class PDDE(Structure):
     _fields_ = [
         ("tam", c_int),
         ("sup", POINTER(noPDDE)),
         ]



 foo = PDDE()

 mylib.func_that_takes_pointer_to_pdde(byref(foo))

edited Jul 19, 2015 at 20:43

answered Jul 19, 2015 at 16:00

deets

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5 Comments

Aurelio Grott Over a year ago

I did exactly as you wrote, but still something missing here.. The first error is "cannot import void_p" , so I used ("x", c_void_p). Ok. now, its telling me that the second element on fields in PDDE class has to be a c type...

deets Over a year ago

I made a mistake, because I obviously can't run your code due to the lack of that weird library of yours, and corrected it. But you will need to figure similar things out on your own, the error-messages is pretty clear, and the documentation on ctypes-Structure as well.

Aurelio Grott Over a year ago

ok, thank you very much @deets ! I had no idea how to declare recursive structures.

Aurelio Grott Over a year ago

I already did @deets , but I just created my account, I guess I don't have enough reputation :/

deets Over a year ago

@eryksun drat. You are correct. I did create a simple example without the call, because I possibly can't have a real c-call that works. But I should have checked with pylint.

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