Obtaining address of custom data-type in C from Python using ctypes

问题

I have a vector struct in C with the following fields,

struct vector {
    unsigned char* data;
    unsigned long size;
    unsigned long elemsize;
    unsigned long capacity;
};

and there are a few functions which correspondingly act on vector instances, such as:

struct vector* vector_new(unsigned long elemsize);
void vector_delete(struct vector* vec);
void vector_push_back(struct vector* vec, void* value, unsigned long elemsize);
void vector_reserve(struct vector* vec, unsigned long cap);
...

and so on (mimicking a c++ style std::vector).

In other sections of my code base I have component structs, for example mirror:

struct mirror {
    double R;
    double T;
    // extra fields omitted - see mirror_wrapper.py below
    struct vector* input[2]; // [vector<beam>*, vector<beam>*]
    struct vector* output[2]; // [vector<beam>*, vector<beam>*]
};

which, amongst others, has the following methods,

struct mirror* mirror_alloc();
int mirror_init(double R, double T, struct mirror* mrr);
// ibn is the "initial-beam-number"
void mirror_set_left_input(struct vector** input, unsigned long ibn, struct mirror* mrr);
void mirror_set_right_input(struct vector** input, unsigned long ibn, struct mirror* mrr);

where one passes the address of another components' output field to these set_input methods to "connect" them.

The input and output fields of every component always store instances of struct beam - a data-type which simply stores a double complex field and a double field.

Currently I am in the process of building wrappers in Python (3.5) to call the various methods in an object-oriented to use later on for easier plotting and so forth; using ctypes to build a shared library of the C code.

The following are the wrappers that I have so far,

vector_wrapper.py

from ctypes import cdll
from ctypes import Structure
from ctypes import c_ubyte
from ctypes import c_ulong
from ctypes import POINTER

class Vector(structure):
    _fields_ = [
        ("data", POINTER(c_ubyte)),
        ("size", c_ulong),
        ("elemsize", c_ulong),
        ("capacity", c_ulong)]

mirror_wrapper.py

from ctypes import cdll
from ctypes import Structure
from ctypes import byref
from ctypes import c_double
from ctypes import c_ubyte
from ctypes import c_ulong
from ctypes import c_bool
from ctypes import POINTER
from ctypes import pointer
from vector_wrapper import Vector
lib = cdll.LoadLibrary('./ctn_lib.so')

class Mirror(Structure):
    _fields_ = [
        ("R", c_double),
        ("T", c_double),
        ("pR", c_double),
        ("pT", c_double),
        ("tuning", c_double),
        ("mass", POINTER(c_double)),
        ("last_time", c_double),
        ("net_force", c_double),
        ("dfcoeff", c_double),
        ("phfcoeff", c_double*2), #phfcoeff is a double complex in c code
        ("rpfcoeff", c_double),
        ("input", POINTER(Vector)*2),
        ("output", POINTER(Vector)*2),
        ("has_left_input", c_bool),
        ("has_right_input", c_bool)]

Is there any way in which I can get the address of an output field of some component (say a mirror), which has the type struct vector**, and pass this to, e.g., some function Mirror.set_left_input as the input parameter?

Also, I assume it is necessary to create all fields in the _fields_ of a python structure corresponding to the fields in the C structs - i.e. is it possible to omit some fields from this descriptor or not?

回答1:

Is there any way in which I can get the address of an output field of some component (say a mirror), which has the type struct vector**, and pass this to, e.g., some function Mirror.set_left_input as the input parameter?

To access the output component of a Mirror given your structures:

>>> m = Mirror()
>>> m.output[0]
<__main__.LP_Vector object at 0x00000199607CA0C8>

To pass it to a function by reference:

>>> mirror_set_left_input(byref(m.output[0]),...)

Also, I assume it is necessary to create all fields in the fields of a python structure corresponding to the fields in the C structs - i.e. is it possible to omit some fields from this descriptor or not?

No, you cannot omit fields from your structure definition, or the binary layout of the underlying C structure won't be correct.

来源：https://stackoverflow.com/questions/41639816/obtaining-address-of-custom-data-type-in-c-from-python-using-ctypes

标签

python

ctypes

python-c-extension