[Numpy-discussion] A question about ctypes and numpy

[william ratcliff]

Hi!  I have been having some difficulty understanding how to deal with
multidimensional ndarrays in structs with ctypes.  Namely, I want to the
define numpy arrays in python and have them operated on in C, and have them
updated on the python side.  Here is a toy example:

c code fragment:
foo.c
typedef struct {
  double X[4][4];
  double Y[4][4];
} matrices;  //there could be other elements in the struct between the
arrays, after them, etc.

int correct(matrices* pt){
 int i=2,j=1;
 pt->Y[i][j]=5.0;
}

python:

import numpy as N
import ctypes

xdata=N.zeros((4,4),dtype='float64')
ydata=N.ones((4,4),dtype='float64')

class fiduc(ctypes.Structure):
    _fields_=[("X",some_ctype),
            ("Y",some_ctype)]

pt=fiduc()
pt.X=xdata.ctypes.data
pt.Y=ydata.ctypes.data
foo = N.ctypeslib.load_library('foo.dll', '.')
foo.correct(ctypes.byref(pt))
print xdata
print ydata

My question is:
If I want to define a class in python and later to have my c code operate on
an instance of it, then I need to know the c_type representation for the
ndarray.  I can't simply blast the data into the structure because due to
memory alignment, structs in C are not guaranteed to have contiguous
elements in memory--I also don't want to take a risk with for example my
mingw generated dll having a different alignment for a structure than what a
ctypes made using msvc expects.   Thanks!!!!!

Cheers,
William
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