"Daniel Platz" <mail.to.daniel.pl...@googlemail.com> wrote in message
news:63ac1a01-8491-4885-bae7-cb884abb5...@34g2000yqp.googlegroups.com...
Hello,
I would like to pass a two dimensional array to C function in a dll. I
use ctypes to call the function.
I compile the dll with visual studio 2008 express and my C source code
looks like this.
#include <stdio.h>
#ifdef __cplusplus
extern "C" { // only need to export C interface if
// used by C++ source code
using namespace std;
#endif
__declspec(dllexport) int print(double** ptr, int ny, int nx)
{
int i, j;
for(i=0; i<ny; i++)
{
for(j=0; j<nx; j++)
{
printf("%.3f \t", *(*(ptr+i)+j));
}
printf("\n");
}
return 0;
}
#ifdef __cplusplus
}
#endif
As you can see the function expects a doube** variable. I tried to
call this function from Python with this code.
import ctypes as ct
matrix = ct.cdll.LoadLibrary('matrix.dll')
getattr(matrix, 'print_matrix')
ptr_type = ct.c_double * 5 * 4
ptr = ptr_type()
matrix.print_matrix(ptr, ct.c_int(4), ct.c_int(5))
I expected to see a matrix showing only zeros since I can address the
single entries in Python by ptr[i][j]. Instead the interpreter returns
with the error message
WindowsError: exception: access violation reading 0x000000
WARNING: Failure executing file: <ctypes_matrix.py>
Furthermore, I am wondering if there is a fast way to use a numpy 2D
array instead or alternatively to cast the ctypes array into a numpy
array.
Has someone an idea to help me?
A two-dimentional array is not equivalent to a double**, so depending on
what your requirements are there are two solutions.
1. To work with your original matrix.c code above, some Python code that
works is:
-------------------------------------
import ctypes as ct
# I like C types in caps
DOUBLE = ct.c_double
PDOUBLE = ct.POINTER(DOUBLE)
PPDOUBLE = ct.POINTER(PDOUBLE)
INT = ct.c_int
matrix = ct.cdll.LoadLibrary('matrix.dll')
print_matrix = getattr(matrix,'print')
# An array of doubles can be passed to a function that takes double*.
DBL5ARR = DOUBLE * 5
# An array of double* can be passed to your function as double**.
PDBL4ARR = PDOUBLE * 4
# Declare double* array.
ptr = PDBL4ARR()
for i in range(4):
# fill out each pointer with an array of doubles.
ptr[i] = DBL5ARR()
for j in range(5):
ptr[i][j] = i + j # just to initialize the actual doubles.
print_matrix(ptr,4,5)
-----------------------------------------------
2. In C, multidimensional arrays are really just single dimentional arrays
where the compiler does the math for you, so if you declare your matrix
print function like this:
----- matrix2.c ---------------------------
#include <stdio.h>
#ifdef __cplusplus
extern "C" { // only need to export C interface if
// used by C++ source code
#endif
__declspec(dllexport) int print(double* ptr, int ny, int nx)
{
int i, j;
for(i=0; i<ny; i++)
{
for(j=0; j<nx; j++)
{
printf("%.3f \t", ptr[i*nx + j]);
}
printf("\n");
}
return 0;
}
#ifdef __cplusplus
}
#endif
-------------------------------------------------
Then the Python code simplifies a bit to:
------------------------------------------
import ctypes as ct
DOUBLE = ct.c_double
PDOUBLE = ct.POINTER(DOUBLE)
INT = ct.c_int
matrix = ct.cdll.LoadLibrary('matrix2.dll')
print_matrix = getattr(matrix,'print')
ptr = (DOUBLE*5*4)()
for i in range(4):
for j in range(5):
ptr[i][j] = i + j
print_matrix(ptr,4,5)
----------------------------------------------
OUTPUT in both cases:
0.000 1.000 2.000 3.000 4.000
1.000 2.000 3.000 4.000 5.000
2.000 3.000 4.000 5.000 6.000
3.000 4.000 5.000 6.000 7.000
HTH,
Mark
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