Calling a function from a dll

Python Help
1

I have a dll which has been created in Visual C++. I am calling functions from the dll in Python.

Simple function that require a double do work fine.

A certain function however requires a pointer to a variable-size FP structure. In the documentation the Floating Point structure is defined as:

typedef struct _FP
{
unsigned short int rows;
unsigned short int cols;
double array[1];
} FP;

The array will actually have a size of rows times cols. Array consists of IEEE 8-byte floats

I fail to call the function defined in the dll. How would you call the function?

2

When posting code in Discourse (and other sites), it’s much easier to read if you format it:

```c
typedef struct _FP
{
  unsigned short int rows;
  unsigned short int cols;
  double array[1];
} FP;
```

In answer to your question, check out ctypes for Python’s interaction with C in shared libraries.

3

Define the FP struct and pass an instance by reference via ctypes.byref(instance). The FP record can be implemented in various ways. Since ctypes arrays are bounds checked, I prefer to use a property to handle a case like FP.array. For example:

class FP(ctypes.Structure):
    _fields_ = (('rows', ctypes.c_ushort),
                ('cols', ctypes.c_ushort),
                ('_array', ctypes.c_double * 1))

    def __init__(
        self, rows, cols, array=None, *,
        USHRT_MAX=((1 << (ctypes.sizeof(ctypes.c_ushort) * 8)) - 1),
    ):
        # ctypes.c_ushort requires int type
        self.rows = rows
        self.cols = cols
        if rows < 1 or cols < 1 or rows > USHRT_MAX or cols > USHRT_MAX:
            raise ValueError('rows and cols must be greater than 0 and '
                             'less than {}'.format(USHRT_MAX + 1))
        length = rows * cols
        if length > 1:
            ctypes.resize(self, ctypes.sizeof(self) +
                ctypes.sizeof(ctypes.c_double) * (length - 1))

        if array is not None:
            self.array[:] = array


    @property
    def array(self):
        length = self.rows * self.cols
        offset = __class__._array.offset
        return (ctypes.c_double * length).from_buffer(self, offset)

>>> fp = FP(2, 2, range(4))
>>> fp.array[:]
[0.0, 1.0, 2.0, 3.0]
>>> fp.array[:] = range(4, 8)
>>> fp.array[:]
[4.0, 5.0, 6.0, 7.0]
4 Likes
4

Brilliant! It works as I would like it. Thanks for your support, I should have asked weeks ago :slight_smile: .
Very much appreciated!

1 Like