Wrapping c functions

[Glenn Pierce]

Hi I have been trying to wrap a c library called FreeImage for python
and am having trouble with a couple of functions.

One function is

FreeImage_Load which takes parameters (enum, const char*, int)

I have wrapped the function with the following code
static PyObject *
freeimage_load(PyObject *self, PyObject *args)
{
int format, flags;
char* filename = "None";
FIBITMAP *dib = NULL;

if (!PyArg_ParseTuple(args, "isi", &format, filename, &flags))
return NULL;

dib = FreeImage_Load(format, filename, flags);

return Py_BuildValue("o", dib);
}

However I am getting a segmentation fault at PyArg_ParseTuple.
Also I have little Idea what to return from the function. FIBITMAP * is
an opaque pointer
that I pass to other FreeImage functions, I pretty certain
Py_BuildValue("o", dib) is wrong.

There is also the function
unsigned char *FreeImage_GetBits(FIBITMAP *dib);

I and not sure how to return the bits to python.

I would probably like to display an image with the gtk function

drawable.draw_rgb_image(*gc*, *x*, *y*, *width*, *height*, *dith*, *rgb_buf*, *rowstride*)

here the RGB Image data is packed in a string as a
sequence of 8-bit RGB pixel triplets
but I have no idea how to get from unsigned char * to that.

Any advice would be greatly appreciated.

Thanks

[Andrew Dalke]

Glenn Pierce wrote:

if (!PyArg_ParseTuple(args, "isi", &format, filename, &flags))
return NULL;
Shouldn't that be &filename ? See
http://docs.python.org/ext/parseTuple.html
for examples.

dib = FreeImage_Load(format, filename, flags); Also I have little Idea what to return from the function. FIBITMAP * is
an opaque pointer
that I pass to other FreeImage functions, I pretty certain
Py_BuildValue("o", dib) is wrong.

If it's truly opaque and you trust your use of the code you can
cast it to an integer, use the integer in the Python code, and
at the Python/C interface cast the integer back to a pointer.
Of course if it no longer exists you'll get a segfault.

If you want more type safety you can use the SWIG approach and
encode the pointers as a string, with type information and
pointer included.

Or use the Python extension API to make a new type.

Searching the archives I see people have tried to write a Python/
FreeImage interface using SWIG.
Andrew
da***@dalkescientific.com

[David M. Cooke]

Andrew Dalke <da***@dalkescientific.com> writes:

Glenn Pierce wrote:

if (!PyArg_ParseTuple(args, "isi", &format, filename, &flags))
return NULL;

Shouldn't that be &filename ? See
http://docs.python.org/ext/parseTuple.html
for examples.

dib = FreeImage_Load(format, filename, flags);

Also I have little Idea what to return from the function. FIBITMAP * is
an opaque pointer
that I pass to other FreeImage functions, I pretty certain
Py_BuildValue("o", dib) is wrong.

If it's truly opaque and you trust your use of the code you can
cast it to an integer, use the integer in the Python code, and
at the Python/C interface cast the integer back to a pointer.
Of course if it no longer exists you'll get a segfault.

If you want more type safety you can use the SWIG approach and
encode the pointers as a string, with type information and
pointer included.

Better yet, use a CObject. That way, a destructor can be added so as
to not leak memory. Type info could be included in the desc field.

return PyCObject_FromVoidPtr(dib, NULL)

(the NULL can be replaced with a routine that will free the image.)

--
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|David M. Cooke
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