LZO decompression utility method on Windows using ctypes for dummies

Hi ctypes-list,

(Sorry if this is a double post, I posted first from an alternate account
(unintentionally), but I could not see my question popping up on the forum).

I would like to implement a
def lzo_decompress(bytestr):
    ....
    return decompressed_bytestr


utility method in a Python module for use in a project using Python 2.5.4 on
a Win XP 32-bit system.

I think I have assembled the pieces for the puzzle, but I cannot figure out
how to put the puzzle together (what to put in the body of the function).

I am an ignorant concerning dlls, C and ctypes, so this is really a tough
problem for me, and I hope you can help.

I have the following:

>From the pytables project an lzo1.dll is available in a zip file here:

http://www.pytables.org/download/lzo-win/

My initial thoughts about getting a fast solution was to use some utility
function in pytables, but there is not one available, see the discussion
here:

http://sourceforge.net/mailarchive/message.php?msg_name=AANLkTim6jGiCcbLEFbh4ybH1X5b0HXPXx7XAcuA0Jc28%40mail.gmail.com

There is also a header file lzo1x.h, which contains the call signature for
the lzo decompress function I should bind to:

/* safe decompression with overrun testing */
LZO_EXTERN(int)
lzo1x_decompress_safe   ( const lzo_byte *src, lzo_uint  src_len,
                                lzo_byte *dst, lzo_uintp dst_len,
                                lzo_voidp wrkmem /* NOT USED */ );

Using a DLL export viewer I have verified that this function is exported in
the DLL and I have also checked that I can load the dll in ctypes and that
the function is available once loaded:

In [11]: dll = ctypes.windll(r"C:\Some\Path\GnuWin32\bin\lzo1.dll")
In [12]: dll.lzo1x_decompress_safe?
Type:           _FuncPtr
Base Class:     <class 'ctypes._FuncPtr'>
String Form:    <_FuncPtr object at 0x00AC6A08>
Namespace:      Interactive
Docstring:
    <no docstring>

Now, my big problem is to understand how I can write wrapper code, which can
call this function with a bytestr and return a bytestr?

Input to this may be the very old (Python 2.2 from 2002) and not maintained
lzo python module:

http://www.oberhumer.com/opensource/lzo/

http://www.oberhumer.com/opensource/lzo/download/LZO-v1/python-lzo-1.08.tar.gz

which I cannot figure out how to compile. This tarball contains a C file,
with what I guess contains CPython code for making a decompress function the
way I would like it using that very same lzo library function,
lzo1x_decompress_safe

The relevant part is here


/***********************************************************************
// decompress
************************************************************************/

static /* const */ char decompress__doc__[] =
"decompress(string) -- Decompress the data in string, returning a string
containing the decompressed data.\n"
;

static PyObject *
decompress(PyObject *dummy, PyObject *args)
{
    PyObject *result_str;
    const lzo_bytep in;
    lzo_bytep out;
    lzo_uint in_len;
    lzo_uint out_len;
    lzo_uint new_len;
    int len;
    int err;

    /* init */
    UNUSED(dummy);
    if (!PyArg_ParseTuple(args, "s#", &in, &len))
        return NULL;
    if (len < 5 + 3 || in[0] < 0xf0 || in[0] > 0xf1)
        goto header_error;
    in_len = len - 5;
    out_len = (in[1] << 24) | (in[2] << 16) | (in[3] << 8) | in[4];
    if ((int)out_len < 0 || in_len > out_len + out_len / 64 + 16 + 3)
        goto header_error;

    /* alloc buffers */
    result_str = PyString_FromStringAndSize(NULL, out_len);
    if (result_str == NULL)
        return PyErr_NoMemory();

    /* decompress */
    out = (lzo_bytep) PyString_AsString(result_str);
    new_len = out_len;
    err = lzo1x_decompress_safe(in+5, in_len, out, &new_len, NULL);
    if (err != LZO_E_OK || new_len != out_len)
    {
        Py_DECREF(result_str);
        PyErr_Format(LzoError, "Compressed data violation %i", err);
        return NULL;
    }

    /* success */
    return result_str;

header_error:
    PyErr_SetString(LzoError, "Header error - invalid compressed data");
    return NULL;
}

How do I implement equivalent ctype pure Python code such that I do not have
to C compile anything?
Best wishes,

Kim

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