help with c <-> python buffer transfer

[John Machin]

tkirke at gmail.com wrote:

> This is the part I need help with. I've also used PyBuffer_...
> subroutines which gave similar problems.  Thanks for all of your other
> comments, but I was hoping someone could just tell me what was wrong
> with the code without having to worry about all of the other things
> that could go wrong.
> For completeness here is the complete c++ module & python output
>
> //=======================================================================
> // Boost Includes
> //==============================================================
> #include <boost/python.hpp>
> #include <boost/cstdint.hpp>
> #include <boost/python/def.hpp>
> #include <boost/python/args.hpp>
> #include <boost/python/overloads.hpp>
>
> // Using
> =======================================================================
> using namespace boost::python;
>
> static PyObject * proc_buf(PyObject *self, PyObject *args) {
>     PyObject *resultobj;
>         char* output_samples;
>         int len;
>     if (!PyArg_ParseTuple(args,"s#|l",&output_samples, &len)) {

Sorry, I read that too quyickly. You need THREE receptors, one for "s",
2nd for "#", 3rd for "l".
e.g. something like:
    int len;
    long third_arg; /* needs initialisation */
    if (!PyArg_ParseTuple(args,"s#|l",&output_samples, &len,
&third_arg)) {

>           return NULL;  /* wrong arguments provided */
>     }
>         for (int i=0;i<len;i++) output_samples[i] *= 2;
>     resultobj = PyString_FromStringAndSize(output_samples, len);
>     return resultobj;
> }
> // Module
> ======================================================================
> BOOST_PYTHON_MODULE(spuctest)
> {
>   def("pass_buf",&proc_buf);
> }
>
> # python code
> ....
>     buffy = mf.read()
>     print type(buffy)
>     buf   = pass_buf(buffy, len(buffy))
>     print type(buf)
> #
>
> #python output
> <type 'buffer'>
> <type 'Nonetype'>

I'd guess the return value of None was fortuitous -- when I tried your
code using C, the call to pass_buf just crashed. With the above fix,
the problem goes away:

|>>> import procbuf
|>>> foo = '\x01\x03\xff'
|>>> x = procbuf.passbuf(foo)
|>>> x
'\x02\x06\xfe'

HTH take2 :-)
John