Re: [Discuss-gnuradio] Problem with my modified IP tunnelling example

[Eric Blossom]

On Mon, Dec 04, 2006 at 04:51:30PM +0800, Jeremy Chew wrote:
> Hi,
> 
> I'm using an older version as this is what that came when I downloaded GR a 
> few months ago and I haven't seen a need to change what I have installed. 
> I'm new to GR, Linux and Python so I'm more eager to debug what I have 
> without introducing even newer stuff.
> 
> Anyway, the earlier problem was due to a C++ bug affecting another thread. 
> This has been fixed. Now there's a problem with passing the packets created 
> in the C++ extension, called sm, to Python.
> 
> The following describes what I do, which is setting the callback, and then 
> calling the callback.
> 
> SETTING THE CALLBACK POINTER IN THE C++ EXTENSION
> 
> The Python call that is to receive the packet from the C++ extension is 
> inherited from null_mac class and is named self.fg.send_pkt(). It is passed 
> from a sub-class of null_mac in Python to the extension with:
> 
> sm.settx2llecallback(self.fg.send_pkt)
> 
> The corresponding C++ call is:
> 
> static PyObject*s_pcTxDnCallback=0;
> ...
> static PyObject*sm_settx2llecallback(PyObject*a_pcSelf,PyObject*a_pcArgs)
> {...
>    PyObject*l_pcCallback=0;
> ...
>    if(PyArg_ParseTuple(a_pcArgs,"O:set_callback",&l_pcCallback))
>    {
>        if(PyCallable_Check(l_pcCallback))
>        {...
>            Py_XINCREF(l_pcCallback);
>            s_pcTxDnCallback=l_pcCallback;
> ...
> }
> 
> When the above executes, PyCallable_Check() seems to run with no error.

OK.

> PASSING PACKETS FROM THE EXTENSION TO PYTHON CODE
> 
> Later, a packet is created in the C++ extension in the form of an unsigned 
> char array. To pass it to the Python IP tunneling code, I do the following:
> 
> BOOL Tx2PyLle(unsigned char*data,UINT16 len)
> {
> ...
>    PyObject*l_pcArgs=0;
> ...
>    
> l_pcArgs=Py_BuildValue("(s#)",reinterpret_cast<char*>(data),static_cast<INT32>(len));
>    if(l_pcArgs )
>    {...
>       assert(s_pcTxDnCallback);
>        ...;
>        PyObject*l_pcRet=PyEval_CallObject(s_pcTxDnCallback,l_pcArgs);
> ...
> }
> 
> This function above has a segmentation fault at PyEval_CallObject().
> 
> My queries are:
> 
> 1) Is the callback to Python passed and set correctly in the extension?

Beat me.  I let SWIG handle all this stuff for me.
FWIW, the latest GNU Radio code uses SWIG directors to enable C++ code
to call back into Python.  It works fine as long as you are using SWIG
1.3.31.

> 2) Is it right to assume that in Python, the packets are Python strings?

The message payloads are strings.  The gr.messages are wrapped C++
objects (not strings).  I'm not sure which type you are trying to pass
to/from C++.

Eric