[pypy-dev] Pointer To Member Problem with rctypes

[Gerald Klix]

Hi all,
I have a problem with GcStruct, Struct and Ptr I can not resolve with my 
own devices.

After some trial with ctypes running on CPython I came up with the 
following lowlevel representation for structures:

     Ptr( GcStruct( "CtypesGcStructure_<ClassName>
             ( "c_data"
                     (Struct "C-Data_<ClassName>
                             *<Fieldefintions>) ) ) )

Samuele and Armin helped to implement rtype_getattr and rtype_setattr 
that delegate all attribute access to the c_data field, by pointing
out how to implement a helper method called generateCDataAccess:

     def generateCDataAccess( self, variable, lowLevelOperations ):
         """
         Answer the C level data sub structure.
         """
         inputargs = [ variable, inputconst( Void, "c_data" ) ]
         return lowLevelOperations.genop(
                 "getfield",
                 inputargs,
                 # Using Ptr makes the C-compiler barf,
                 # omitting Ptr leeds to specialisation errors
                 Ptr( self.c_data_lowleveltype ) )

     def rtype_setattr( self, highLevelOperation ):
         c_data = self.generateCDataAccess(
                 highLevelOperation.inputarg( self, 0 ),
                 highLevelOperation.llops )
         inputargs = highLevelOperation.inputargs(
                     *highLevelOperation.args_r[ :3 ] )
         inputargs[ 0 ] = c_data
         print "inputargs:", inputargs
         print "r_result:", highLevelOperation.r_result
         highLevelOperation.genop( "setfield", inputargs )

Obviously generateCDataAccess needs to return a Ptr to the embedded
C-structure. When I use Ptr as in the example above, I get the following
traceback:

     c:\Dokumente und Einstellungen\root\Lokale 
Einstellungen\Temp\usession-353\testi
     ng_2\testing_2.c(894) : error C2440: '=': 'pypy_C_Data_tagpoint0' 
kann nicht in
     'pypy_C_Data_tagpoint0 *' konvertiert werden

Which means, that a 'pypy_C_Data_tagpoint0' can not be converted to a
'pypy_C_Data_tagpoint0 *'. A brief look at the generated C-source,
shows that the compiler is right here:

     struct pypy_CtypesStructure_tagpoint0 
*pypy_g__py_test_compile_struct(struct pypy_CtypesStructure_tagpoint0 
*l_p_0, long l_x_1, long l_y_1) {
             /*void l_v111;*/ /*void l_v112;*/
             struct pypy_C_Data_tagpoint0 *l_v98;
             struct pypy_C_Data_tagpoint0 *l_v99;
             struct pypy_CtypesStructure_tagpoint0 *l_v110;
             pypy_IncRf_CtypesStructure_tagpoint0(l_p_0);

         block0:
             l_v98 = l_p_0->cs_c_data;
             l_v98->cd_x = l_x_1;
             l_v99 = l_p_0->cs_c_data;
             l_v99->cd_y = l_y_1;
             l_v110 = l_p_0;
             goto block1;


         block1:
             return l_v110;
      }

If I ommit the Ptr construction so that, generateCDataAccess is defined 
as follows:

     def generateCDataAccess( self, variable, lowLevelOperations ):
         """
         Answer the C level data sub structure.
         """
         inputargs = [ variable, inputconst( Void, "c_data" ) ]
         return lowLevelOperations.genop(
                 "getfield",
                 inputargs,
                 self.c_data_lowleveltype )

I get the following error:

     [D:\svnwork\work\dist\pypy\rpython\lltypesystem\lltype.py:146]
     _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

         def _nofield(self, name):
             raise AttributeError, 'struct %s has no field %r' % 
(self._name,
     E                                                            name)
     >       AttributeError: struct C-Data_tagpoint has no field 'TO'

This is no surprise at all. The getfield operation obviosly needs a 
pointer to
work with.

So it boils down to the question:
How can I access the members of an embedded structure?
Is there a possibility to get the address of substructure, just like
C's ampersand (&) operator does?


Any help is welcome.

Best regards,
Gerald