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Re: [rtems-users] C++ Virtual Functions



Joel Sherrill <joel@OARcorp.com> wrote, On 2/3/2005 10:57 AM:

Sergei Organov wrote:

"Smith, Gene" <gene.smith@siemens.com> writes:



Sergei Organov wrote, On 2/2/2005 12:10 PM:



"Smith, Gene" <Gene.Smith@siemens.com> writes:


I am having some problems running code containing C++ virtual
functions in rtems. When the code tries to branch to a virtual
function address using "bctrl" instruction, the address (in ctr) is
way out of range and results in a machine check exception.

Is there anything I need to do special in rtems for virtual functions to
work? Is anyone using virtual functions, pure or impure, in rtems? I am
still using rtems 4.6.1 and my ppc40x-derived bsp.

Besides what others have already suggested to check for, it could be the

case that you call a virtual member function of a static or global
object for which constructor hasn't been invoked (yet). In this case
virtual table pointer in the object isn't initialized. Global
constructors are invoked by special startup code so you need to check if
this code is linked in and executed. Check for .ctors section in the
executable as well.

Yes, that is what I just concluded in my previous post to this thread. I could see *no* .ctors or .dtors sections in my original output file. The .ctors and .dtors sections were supposedly placed into the .text section (see the attached linkcmds on previous msg). I then moved them out of .text into their own sections following .got2 (as shown in referenced example linkcmds, psim and motorola) and now I do see the sections .c(d)tors in the output following .got2. However, I still don't seem to see any initialization (construction) of the global object in question (its constructor is never hit when a breakpoint is put on it).


The code that calls constructors doesn't in fact use .ctors section in
any way, it uses __CTOR_LIST__  and probably __CTOR_END__ symbols that
seem to be correctly defined by your linker script, so moving all that
from .text to separate .ctors won't help.

Most probably you don't link in correct C++ startup code or somehow it's
not called. There are guys on the list that know much better than me how
C++ startup code is linked in and invoked in RTEMS. I can't tell out of
my head as for historical reasons I do it manually in my projects, not
using RTEMS standard ways. I.e., I have my own routine that calls all
the functions put into the table referenced to by __CTOR_LIST__ and call
it before main().



psim in the CVS tree and head of the 4.6 branch works for C++ and exceptions. The trick is in the linkcmds and bsp_specs. If you get those right, it should just work.

bsp_specs
==========
The *startfile section needs to include ecrti, rtems_crti, and crtbegin.

The *endfile section needs to include crtend and ecrtn.

CHeck out psim/bsp_specs on the head of the 4.6 branch or the trunk.
It looks different from the gen405/bsp_specs.

linkcmds
=========
Look for the .init and .fini sections and the CTOR/DTOR (caps) related
sections.

--joel


I think Sergei and Phil are saying this is probably the right way to go. However, I still have a bit of a problem. The rtems_crti.o file is flagged during link as not present. I found a rtems_crti.S in the libbsp/powerpc/shared area but it is is not being assembled in my gen40x/helas403 hybrid bsp. I moved it into my bsp and assembled it along with my startup assembly file but ld still can't find it. Looks like it needs to be somewhere in the region of crtbegin.o to be found by ld. I suppose rtems_crti is important?
-gene