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



Sergei Organov wrote, On 2/3/2005 9:45 AM:

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


Sergei Organov wrote, On 2/3/2005 8:38 AM:


"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().

I went back a read everything I could find on this list regarding this
and several people have reported this problem. The only one who
reported a resolution did like you and wrote their own initializer
(Phil Torre). However there was a lot of discussion about "eabi" that
got into the ctor init and pointed out that it occurs in a call to
_init() in _Thread_Handler when __USE_INIT_FINI__ is defined. The
_init() call occurs on my system as described which in turn calls
etext and eventually __do_global_ctors_aux__ but I never see any
constructors actually called before _init() returns. I assume it is
supposed to call the constructors for any globally or statically
defined c++ objects somewhere inside the _init() call?


Well, all that is in fact some g++ magic that I still don't quite
understand, but the essence is that the routines which addresses are
given in the ctor table should be called in turn at startup. Here is how
I do it. Please note that my ctor table is built somewhat different,
i.e., in the linker script:

  /* Table of global constructors (to be called on startup). */
  .ctors : {
    __CTOR_LIST__ = .;
    KEEP(*(SORT(.ctors.*)))
    KEEP(*(.ctors))
    __CTOR_END__ = .;
  } > ROM

so that there is no "number of pointers" field in the beginning of the
table. Please also note that my code calls them top-to-bottom, and AFAIK
g++'s way is to call bottom-to-top.

Here is the code:

/* Single entry in the CTOR table.  */
typedef void (*Ctor) (void);

/* Begin and end markers of CTORS table. */
extern Ctor const __CTOR_LIST__[];
extern Ctor const __CTOR_END__[];

/* Call global constructors according to CTOR table */
static void ctors(void)
{
  Ctor const* ptr = __CTOR_LIST__;
  Ctor const* end = __CTOR_END__;
  for ( ; ptr != end; ptr++)
    if (*ptr)
      (*ptr)();
}


Also, are these c++ objects supposed to be in the .bss section?


I don't believe it's essential, what's essential is that the section
must be read-write, i.e., you can't put C++ objects but PODs into ROM.

PODs?

Anyhow, thanks for the code. I put it in my init.c and I now see some constructors being called by it. However, for some reason, I don't see the constructor for the class with the virtual function that I am wanting to be called. The class variable is declared static and it's constructor is in the the same cpp file. When I dump the object for the cpp file I see a .ctor section of size 4 and the constructor of interest is in a ".gnu.linkonce.t..." type of section. But the address of the constructor never seems to make it to the final .ctor table since when it goes through your code it never calls the constructor of interest.

I also changed my linkcmds file (in the libbsp area) as you show. All I did was re-install rtems and it put linkcmds to the right place in the install area, /opt/rtems-4.6. Then I forced a relink of my app. That seem like enough but maybe I need to make clean all, make install rtems and rebuild clean my app. Takes a while but I will do it now.

Tks,
-gene