Re: [Jack-Devel] [Fwd: Re: [Guitarix-developer] jack session crash]

On Wed, 2 Nov 2011 15:53:07 -0400
Paul Davis <[hidden]> wrote:

> On Wed, Nov 2, 2011 at 3:47 PM, hermann <[hidden]> wrote:
> > Am Mittwoch, den 02.11.2011, 15:33 -0400 schrieb Paul Davis:
> >> if (function_name)
> >>
> >> will evaluate to false.
> >>
> >> ardour is doing this all the time.
> >
> > Well, we come to a something similar solution,
> 
> hopefully just involving:
> 
> 
>     #include <jack/weakjack.h>

Hi Paul, I'm the original author of the mail that hermann forwarded.
Which part of my mail do you not understand? I thought I explained the
problem :-)

AFAICS jackweak.h defines JACK_OPTIONAL_WEAK_EXPORT and
JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT, whereas the mentioned functions
from jack/session.h use JACK_WEAK_EXPORT, which is defined in
jack/weakmacros.h, which is included by session.h.

Anyhow, just to be sure I included weakjack.h in the test program,
but it doesn't change the behaviour.

Short summary:

If the jack function with weak attribute is in the shared lib that
is used when building the executable, an ELF PLT entry is generated.
When the executable is used with a shared lib that doesn't contain
the function, the the function address is still the PLT entry (!= 0)
but when called will indirectly jump to adress 0.

Btw. I'm using gcc 4.6.1, but I remember the same behaviour with older
gcc version.

ciao
Andreas

1320265775.28621_0.ltw:2,a <20111102212840.733213b6 at pluto dot noname>