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

Hi

We have a wired problem with jack_session_manager diffs in 
jack1 <-> jack2 
for functions:

A)  jack_client_get_uuid
B)  jack_get_uuid_for_client_name

Therefore I forward our results to you. (attached)

Is that a known issue ?

regards
hermann
> Could nail it on
> string GxJack::get_uuid_insert()
>
> replacing
> char* uuid = jack_client_get_uuid(client_insert);
> with
> const char* uuid = jack_get_uuid_for_client_name(client_insert,
> client_insert_name.c_str());
>
> works here.
> I'm on jackd2, but as rosea already reported a crash with jackd1 and the
> old version, there seems to be a incompatibility in both versions of
> jack-session manager (jack1 <->jack2). That's more then strange. . .  .

yes, we have incompatibilities in all directions, that's really bad. I tested
with ubuntu jackd1 (1:0.121.0+svn4) and jackd2 (1.9.7~dfsg-1ubuntu2)

functions:

A)  jack_client_get_uuid
B)  jack_get_uuid_for_client_name

Both functions are defined with weak linkage in the header files of both
packages. But jackd1 has only A in its shared library, while jackd2 has
only B.

To make the mess complete, the weak linking doesn't work as
advertised by the jackd developers. At least my understanding and
that of the ubuntu / debian packagers seems to be:

 1) you can compile and link with either package (works)
 2) when the function is not in the shared library used at
     runtime, the function address will be 0. Does NOT work.

2) only works when the shared lib used at runtime is the same as
the one used when linking. The compiler / linker will set the address
of a weak function that it does not find in the libraries supplied to
constant 0. Even when the funktion is in the library which is used at
runtime, there will be no runtime linking.

Even worse, when the funktion is found while building the
executable, there will be an entry in the ELF PLT (Procedure Linkage
Table). The address of the function in the program will be that of
the PLT entry. So the function address is != 0 even when the
runtime linker can't resolve the function and produces a jump
to address 0 when calling the funktion.

To solve the problem for guitarix, I had to make some ugly
constructions:

 - the functions must never be used directly (or you will get
   a PLT entry so that dlsym() will only reflect the state while
   building the executable)

 - define typedefs for the function pointers that must be kept
   in sync with the definitions of the functions in the jack header

 - define global variables that are initialized with the function
   addresses via dlsym().

Example:

typedef char *(*jack_client_get_uuid_type)(jack_client_t *);

jack_client_get_uuid_type jack_client_get_uuid_fp =
    reinterpret_cast<jack_client_get_uuid_type>(
	dlsym(RTLD_DEFAULT, "jack_client_get_uuid"));

Test program to verify the described behaviour:

----------------------------------------------------------------
#include <stdio.h>
#include <jack/session.h>

int main() {
    printf("jack_client_get_uuid = %p, jack_get_uuid_for_client_name = %p\n",
	   jack_client_get_uuid, jack_get_uuid_for_client_name);
    return 0;
}
----------------------------------------------------------------

test run (the ## lines represent deinstallation of jackd
and installation of the displayed jackd version):

----------------------------------------------------------------
## libjack0            1:0.121.0+svn4
$ gcc t.c -ljack
$ ./a.out
jack_client_get_uuid = 0x8048430, jack_get_uuid_for_client_name = (nil)

## libjack-jackd2-dev  1.9.7~dfsg-1ubuntu2
$ ./a.out
jack_client_get_uuid = 0x8048430, jack_get_uuid_for_client_name = (nil)
$ gcc t.c -ljack
$ ./a.out
jack_client_get_uuid = (nil), jack_get_uuid_for_client_name = 0x8048420

## libjack0            1:0.121.0+svn4
$ ./a.out
jack_client_get_uuid = (nil), jack_get_uuid_for_client_name = 0x8048420
----------------------------------------------------------------

ciao
Andreas

1320261339.20695_0.ltw:2,a <1320261298.2111.31.camel at box>