7 Mar
2012
7 Mar
'12
5:41 p.m.
<snip>
> Yes! I'll pick an obvious example, KHTML. I
would like to replace the
broken KHTML engine with Webkit using Qt4. Problem is, I
can't link a
Qt4
> library into a TDE library or program due to
symbol conflicts. Even if
you trick the compiler and linker, the program will crash
at runtime
because the C++ runtime will not know which symbols (Qt3 or Qt4) to use
when function names and static members have the same name.
But that is what the linker does. Isn't it? Looks
at symbolic names and
puts addresses instead. Therefore you shouldn't have name
conflicts at runtime. The crash you may encounter is
because the linker
chosen wrong symbols to link together in one algorithm. But this
have
obvious solution. Put the code you want to link with Qt4 in separate
module and link only with Qt4. I still don't understand how is renaming
better solves this. Could you please refer exact source file and
function?
Actually the linker creates a reference to a "mangled" symbol--see
http://en.wikipedia.org/wiki/Name_mangling#Name_mangling_in_C.2B.2B
Let's take a simple example, qWarning. This becomes _Z8qWarningPKcz in
libqt-mt.so.3.3.8 (nm -D /usr/lib/libqt-mt.so | grep qWarning), and
_Z8qWarningPKcz in /usr/lib/libQtCore.so (nm -D /usr/lib/libQtCore.so |
grep qWarning). If my program references Z8qWarningPKcz, which library
will the symbol resolver choose at runtime? They are not binary
compatible with each other, and when the symbol resolver chooses the wrong
one at runtime (which it will do quite often) the entire program will
crash.
<snip>
I suppose this could be done in more easy way without
renaming.
Not really, see above. :-)
And if you want to work on a "pure" KDE 3.5.10 fork you have that right;
just be aware that you will hit a dead end far before TDE will. ;-)
Tim
7 Mar
7 Mar
7:21 p.m.
New subject: [trinity-devel] QT API renaming! Why?!
On 03/07/2012 11:41 AM, Timothy Pearson wrote:
And if you want to work on a "pure" KDE
3.5.10 fork you have that right;
just be aware that you will hit a dead end far before TDE will. ;-)
No doubt. That just makes all the more impressive what Ilya has done for opensuse.
My primary desktops are TDE 3.5.12 (on Arch) and KDE 3.5.10 on opensuse 11.4. To
be honest - from a user and function standpoint (sftp aside), they both work
phenomenally. TDE R13 & R14 will as well, but, at least for Arch, there is tons
of work to do to get all the features building.
Renaming/separate Qt4 modules -- it doesn't really matter to the user. However,
it probably matters a lot from a long-term maintainability standpoint. From what
I can tell at this point, tqt3 is doing a fine job. So much so, that when I
opened konqueror for the first time in 'tree view' the (Name, Size, File Type,
and Modified) columns all fit neatly inside the file list window while
(Permissions, Owner, Group, Link) all neatly scrolled off the right-side and
were easily viewable via the horizontal scroll. That is something that Qt4 has
never been able to accomplish without the existing column sizes going all to hell.
From what I've seen, if TQt3 is maintainable, that should be the long-term
solution for the foreseeable future -- until (and more importantly, if) Qt4
matures to the point that it can provide a workable replacement. It is far from
there yet and IMHO most widgets still look like they were created by some
teenager hopped up on Jolt soda saying "look Bevis -- see what I can make it
do!" ;-)
--
David C. Rankin, J.D.,P.E.
9:14 p.m.
New subject: [trinity-devel] QT API renaming! Why?!
On Wed March 7 2012 09:41:23 Timothy Pearson wrote:
Let's take a simple example, qWarning. This
becomes _Z8qWarningPKcz in
libqt-mt.so.3.3.8 (nm -D /usr/lib/libqt-mt.so | grep qWarning), and
_Z8qWarningPKcz in /usr/lib/libQtCore.so (nm -D /usr/lib/libQtCore.so |
grep qWarning). If my program references Z8qWarningPKcz, which library
will the symbol resolver choose at runtime? They are not binary
compatible with each other, and when the symbol resolver chooses the wrong
one at runtime (which it will do quite often) the entire program will
crash.
Could the problem you describe be addressed more easily with C++ namespaces?
--Mike Bird
10:39 p.m.
New subject: [trinity-devel] QT API renaming! Why?!
On Wed March 7 2012 09:41:23 Timothy Pearson wrote:
No, unfortunately. The methods and variables that are in conflict are
either static global or due to identical class and method naming. Also,
using namespaces that heavily will introduce other problems.
Tim
Let's take a simple example, qWarning. This
becomes _Z8qWarningPKcz in
libqt-mt.so.3.3.8 (nm -D /usr/lib/libqt-mt.so | grep qWarning), and
_Z8qWarningPKcz in /usr/lib/libQtCore.so (nm -D /usr/lib/libQtCore.so |
grep qWarning). If my program references Z8qWarningPKcz, which library
will the symbol resolver choose at runtime? They are not binary
compatible with each other, and when the symbol resolver chooses the
wrong
one at runtime (which it will do quite often) the entire program will
crash.
Could the problem you describe be addressed more easily with C++
namespaces?
--Mike Bird 
8 Mar
8 Mar
9:59 a.m.
New subject: [trinity-devel] QT API renaming! Why?!
On Wed, Mar 7, 2012 at 9:41 PM, Timothy Pearson
<kb9vqf(a)pearsoncomputing.net> wrote:
<snip>
^^^
> Yes! I'll pick an obvious example,
KHTML. I would like to replace the
broken KHTML engine with Webkit using Qt4.
Problem is, I can't link a
Qt4
> library into a TDE library or program due to
symbol conflicts. Even if
you trick the compiler and linker, the program will
crash at runtime
because the C++ runtime will not know which symbols (Qt3 or Qt4) to use when function names and static members have the same
name.
> But that is what the linker does. Isn't it?
...
Actually the linker creates a reference to a "mangled" symbol--see
http://en.wikipedia.org/wiki/Name_mangling#Name_mangling_in_C.2B.2B
Let's take a simple example, qWarning. This becomes _Z8qWarningPKcz in
libqt-mt.so.3.3.8 (nm -D /usr/lib/libqt-mt.so | grep qWarning), and
_Z8qWarningPKcz in /usr/lib/libQtCore.so (nm -D /usr/lib/libQtCore.so |
grep qWarning). If my program references Z8qWarningPKcz, which library
will the symbol resolver choose at runtime? They are not binary
compatible with each other, and when the symbol resolver chooses the wrong
one at runtime (which it will do quite often) the entire program will
crash.
Ok, you are telling about dynamic (or runtime) linking then. That is
not 'C++ runtime'. Saying this had lead me to misunderstanding,
because C++ runtime is exception and type handling.
Well, I understand now that the problem is a bit more complex. But
I've found two solution candidates so far! Obvious one and cunning
one. The obvious is to compile Qt4 in different namespace
(-qtnamespace option).
The cunning one is to make run-time loader to correctly do symbol
mapping. Since ld.so loader yet doesn't support direct binding
(http://en.wikipedia.org/wiki/Direct_binding) this can be done with
dlopen(RTLD_DEEPBIND) at SO init time. I've put some example in bug
900. Please, look at lib_vinegret.tar.gz.
You'll find in 'lib_vinegret' dir the simplified matter of problem:
lib_v1.c: Qt3 library
lib_v1_user.c: Most TDE code
lib_v2.c: Qt4 library
lib_v2_user.c: TDE code that wants to use Qt4
When you intermix 'lib_v1' and 'lib_v2' in one executable you will get
'lib_v2_user' executing 'lib_v1' instead of 'lib_v2'.
Please, type 'make', then './run.sh' to look at the results:
This is v1 user
This is v1 library!
This is v2 user
This is v1 library!
You can examine why this happened in generated rtld.log. The
interesting lines are:
4885: object=./lib_v1_user.so [0]
4885: scope 0: ./executable ./lib_v1_user.so
./lib_v2_user.so /lib/x86_64-linux-gnu/libc.so.6 ./lib_v1.so
./lib_v2.so /lib64/ld-linux-x86-64.so.2
4885:
4885: object=./lib_v2_user.so [0]
4885: scope 0: ./executable ./lib_v1_user.so
./lib_v2_user.so /lib/x86_64-linux-gnu/libc.so.6 ./lib_v1.so
./lib_v2.so /lib64/ld-linux-x86-64.so.2
Note the 'scope 0'. It is same for all objects and called 'global
scope'. The order in this scope shows the actual order of symbol
searches. So the 'lib_function' symbol will be found always from
./lib_v1.so. Further entries in rtld.log confirm that.
Sadly enough, there is no way to force dynamic linker to load
different objects with different lookup scopes. But here comes
RTLD_DEEPBIND to the stage. Please, look at this diff:
http://bugs.pearsoncomputing.net/attachment.cgi?id=468&action=diff
I made some wrapper 'lib_v2_user_scoper' around the code we want to
load with different library. If you run this variant, you should see
this:
This is v1 user
This is v1 library!
This is v2 user
This is v2 library!
This is v1 user
This is v1 library!
This is v2 user
This is v2 library!
Voila! :-)
If per chance you will remove RTLD_DEEPBIND flag you will see the old
flat way of resolution:
This is v1 user
This is v1 library!
This is v2 user
This is v1 library!
This is v1 user
This is v1 library!
This is v2 user
This is v1 library!
Alex
<snip>
I suppose this could be done in more easy way
without renaming.
Not really, see above. :-)
And if you want to work on a "pure" KDE 3.5.10 fork you have that right;
just be aware that you will hit a dead end far before TDE will. ;-)
Tim
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4704
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4 comments
4 participants
participants (4)
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Aleksey Midenkov -
David C. Rankin -
Mike Bird -
Timothy Pearson