Some time ago I wrote a description of the
-Bsymbolic
linker flag which could do with some further explanation. The
original
article is a good starting place.
One interesting point that I didn't go into was the potential for
code optimisation
-Bsymbolic brings about. I'm not sure if I
missed that at the time, or the toolchain changed, both are probably
equally likely!
Let me recap the example...
ianw@jj:/tmp/bsymbolic$ cat Makefile
all: test test-bsymbolic
clean:
rm -f *.so test testsym
liboverride.so : liboverride.c
$(CC) -Wall -O2 -shared -fPIC -o liboverride.so $<
libtest.so : libtest.c
$(CC) -Wall -O2 -shared -fPIC -o libtest.so $<
libtest-bsymbolic.so : libtest.c
$(CC) -Wall -O2 -shared -fPIC -Wl,-Bsymbolic -o $@ $<
test : test.c libtest.so liboverride.so
$(CC) -Wall -O2 -L. -Wl,-rpath=. -ltest -o $@ $<
test-bsymbolic : test.c libtest-bsymbolic.so liboverride.so
$(CC) -Wall -O2 -L. -Wl,-rpath=. -ltest-bsymbolic -o $@ $<
$ cat liboverride.c
#include <stdio.h>
int foo(void)
printf("override foo called\n");
return 0;
$ cat libtest.c
#include <stdio.h>
int foo(void)
printf("libtest foo called\n");
return 1;
int test_foo(void)
return foo();
$ cat test.c
#include <stdio.h>
int test_foo(void);
int main(void)
printf("%d\n", test_foo());
return 0;
In words;
libtest.so provides
test_foo(), which
calls
foo() to do the actual work.
libtest-bsymbolic.so is simply built with the flag in
question,
-Bsymbolic.
liboverride.so provides the
alternative version of
foo() designed to override the
original via a
LD_PRELOAD of the library.
test is built against
libtest.so,
test-bsymbolic against
libtest-bsymbolic.so.
Running the examples, we can see that the
LD_PRELOAD does
not override the symbol in the library built with
-Bsymbolic.
$ ./test
libtest foo called
1
$ ./test-bsymbolic
libtest foo called
1
$ LD_PRELOAD=liboverride.so ./test
override foo called
0
$ LD_PRELOAD=liboverride.so ./test-bsymbolic
libtest foo called
1
There are a couple of things going on here. Firstly, you can see
that the SYMBOLIC flag is set in the dynamic section, leading to the
dynamic linker behaviour I explained in the
original
article:
ianw@jj:/tmp/bsymbolic$ readelf --dynamic ./libtest-bsymbolic.so
Dynamic section at offset 0x550 contains 22 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x00000010 (SYMBOLIC) 0x0
...
However, there is also an effect on generated code. Have a look at
the PLTs:
$ objdump --disassemble-all ./libtest.so
Disassembly of section .plt:
[... blah ...]
0000039c <foo>:
39c: ff a3 10 00 00 00 jmp *0x10(%ebx)
3a2: 68 08 00 00 00 push $0x8
3a7: e9 d0 ff ff ff jmp 37c <_init+0x30>
$ objdump --disassemble-all ./libtest-bsymbolic.so
Disassembly of section .plt:
00000374 <__gmon_start__@plt-0x10>:
374: ff b3 04 00 00 00 pushl 0x4(%ebx)
37a: ff a3 08 00 00 00 jmp *0x8(%ebx)
380: 00 00 add %al,(%eax)
...
00000384 <__gmon_start__@plt>:
384: ff a3 0c 00 00 00 jmp *0xc(%ebx)
38a: 68 00 00 00 00 push $0x0
38f: e9 e0 ff ff ff jmp 374 <_init+0x30>
00000394 <puts>:
394: ff a3 10 00 00 00 jmp *0x10(%ebx)
39a: 68 08 00 00 00 push $0x8
39f: e9 d0 ff ff ff jmp 374 <_init+0x30>
000003a4 <__cxa_finalize@plt>:
3a4: ff a3 14 00 00 00 jmp *0x14(%ebx)
3aa: 68 10 00 00 00 push $0x10
3af: e9 c0 ff ff ff jmp 374 <_init+0x30>
Notice the difference? There is no PLT entry for
foo()
when
-Bsymbolic is used.
Effectively, the toolchain has noticed that
foo() can
never be overridden and optimised out the PLT call for it. This is
analogous to using "hidden" attributes for symbols, which I have
detailed in another article on
symbol
visiblity attributes (which also goes into PLT's, if the above
meant nothing to you).
So
-Bsymbolic does have some more side-effects than just
setting a flag to tell the dynamic linker about binding rules -- it
can actually result in optimised code. However, I'm still struggling
to find good use-cases for
-Bsymbolic that can't be better
done with Version scripts and visibility attributes. I would
certainly recommend using these methods if at all possible.
Thanks to Ryan Lortie for comments on the original article.
Note: I wrote most of this before Neil Williams' recent comments on the 3.0 family of formats, so despite the timing this isn't really a reaction to that although I do have a couple of responses. On the whole I think I agree that the Lintian message is a bit heavy-handed and I'm not sure I'm thrilled about the idea of the default source format being changed (though I can see why the dpkg maintainers are interested in that). That said, as far as I personally am concerned, there is a vast cognitive benefit to me in having as much as possible be common to all my packages. Once I have more than a couple of packages that require patching and benefit from the 
Regarding my 
then they are really worth the extra expense, I have no regrets at all about spending about $320US (including tax) on my Bose QC-15. The Bauhn product is good for when you want something cheap, for example a set to be used in a server room, or for the use of children. I bought a Bauhn headset for a friend who is a pilot, he spent $1,100 on a noise-canceling headset for his plane but had never got around to buying one for recreational use I expect that he will allow his children to use his new Bauhn headphones, if they get broken it s only a $69 expense.
The second cheapest NC headphones I ve seen on offer in Australia is 
for $50US but doesn t seem to ship them outside the US (at least not to Australia).
Maintaining stable life with unstable distro?
This post may sound ludicrous to some of you but please hear me out. I ve been on Debian unstable/Sid for awhile now and it was all cool until yesterday.
Besides my work on Debian DebConf/some devel, I m university student, also I do web devel which is my primary source of income, besides all those I try to go out on weekends and have some kind of social life. Somehow I manage to do all these things and I d say I manage them pretty successfully too.
Now I m hardcore to the bone, but this is just too much, last week started working on immense web devel project didn t sleep more then 2hrs/day whole week; I also didn t shut down my notebook and just kept it on suspend as I usually do. Either way I was about to print/scan something and I connect my printer and what happens? My whole system freezes, now I ll let you figure out the result of doing hard reset and having a lot of gedit docs open at that time. And yes, I did had that bug reported awhile ago, can t find it now, but it was fixed, seems like it s back again.
In the end I m left with weekend completing the lost work + late school work + eclipse 3.5.2-2 ought to be uploaded. I do almost all the work (didn t upload eclipse) and literally pass out on Sunday. Wake up sometime on Monday, wasn t able to get up from bed.
Today is March 24th, that means 
Thanks to everyone who wished me a happy birthday. This is my present -- completely vegetarian tiramisu. (I'm getting a longer-lasting present too but it hasn't arrived yet.)
The shiny new 0.2.2 release of
by Michael Parenti. It seems that Caesar was more of a populist than a despot.
I'm now on my way back from The first miniDebCamp and miniDebconf that
happened from March 13th to March 19th in Khon Kaen, Thailand.
This even was organized locally by a team of very motivated Thai
Debian enthusiasts and contributors, such as Theppitak Karoonboonyanan
(*the* Thai DD), Neutron Soutmun, Kitt Tientanopajai, and all those whose name I'm
not remembering as of now (I hope they won't mind).
The even had kinda the structure of DebConfs, with a few days of "Debcamp"
to begin. See the 
So, a short while ago, the battery of my N900 had run flat. As in,
completely, utterly flat. In itself, that's no big deal -- you connect
it to a power outlet, wait until it's gone past the low-battery charging
bits, then boot and use as usual.
Except that it wouldn't. I'd recently installed a firmware update
from Nokia (which contained some bug fix that I really was waiting for),
and now the bloody thing wouldn't boot. Rather than do the low-battery
charging dance, it thought the battery had enough power that it could
just go ahead and boot.
Of course it was wrong, so whenever I powered it on, it would sit
there fore 30 seconds, try to boot, fail, power off for about a second,
and start over. I left it like that for a night, but it didn't
recharge.
Needless to say, that was quite annoying.
So I sent it in for repair, and today there was a package in the
mail, containing my (repaired) N900.
They'd reflashed it. A reasonable course of action, I guess, but of
course that meant all data was gone. Good thing it has a backup
application, and good thing I stored those backups on an external
micro-SD card.
Restoring those back-ups was a breeze; they'd also stored the
contents of sources.list and have done something akin to "dpkg
--get-selections", since now the application manager kindly asks me
whether I wish to reinstall the applications that I had installed.
Cute.
At any rate, I'm very happy I finally got rid of this awful
Samsung "cellphone". It's a SGH-C140 that I had as a replacement until
my N900 got back, but there was something very very wrong with it; I
don't know whether it's a design flaw or just a wear-and-tear issue
(these replacement phones aren't usually handled in the best possible
way), but it managed to lose connection to the network every once in a
while. So people who would call me would immediately get my voicemail,
and then they'd be angry with me later because I was unavailable. Well,
duh.
Anyway, that's over now. Whee.
For several years now we've been buying our groceries online. It's worked well, and for the last couple of christmases I remember Heather adding a six-pack into the pre-christmas order so she could pull it out and hand it off to the delivery guy.
Fair enough too, because he was their front-line man. He was the guy who had to actually meet the customer, and even if only for two minutes face time, the impression he gave with his cheery "seeya mate" on the way out, and his always-happy smile, was that getting the groceries delivered was fun.
Well that ongoing encouragement has gone now. His surly replacement who moans about having to carry the boxes up a flight of stairs does nothing to promote a repeat experience, and he brings into sharp focus the disadvantages of online grocery ordering: sometimes the wrong thing is substituted, or sent. Or the fruit doesn't travel quite so well as it does with the personal loving kindness we supply in our own padded wagon.
Two visits from the new guy, and we're back to driving to the supermarket ourselves, patrolling the aisles once more in a cereal haze of consumer demand. Distress-purchasing done, and back to the car A.S.A.P. Carrying our own boxes up the stairs.
I hate it, which is why I promoted the idea that we try this online grocery thing a few years ago, and it's worked just fine until this sullen bastard came along and destroyed the fun.
I wonder how 
] men in there helps build connections into the rest of the project. Make it clear that mistakes are ok. On retention recognise first patches, first commits in newsletters and the like. Call out big things or long wanted features by the person that helped. Regular discussion of patches and fixes rather than just the changelog. CMU did a study on undergrad women participation in CS : Lack of confidence preceeds lack of interest/partipation . Engagement with what they are doing is a key thing too. Women are consistently undervaluing their worth to the free software community . Its the personal touch that seems to make a huge difference . More projects should do a code of conduct kudos to Ubuntu for doing it Chris Ball.
I found the mentoring and women-in-free-software talks to have extremely similar themes which is perhaps confirmation or something but it wasn t surprising to me. They were both really good talks though!
And thats my coverage of LibrePlanet I m catching a plane after lunch 
. Its a good low-key conference, and well put together.
