Re: [PATCH 1/6] perf tools: Add -lutil in python lib list for broken python-config

From: Wangnan (F)
Date: Mon Jan 11 2016 - 04:51:06 EST




On 2016/1/11 17:44, Wangnan (F) wrote:


On 2016/1/11 17:28, Jiri Olsa wrote:
On Fri, Jan 08, 2016 at 02:23:56PM +0000, Wang Nan wrote:
On some system the perf-config is broken, causes link failure like this:

/usr/lib64/python2.7/config/libpython2.7.a(posixmodule.o): In function `posix_forkpty':
/opt/wangnan/yocto-build/tmp-eglibc/work/x86_64-oe-linux/python/2.7.3-r0.3.1/Python-2.7.3/./Modules/posixmodule.c:3816: undefined reference to `forkpty'
/usr/lib64/python2.7/config/libpython2.7.a(posixmodule.o): In function `posix_openpty':
/opt/wangnan/yocto-build/tmp-eglibc/work/x86_64-oe-linux/python/2.7.3-r0.3.1/Python-2.7.3/./Modules/posixmodule.c:3756: undefined reference to `openpty'
collect2: error: ld returned 1 exit status
make[1]: *** [/home/wangnan/kernel-hydrogen/tools/perf/out/perf] Error 1
make: *** [all] Error 2

$ python-config --libs
-lpthread -ldl -lpthread -lutil -lm -lpython2.7
so your 'python-config --libs' shows -lutil and you still get the build failure?

after your fix I can see double -lutil in PYTHON_EMBED_LIBADD,
which is probably not a problem but could be ommited I think

In my python-config:

# python-config
-lpthread -ldl -lpthread -lutil -lm -lpython2.7

There are multiple -lpthread already. In fact multiple '-l' options is not
a problem at all. Please see below.

In this case a '-lutil' should be appended to -lpython2.7.

(I know we have --start-group and --end-group. I can see them in
command line of collect2 by strace. However it doesn't work. Seems
I have a broken environment?)
hum, how is this related to start/end -group options?

Please check man page of ld:

<quote>
--start-group archives --end-group
The archives should be a list of archive files. They may be either explicit file names, or -l options.

The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved.

Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.
</quote>

Without --start/end-group, if we provide -l in this order:

... -la -lb -lc ...

and libb.a requires a symbol in liba.a, link would fail. The simplest way
to avoid this is to add another -la after -lb:

... -la -lb -la -lc ...

In my environment it works as if we don't provide -Wl,--start-group and -Wl,--end-group.
I don't know why.


I mean the python case before I add -lutil after python libs. -Wl,--start-group and -Wl,--end-group
seems not work for me. They are issued, I confirm this in strace output.

Thank you.

Thank you.