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Date: Mon, 11 Jan 2016 17:44:23 +0800
From: "Wangnan (F)" <wangnan0@huawei.com>
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To: Jiri Olsa <jolsa@redhat.com>
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        <lizefan@huawei.com>, <pi3orama@163.com>,
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Subject: Re: [PATCH 1/6] perf tools: Add -lutil in python lib list for broken
 python-config
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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.

Thank you.