[CMake] How to specify -fpic ?

[Michael Wild]

On 30. Sep, 2010, at 13:30 , David Aldrich wrote:

> Hi Michael
> 
>> So, the options are:
>> 
>> - use -whole-archive as you do now
>> - make the Kernel library shared
>> - link the dlopen'ed libraries against Kernel
> 
> Thanks very much - I understand. I think I will keep the -whole-archive method.
> 
> However, now I'm worried about how I link in the Python library:
> 
> /usr/bin/c++ -O3 -DNDEBUG -Wall -m64 -o myProj -rdynamic -Wl,-whole-archive Kernel/libKernel.a -Wl,-no-whole-archive -ldl -lpython2.4
> 
> The dlopen'ed libraries may use the Python library. I guess I could include it in the -whole-archive part, but perhaps it would be better to link each shared library against Python. Would you agree?
> 

Unless the python library is static (which it AFAIK almost never is) you should be safe.

> Sorry that I am now off topic w.r.t cmake.
> 
> By the way, I think I am pretty much there with changing our build system from manually coded makefiles to cmake. I like cmake!  Thanks for all your help in getting there.  The support on this list is excellent.

;-) well, CMake does have its quirks and dark corners, but in general I find it to be much preferable over anything else I've ever seen so far...

Michael

> 
> Best regards 
> 
> David 
> 
>> -----Original Message-----
>> From: Michael Wild [mailto:themiwi at gmail.com]
>> Sent: 30 September 2010 12:20
>> To: David Aldrich
>> Cc: cmake at cmake.org
>> Subject: Re: [CMake] How to specify -fpic ?
>> 
>> 
>> On 30. Sep, 2010, at 13:05 , David Aldrich wrote:
>> 
>>> Hi
>>> 
>>> Ok, by following the link to the wiki suggested by Michael Loose, I used:
>>> 
>>> target_link_libraries( myProj -Wl,-whole-archive Kernel
>>> -Wl,-no-whole-archive)
>>> 
>>> This has fixed my problem. The executable links and runs correctly.
>>> 
>>> My link command is now:
>>> 
>>> /usr/bin/c++   -O3 -DNDEBUG -Wall -m64  -Wl,--export-dynamic   -o myProj -
>> Wl,-whole-archive Kernel/libKernel.a -Wl,-no-whole-archive -ldl -lpython2.4
>>> 
>>> I don't need portability so I think this is ok.
>>> 
>>> Michael Wild wrote:
>>> 
>>>> The -whole-archive flag is pretty useless with executables (unless,
>>>> you plan to use it as a library too, but that is outlandish).
>>>> --export-dynamic may be also necessary, if your dlopen'ed libraries
>>>> use symbols in your executable.
>>> 
>>> So my concern is now: am I being 'outlandish' ?  The dlopen'ed libraries do
>> use functions in libKernel.a that is linked to the executable. Is this
>> outlandish? Is there a better way?
>> 
>> The "outlandish" referred to using an executable also as a library, so you're
>> fine :-) But I think I found the reason for why you need this -whole-archive
>> flag: Since your executable doesn't use all of the functions in the Kernel
>> library, the linker throws them away. This results in your dlopen'ed
>> functions to fail.
>> 
>> So, the options are:
>> 
>> - use -whole-archive as you do now
>> - make the Kernel library shared
>> - link the dlopen'ed libraries against Kernel
>> 
>> Michael
>> 
> 

--
There is always a well-known solution to every human problem -- neat, plausible, and wrong.
H. L. Mencken

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