[CMake] SHARED library containing OBJECT library: Missing -fPIC

[Andreas Naumann]

I think the latter is the case. It should not be allowed to compose a 
shared library from OBJECT libraries.
What does the cmake developer think about this problem?

Regards,
Andreas

Am 22.06.2012 11:14, schrieb Benjamin Eikel:
> Hello Andreas,
>
> Am Freitag, 22. Juni 2012, 11:09:36 schrieb Andreas Naumann:
>    
>> Hello Benjamin,
>>
>> if you wants to use an object file for a shared library, this object
>> file has to be compiled with -fPIC. I don't think, that it is possible
>> to create a shared library from such object files.
>>      
> I know that this is the case. My question is not directed to -fPIC in general,
> but to CMake's handling of these files. I want to use CMake's new OBJECT
> library feature to structure the build of a library. The library is built
> statically as well as dynamically. When building statically, there is no
> problem in using the OBJECT libraries of CMake. But when building dynamically,
> the problem described in my original e-mail arises. CMake's documentation does
> not say that it is not allowed to use OBJECT libraries to compose shared
> libraries.
>
> Kind regards
> Benjamin
>
>    
>> Regards,
>> Andreas
>>
>> Am 22.06.2012 09:50, schrieb Benjamin Eikel:
>>      
>>> Hello,
>>>
>>> I have a problem using an OBJECT library that I want to compile into a
>>> SHARED library using CMake version 2.8.8.
>>>
>>> Here is a small example that demonstrates my problem:
>>>
>>> # --------------- CMakeLists.txt ---------------
>>> cmake_minimum_required(VERSION 2.8.8)
>>> project(CMakeTest CXX)
>>> add_library(MyLibSub OBJECT
>>>
>>>           ClassA.cpp
>>>
>>> )
>>> add_library(MyLib SHARED
>>>
>>>           $<TARGET_OBJECTS:MyLibSub>
>>>           ClassB.cpp
>>>
>>> )
>>>
>>> The content of the other four files is more or less irrelevant. To make
>>> the example complete, I added them at the end of this e-mail.
>>>
>>> When I want to build this example, I get the following error:
>>>
>>> $ mkdir build&&   cd build&&   cmake ..&&   make
>>> -- The CXX compiler identification is GNU 4.7.0
>>> -- Check for working CXX compiler: /usr/bin/c++
>>> -- Check for working CXX compiler: /usr/bin/c++ -- works
>>> -- Detecting CXX compiler ABI info
>>> -- Detecting CXX compiler ABI info - done
>>> -- Configuring done
>>> -- Generating done
>>> -- Build files have been written to: /home/benjamin/Desktop/CMake
>>> test/build Scanning dependencies of target MyLibSub
>>> [ 50%] Building CXX object CMakeFiles/MyLibSub.dir/ClassA.cpp.o
>>> [ 50%] Built target MyLibSub
>>> Scanning dependencies of target MyLib
>>> [100%] Building CXX object CMakeFiles/MyLib.dir/ClassB.cpp.o
>>> Linking CXX shared library libMyLib.so
>>> /usr/bin/ld: CMakeFiles/MyLibSub.dir/ClassA.cpp.o: relocation R_X86_64_32
>>> against `.rodata' can not be used when making a shared object; recompile
>>> with -fPIC
>>> CMakeFiles/MyLibSub.dir/ClassA.cpp.o: could not read symbols: Bad value
>>> collect2: error: ld returned 1 exit status
>>> make[2]: *** [libMyLib.so] Error 1
>>> make[1]: *** [CMakeFiles/MyLib.dir/all] Error 2
>>> make: *** [all] Error 2
>>>
>>> When I add the line
>>> set_target_properties(MyLibSub PROPERTIES COMPILE_FLAGS "-fPIC")
>>> to the CMakeLists.txt, everything works fine. Am I doing something wrong?
>>> Should CMake add "-fPIC" automatically in this case? Your feedback is
>>> greatly appreciated.
>>>
>>> Kind regards
>>> Benjamin
>>>
>>>
>>>
>>> // --------------- ClassA.cpp ---------------
>>> #include "ClassA.h"
>>> #include<iostream>
>>>
>>> void ClassA::printName() {
>>>
>>>           std::cout<<   "ClassA"<<   std::endl;
>>>
>>> }
>>> // --------------- ClassA.h ---------------
>>> #ifndef CLASSA_H
>>> #define CLASSA_H
>>>
>>> struct ClassA {
>>>
>>>           void printName();
>>>
>>> };
>>>
>>> #endif /* CLASSA_H */
>>> // --------------- ClassB.cpp ---------------
>>> #include "ClassB.h"
>>> #include<iostream>
>>>
>>> void ClassB::printName() {
>>>
>>>           std::cout<<   "ClassB"<<   std::endl;
>>>           a.printName();
>>>
>>> }
>>> // --------------- ClassB.h ---------------
>>> #ifndef CLASSB_H
>>> #define CLASSB_H
>>>
>>> #include "ClassA.h"
>>>
>>> struct ClassB {
>>>
>>>           void printName();
>>>           ClassA a;
>>>
>>> };
>>>
>>> #endif /* CLASSB_H */
>>>
>>> --
>>>
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>> --
>>
>> Powered by www.kitware.com
>>
>> Visit other Kitware open-source projects at
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>>
>> Please keep messages on-topic and check the CMake FAQ at:
>> http://www.cmake.org/Wiki/CMake_FAQ
>>
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>>      
> --
>
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html
>
> Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ
>
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