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

[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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> > 
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