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

[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.

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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