Linking error using CMake to create a shared library in C++ because multiple definition of symbol -

i'm having problems compile code, links poco c++ libraries, create shared library linux (mint 13, in case). reduced environment.

i have these 3 files:

il_notify.h

#ifndef __il_notify_h__ #define __il_notify_h__  #include <string> #include "poco/logger.h" #include "poco/logstream.h"  using poco::logger; using poco::logstream;  namespace mynamespace {     // logger. inherits root channel     logstream lstr(logger::get("mylogger"));      // other class, call logger way:     //lstr << "this test" << std::endl; }  #endif 

il_class1.cpp

#include "il_class1.h" #include "il_notify.h"  namespace mynamespace {      void il_class1::foo()     {         // stuff...          lstr << "this test msg il_class1::foo" << std::endl;     } } 

il_class2.cpp

#include "il_class2.h" #include "il_notify.h"  namespace mynamespace {      void il_class2::bar()     {         // stuff...          lstr << "this test msg il_class2::bar" << std::endl;     } } 

il_class1 , il_class2 declared @ il_class1.h , il_class2.h. no inclussion of il_notify.h inside headers.

and cmakelists.txt file

# set search path find pococonfig.cmake set(cmake_prefix_path ${cmake_prefix_path} ${cmake_source_dir}/cmake/modules/)   # needed packages find_package(poco required)  # now, can use poco include_directories(     ${poco_include_dirs}     ${cmake_current_source_dir}/include )  # libraries linking against link_directories(     ${poco_library_dirs} )  # cpp files build library file(glob_recurse all_sources     ${cmake_current_source_dir}/src/il_class1.cpp     ${cmake_current_source_dir}/src/il_class2.cpp )  # library creation add_library(mylib shared ${all_sources}) 

i can execute cmake successfully, create makefile. when run make, i error

cmakefiles/mylib.dir/src/il_class2.cpp.o:(.bss+0x0): multiple definition of `mynamespace::lstr' cmakefiles/mylib.dir/src/il_class1.cpp.o:(.bss+0x0): first defined here 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 

so, when i'm preventing headers included twice, via #ifndef, make can't compile code because symbol declared in header detected twice. why?

after seeing question duplicated, got right solution.

my il_notify.h

#ifndef __il_notify_h__ #define __il_notify_h__  #include <string> #include "poco/logger.h" #include "poco/logstream.h"  using poco::logger; using poco::logstream;  namespace openil {     // logger. inherits root channel     extern logstream lstr; }  #endif 

my new il_notify.cpp

#include "il_notify.h"  namespace openil {     // todo: more flexibility here: more channels, different formatting...     // maybe il_utils function should      // logger. inherits root channel     logstream lstr(logger::get("openillogger"));      // other class, call logger way:     //lstr << "this test" << std::endl; }