[SOLVED] Boost undefined symbol when building .so library with CMake

Issue

I'm building a shared library (.so) on Linux with CMake which uses Boost 1.75.0.

In CMakeLists.txt, Boost is added the following way:

find_package(Boost REQUIRED COMPONENTS system chrono filesystem date_time log iostreams program_options)

and added to the target:

target_link_libraries(mytarget PUBLIC ${Boost_LIBRARIES})

CMake finds Boost, so no problem here.

Boost is linked as a static library. When I list the linked object files, I get the correct libraries:

-- LIB_FILES: ...;/path/to/boost/debug/lib/libboost_filesystem.a;...

The library compiles and links without error.

However, when I use a simple function of the library, I get the following error:

undefined symbol: _ZNK5boost10filesystem16filesystem_error4whatEv

This symbol is indeed undefined in the library:

$ nm -g libmytarget.so | grep _ZNK5boost10filesystem16filesystem_error4whatEv
                 U _ZNK5boost10filesystem16filesystem_error4whatEv

The demangled name is boost::filesystem::filesystem_error::what() const, which is defined in boost/filesystem/exception.hpp as

namespace boost
{
  namespace filesystem
  {
    class BOOST_FILESYSTEM_DECL filesystem_error : public std::exception
    {
    public:
      virtual const char * what() const throw();
...

Note that my code does not call this method.

The symbol is defined in libboost_filesystem.a, which is used by the linker, as a symbol in the text (code) section:

$ nm -g /path/to/boost/debug/lib/libboost_filesystem.a | grep _ZNK5boost10filesystem16filesystem_error4whatEv
00000000000003fa T _ZNK5boost10filesystem16filesystem_error4whatEv

My question:

I don't understand how it is possible that this symbol is undefined in the compiled library, when it is present in the statically linked lib (file libboost_filesystem.a), which is recognized and used by the linker.

Solution

Enabling INTERPROCEDURAL_OPTIMIZATION in CMake solved the problem:

set_target_properties(mytarget PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)

Now, the symbol is properly linked:

$ nm -g libmytarget.so | grep filesystem_error4what
00000000001cfe22 T _ZNK5boost10filesystem16filesystem_error4whatEv

But I still don't know what's going on behind the scenes and why this error pops up in the first place.

Answered By - WolfgangP