CMake linking error (undefined reference to) - cmake

Question

I am working with the SSL-Vision software. It has an example client that I've been trying to separate from the whole project. I found the sources needed to edit a client myself, so I just copied them from the software and used CMake to build my client.
The project structure below is simplified, narrowing to the issue (I believe!).
.
├── CMakeLists.txt
├── main.cc
├── build
│ ├── CMakeLists.txt
│ └── messages_ssl_... (.cc/.h, 4 each)
└── src
├── CMakeLists.txt
└── (Other subdirs and sources/headers)
./CMakeLists.txt:
cmake_minimum_required(VERSION 2.6)
project( TestClient )
find_package( PkgConfig REQUIRED )
pkg_check_modules( QTCORE_PKG QtCore )
include_directories( ${QTCORE_PKG_INCLUDE_DIRS} )
include(FindProtobuf)
find_package( Protobuf REQUIRED )
include_directories( ${PROTOBUF_INCLUDE_DIRS} )
find_package( PkgConfig REQUIRED )
pkg_check_modules( GLIB_PKG glib-2.0 )
include_directories( ${GLIB_PKG_INCLUDE_DIRS} )
include_directories( "src" )
add_subdirectory( src )
include_directories( "build" )
add_subdirectory( build )
add_executable( clientTest clientTest.cc )
target_link_libraries( clientTest robocup_ssl_client messages_robocup_ssl_detection.pb messages_robocup_ssl_geometry.pb messages_robocup_ssl_wrapper.pb messages_robocup_ssl_refbox_log.pb netraw robocup_ssl_client protobuf QtCore )
./build/CMakeLists.txt:
add_library( messages_robocup_ssl_detection.pb SHARED messages_robocup_ssl_detection.pb.cc )
add_library( messages_robocup_ssl_refbox_log.pb SHARED messages_robocup_ssl_refbox_log.pb.cc )
add_library( messages_robocup_ssl_geometry.pb SHARED messages_robocup_ssl_geometry.pb.cc )
add_library( messages_robocup_ssl_wrapper.pb SHARED messages_robocup_ssl_wrapper.pb.cc )
It could be a missing #include in the messages_ssl_... files, but they all are auto-generated and seems to be correct.
In messages_robocup_ssl_detection.pb.h and messages_robocup_ssl_detection.pb.h there are only protobuf includes.
In messages_robocup_ssl_refbox_log.pb.h:
#include "messages_robocup_ssl_detection.pb.h"
// Other protobuf includes
In messages_robocup_ssl_wrapper.h:
#include "messages_robocup_ssl_detection.pb.h"
#include "messages_robocup_ssl_geometry.pb.h"
// Other protobuf includes
In each .cc file is included only its header and other protobuf libs.
Finally, when I do make, the following error is generated:
Linking CXX executable clientTest
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::ByteSize() const'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::MergeFrom(SSL_GeometryData const&)'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `protobuf_AddDesc_messages_5frobocup_5fssl_5fgeometry_2eproto()'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::Clear()'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::SSL_GeometryData()'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::default_instance()'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::SerializeWithCachedSizesToArray(unsigned char*) const'
build/libmessages_robocup_ssl_wrapper.pb.so: undefined reference to `SSL_GeometryData::MergePartialFromCodedStream(google::protobuf::io::CodedInputStream*)'
collect2: ld returned 1 exit status
make[2]: ** [clientTest] Erro 1
make[1]: ** [CMakeFiles/clientTest.dir/all] Erro 2
make: ** [all] Erro 2
I've been trying to fix this for some time.
Why is the libmessages_robocup_ssl_wrapper.pb.so showing errors if it has already been built before the linkage?

Answer 1

It could well be the linking order.
It looks like messages_robocup_ssl_wrapper.pb depends on messages_robocup_ssl_geometry.pb. If so, wrapper should come before geometry in the link line.
target_link_libraries( clientTest robocup_ssl_client
messages_robocup_ssl_detection.pb
messages_robocup_ssl_wrapper.pb
messages_robocup_ssl_geometry.pb
messages_robocup_ssl_refbox_log.pb
netraw
robocup_ssl_client
protobuf
QtCore )
Even better, let CMake take care of dependencies like this.
If you add...
target_link_libraries( messages_robocup_ssl_wrapper.pb
messages_robocup_ssl_geometry.pb )
then CMake will automatically retain that dependency when messages_robocup_ssl_wrapper.pb is specified as a dependency of another target. If you do this, then you can choose to omit messages_robocup_ssl_geometry.pb from the target_link_libraries( clientTest ... ) call.

Answer 2

Yet another reason to observe this error, maybe a less common one though, is that the ABIs are not compatible.
For instance, one of the library files could be built with the flag -D_GLIBCXX_USE_CXX11_ABI=0 while the project is not. This can be nasty to debug, especially when not being aware that this can be the problem.
I realize that this is not the issue in your specific scenario, but this answer might help other stumbling on it.

Answer 3

Another reason for undefined reference to ... can be a function marked as inline in a source file that is not a header. When optimization is enabled, the compiler can actually inline the function and skip generating the symbol leading to the error.
For this case, the solution is to either move inline functions to headers or remove the inline mark.