Unable to link libssh in Cmake - c++

Question

I am trying to compile C++ Qt app that needs a connection to a ssh server, but I am unable to do it, as every time I try to compile it, I am getting the following error code:
[1/1] Linking CXX executable SSH-Manager
FAILED: SSH-Manager
: && /usr/bin/c++ -g CMakeFiles/SSH-Manager.dir/SSH-Manager_autogen/mocs_compilation.cpp.o CMakeFiles/SSH-Manager.dir/src/main.cpp.o CMakeFiles/SSH-Manager.dir/src/mainwindow.cpp.o CMakeFiles/SSH-Manager.dir/src/ssh_connection.cpp.o -o SSH-Manager -Wl,-rpath,/home/<username>/Qt/6.4.1/gcc_64/lib: -L -lssh /home/<username>/Qt/6.4.1/gcc_64/lib/libQt6Widgets.so.6.4.1 /home/<>username/Qt/6.4.1/gcc_64/lib/libQt6Gui.so.6.4.1 /home/<username>/Qt/6.4.1/gcc_64/lib/libQt6Core.so.6.4.1 /usr/lib/x86_64-linux-gnu/libGL.so && :
/usr/bin/ld: CMakeFiles/SSH-Manager.dir/src/ssh_connection.cpp.o: in function `SSH_connection::SSH_connection()':
/home/<username>/Programming/Projects/SSH-Manager/src/ssh_connection.cpp:11: undefined reference to `ssh_new'
.
.
.
/usr/bin/ld: /home/<username>/Programming/Projects/SSH-Manager/src/ssh_connection.cpp:17:
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
And the relevant part of CMakeLists.txt:
find_package(LIBSSH)
if (LIBSSH_FOUND)
message(${LIBSSH_VERSION})
include_directories(${LIBSSH_INCLUDE_DIR})
link_directories(${LIBSSH_LIBRARY_DIR})
target_link_libraries(SSH-Manager PRIVATE -L${LIBSSH_LIBRARY} -lssh)
else ()
message(Unable to find libssh!)
endif ()
What have I already tried:
purged and reinstalled openssh-server and libssh-dev
found and verified libssh.h file
included LIBSSH_DIR
What am I doing wrong?

Answer 1

If you look at what is being passed to the compiler, the error becomes pretty clear:
... -Wl,-rpath,/home/<username>/Qt/6.4.1/gcc_64/lib: -L -lssh
As you can see -L is empty.
To fix this let's start with the correct usage of target_link_libraries, this is NOT the correct usage:
target_link_libraries(SSH-Manager PRIVATE -L${LIBSSH_LIBRARY} -lssh)
Do not pass anything like -Lxxx -lxxx to this function, the function doesn't serve as a function that passes compiler options. If you check the libssh site you will see the variables which are populated, i.e. find_package(package) populates certain variables that you then can use in your CMakeLists.txt, in your case you are mainly interested in ${LIBSSH_LIBRARIES}, also note that you shouldn't need to use link_directories(). So instead of
target_link_libraries(SSH-Manager PRIVATE -L${LIBSSH_LIBRARY} -lssh)
# it should look like this
target_link_libraries(SSH-Manager PRIVATE ssh) #OR ${LIBSSH_LIBRARIES}
Also bare in mind that the name of the package might be case-sensitive if you are on Linux
EDIT: The authors also have an alias, so target_link_libraries(project ssh) is sufficient. Check the link provided to see the correct usage of it in your project.