"Cannot find -l<directory>" error - CMake - c++

Question

My program includes a subdirectory, because a header and a source file are there which are needed. My project looks like this:
EDIT: Perhaps I didn't specify: my goal is to link frame_cap.cpp to the header grab_cut.h, as I require its functions.
[-] raspicam
| [-] main_folder
| frame_cap.cpp
| CMakeLists.txt
| [-] opencv-plus
| grab_cut.cpp
| grab_cut.h
| CMakeLists.txt
Now, I have already linked the two using the CMakeLists.txt in main_folder. It looks as follows:
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(frame_cap)
SET(OpenCVPLUS_DIR "/home/pi/Desktop/raspicam/main_folder/opencv-plus")
FIND_PACKAGE( OpenCV REQUIRED )
SET(OpenCV_PACKAGE ${OpenCV_LIBRARIES})
INCLUDE_DIRECTORIES( ${OpenCV_INCLUDE_DIRS} )
INCLUDE_DIRECTORIES(${OpenCVPLUS_DIR})
LINK_DIRECTORIES(${OpenCVPLUS_DIR})
ADD_SUBDIRECTORY(${OpenCVPLUS_DIR})
ADD_EXECUTABLE( frame_cap frame_cap.cpp )
TARGET_LINK_LIBRARIES( frame_cap ${OpenCV_LIBS} ${OpenCVPLUS_DIR})
As you can see, OpenCVPLUS_DIR is, well, the opencv-plus directory, where both the header and source file are located. The CMakeLists.txt file in the opencv-plus folder looks as follows:
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(grab_cut)
SET(OpenCVPLUS_DIR "/home/pi/Desktop/raspicam/main_folder/opencv-plus")
FIND_PACKAGE(OpenCV REQUIRED)
INCLUDE_DIRECTORIES(${OpenCV_INCLUDE_DIRS})
SET(SOURCES grab_cut.cpp ${OpenCVPLUS_DIR}/grab_cut.h)
ADD_EXECUTABLE(grab_cut grab_cut.cpp ${SOURCES})
TARGET_LINK_LIBRARIES(grab_cut ${OpenCV_LIBS})
Finally, this is the exact error I get:
[ 25%] Linking CXX executable frame_cap
/usr/bin/ld: cannot find -lraspicam/main_folder/opencv-plus/
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/frame_cap.dir/build.make:102: frame_cap] Error 1
make[1]: *** [CMakeFiles/Makefile2:73: CMakeFiles/frame_cap.dir/all] Error 2
make: *** [Makefile:84: all] Error 2
I'd be happy to hear your suggestions as I'm a newbie at CMake and am not really sure what I'm doing wrong.

Answer 1

The error:
/usr/bin/ld: cannot find -lraspicam/main_folder/opencv-plus/
occurs because you try to link a directory to your frame_cap executable here:
TARGET_LINK_LIBRARIES( frame_cap ${OpenCV_LIBS} ${OpenCVPLUS_DIR})
This doesn't make sense; only libraries should be linked to your executable.
From your comments, it sounds like you want to build two executables, but use the grab_cut functions in both. This is certainly achievable with CMake. But you appear to have a design problem, because compiling the grab_cut.cpp file with the frame_cap executable will probably cause issues, assuming both files contain a main function. You'll need to split the main function into a separate file (say, main.cpp), then you can include grab_cut.cpp in the compilation of both executables.
BTW, you can use CMAKE_CURRENT_LIST_DIR to refer to the directory of the current CMakeLists.txt file, so there is no need to list out directories in your source-tree explicitly. Also, I pray you are using a newer CMake than version 2.8. Some of the steps in your CMake files seemed unnecessary, so here are the simplified files that might work for you.
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(frame_cap)
# There is no need to spell out this directory since it exists
# in the source-tree already.
SET(OpenCVPLUS_DIR "/home/pi/Desktop/raspicam/main_folder/opencv-plus")
FIND_PACKAGE( OpenCV REQUIRED )
# Seems unused, remove it.
SET(OpenCV_PACKAGE ${OpenCV_LIBRARIES})
# Include the OpenCV and opencv-plus headers in this executable.
INCLUDE_DIRECTORIES(${OpenCV_INCLUDE_DIRS} ${CMAKE_CURRENT_LIST_DIR}/opencv-plus)
# There is no library to link from this directory, so we don't need this call.
LINK_DIRECTORIES(${OpenCVPLUS_DIR})
ADD_SUBDIRECTORY(opencv-plus)
# Add the grap_cut.cpp file to the frame_cap executable.
ADD_EXECUTABLE( frame_cap frame_cap.cpp opencv-plus/grab_cut.cpp)
# We don't link anything from opencv-plus, we compiling it directly.
TARGET_LINK_LIBRARIES( frame_cap ${OpenCV_LIBS} ${OpenCVPLUS_DIR})
And your opencv-plus/CMakeLists.txt file can be simplified to something like this:
# If this CMake file is always called as part of the top-level CMake
# file, don't need this.
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(grab_cut)
# Don't need this, as explained earlier.
SET(OpenCVPLUS_DIR "/home/pi/Desktop/raspicam/main_folder/opencv-plus")
# The top-level CMake file already found OpenCV, so the variables
# we need are already defined.
FIND_PACKAGE(OpenCV REQUIRED)
# The include directories get initialized from the parent CMake file, so no need
# for this call if the include directories are the same for both executables.
INCLUDE_DIRECTORIES(${OpenCV_INCLUDE_DIRS})
# Separate 'main' function into separate file, and add it here.
SET(SOURCES grab_cut.cpp grab_cut.h main.cpp)
# Don't need to add 'grab_cut.cpp' to the executable twice! It was
# already added using the 'SOURCES' variable.
ADD_EXECUTABLE(grab_cut grab_cut.cpp ${SOURCES})
TARGET_LINK_LIBRARIES(grab_cut ${OpenCV_LIBS})