Undefined reference in libfreenect c++ wrapper

I want to print the number of connected devices with libfreenect in c++. As described in https://openkinect.org/wiki/C%2B%2B_Wrapper i include the libfreenect.hpp header file in my TestKinectConnection.cpp.

My TestKinectConnection.cpp:

#include <iostream>
#include "libfreenect.hpp"

using namespace std;

int main(void) {
    Freenect::Freenect nect;
    freenect_context *f_ctx;
    cout << nect.deviceCount() << endl;
    return(0);
}

When i build with cmake --build build -- -j3 the terminal shows

CMakeFiles/projektinf.dir/src/main/TestKinectConnection.cpp.o: In function `Freenect::Freenect::Freenect()':
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectC2Ev[_ZN8Freenect8FreenectC5Ev]+0x40): undefined reference to `freenect_init'
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectC2Ev[_ZN8Freenect8FreenectC5Ev]+0x90): undefined reference to `freenect_select_subdevices'
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectC2Ev[_ZN8Freenect8FreenectC5Ev]+0xb0): undefined reference to `pthread_create'
CMakeFiles/projektinf.dir/src/main/TestKinectConnection.cpp.o: In function `Freenect::Freenect::~Freenect()':
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectD2Ev[_ZN8Freenect8FreenectD5Ev]+0xa5): undefined reference to `pthread_join'
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectD2Ev[_ZN8Freenect8FreenectD5Ev]+0xb4): undefined reference to `freenect_shutdown'
CMakeFiles/projektinf.dir/src/main/TestKinectConnection.cpp.o: In function `Freenect::Freenect::deviceCount()':
TestKinectConnection.cpp:(.text._ZN8Freenect8Freenect11deviceCountEv[_ZN8Freenect8Freenect11deviceCountEv]+0x17): undefined reference to `freenect_num_devices'
CMakeFiles/projektinf.dir/src/main/TestKinectConnection.cpp.o: In function `Freenect::Freenect::operator()()':
TestKinectConnection.cpp:(.text._ZN8Freenect8FreenectclEv[_ZN8Freenect8FreenectclEv]+0x4f): undefined reference to `freenect_process_events_timeout'
collect2: error: ld returned 1 exit status
CMakeFiles/projektinf.dir/build.make:95: recipe for target '../bin/projektinf' failed
make[2]: *** [../bin/projektinf] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/projektinf.dir/all' failed
make[1]: *** [CMakeFiles/projektinf.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

My CMakeLists.txt:

# Specify the minimum version for CMake
cmake_minimum_required(VERSION 3.10)

# Project's name
project(projektinf)

# Set the output folder where your program will be created
set(CMAKE_BINARY_DIR ${CMAKE_SOURCE_DIR}/bin)

set(EXECUTABLE_OUTPUT_PATH ${CMAKE_SOURCE_DIR}/bin)
set(LIBRARY_OUTPUT_PATH ${CMAKE_SOURCE_DIR}/lib)
set(PROJECT_SOURCE_DIR ${CMAKE_SOURCE_DIR})
set(EXTERNAL_INSTALL_LOCATION ${CMAKE_SOURCE_DIR}/lib)

# The following folder will be included
include_directories("${PROJECT_SOURCE_DIR}/src/include")

add_executable(projektinf ${PROJECT_SOURCE_DIR}/src/main/TestKinectConnection.cpp)

add_library(libfreenect ${PROJECT_SOURCE_DIR}/src/include/libfreenect.hpp)
target_link_libraries(projektinf PUBLIC libfreenect)
set_target_properties(libfreenect PROPERTIES LINKER_LANGUAGE CXX)

https://github.com/OpenKinect/libfreenect/blob/master/wrappers/cpp/libfreenect.hpp

https://github.com/OpenKinect/libfreenect/blob/master/include/libfreenect.h

In short summary, libfreenect.hpp tries to include libfreenect.h und completely fails.

Solution

From your question it is unclear whether you installed libfreenect globally or simply bundled the library with your code.

If you installed it globally, you forgot to tell CMake that the libfreenect target needs to link with libfreenect.so using the -lfreenect linker flag. Adding the following should fix that:

set_property(TARGET libfreenect PROPERTY INTERFACE_LINK_LIBRARIES -lfreenect)

The proper approach is to make libfreenect an IMPORTED target, as documented in "It's time to do CMake right".

If you bundled the .cpp with your code, you need to add the .cpp file to the add_library statement that defines the libfreenect target.