The following variables are used in this project, but they are set to NOTFOUND

Question

I am working with CMake for my first time and have some understanding problems. Can anyone explain to me what the ERROR means, and why and how to fix it?

For my project, I downloaded the git-libraries "libubox-master" and "ubus-head" and put them into the folders "libubox" and "ubus" respectively in my project directory. Those libraries already have different CMakeList.txt files. In my project directory, I created the files client.c, server.c, server.h and CMakeLists.txt on my own.

My folder structure:

project/
--------cmake-build-debug/ ...
--------libubox/ ... provided files and folders, including CMakeFiles
--------ubus/ ... provided files and folders, including CMakeFiles
--------client.c
--------CMakeLists.txt
--------server.c
--------server.h

My CMakeList.txt file looks like:

   cmake_minimum_required(VERSION 3.7)
   project (ubusDemo)

   add_subdirectory(libubox)
   add_subdirectory(ubus)

   add_executable(Client client.c)
   add_executable(Server server.c)

So it's just a simple file - did I forget something necessary?

The error message I got:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
blob_library
    linked by target "ubusd" in directory /home/matthias/Workspace/ubusDemo/ubus
    linked by target "cli" in directory /home/matthias/Workspace/ubusDemo/ubus
    linked by target "server" in directory /home/matthias/Workspace/ubusDemo/ubus/examples
ubox_include_dir
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/lua
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
   used as include directory in directory /home/matthias/Workspace/ubusDemo/ubus/examples
ubox_library
    linked by target "ubus" in directory /home/matthias/Workspace/ubusDemo/ubus
    linked by target "ubusd" in directory /home/matthias/Workspace/ubusDemo/ubus
    linked by target "cli" in directory /home/matthias/Workspace/ubusDemo/ubus
    linked by target "server" in directory /home/matthias/Workspace/ubusDemo/ubus/examples
    linked by target "client" in directory /home/matthias/Workspace/ubusDemo/ubus/examples

Where do I have to make changes? Just in my CMakeLists.txt file, or do I have to add some dependencies in the other CMakeLists.txt files?

And how can I use some header files from e.g. libubox like the blob.h in my client.c File? Is it correct to write #include <libubox/blob.h>?

Unfortunately, I haven't work with Makefiles (neither CMake nor Make) before. I hope, someone can help me.

Answer 1

After searching for these unset variables in each of the external repositories, I found them here in the ubus CMake file:

IF(BUILD_STATIC)
  FIND_LIBRARY(ubox_library NAMES ubox.a)
  FIND_LIBRARY(blob_library NAMES blobmsg_json.a)
ELSE(BUILD_STATIC)
  FIND_LIBRARY(ubox_library NAMES ubox)
  FIND_LIBRARY(blob_library NAMES blobmsg_json)
ENDIF(BUILD_STATIC)

FIND_PATH(ubox_include_dir libubox/usock.h)
INCLUDE_DIRECTORIES(${ubox_include_dir})

This code in ubus is looking for the headers and built library files provided by the libubox package. It assumes that libubox is already installed on your machine, and that the searched components can be found in the standard system paths.

Using add_subdirectory() to include libubox in your CMake project will not build it soon enough; rather, libubox needs to be built and installed when you call add_subdirectory(ubus).

So, you will have to manually build and install the libubox package separately, before running your CMake project. Or, you can try to let CMake handle these steps within your project, with something like ExternalProject_Add() and execute_process, or FetchContent, as documented in some of the answers to this question. If you choose the latter, I encourage you to research each of these to determine which solution works best for your project.

Also, if just getting started with CMake, the CMake tag has a few useful CMake tutorials and resources.