CMake and FIND PACKAGE

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One of the most important features of CMake is that it allows to configure your project to include/link additional packages (libraries). To do this you have to tell CMake you want to use an additional package, you do this with the FIND_PACKAGE macro. I found FIND_PACKAGE a bit confusing so I try to explain here how it works and how we have been using it on YARP.

Note: I here explain the functionalities of FIND_PACKAGE that are used in YARP, go to http://www.cmake.org/HTML/Documentation.html for a more complete explanation.

FIND_PACKAGE(<name>) asks CMake to find and load settings from the external projects/packages called <name>. This works in a few steps:

  • CMake searches the directories listed in CMAKE_MODULE_PATH for a file called Find<name>.cmake. If found, this macro is executed and it is responsible for finding the package (examples of similar macro are: FindACE.cmake, FindBoost.cmake, FindOpenCV.cmake that are located in your $YARP_ROOT/conf directories). "Find" macros define variables which allow you to check if the package has been found and use it in your project. Unfortunately different macros behave differently so you have to check the code to find out the exact names of those variables. A good idea would be to define:
  <name>_FOUND
  <name>_INCLUDE_DIR
  <name>_LIBRARIES
  <name>_LINK_DIRECTORIES

For example, suppose you want to include the ACE package in a new project:

  #### assume FindOpenCV.cmake is in one of the CMAKE_MODULE_PATH folders.
  FIND_PACKAGE(OpenCV)
  
  # searches for all .cpp and .h files in the current directory and add them 
  # to the current project
  FILE(GLOB folder_source *.cpp)
  FILE(GLOB folder_header *.h)
  SOURCE_GROUP("Source Files" FILES ${folder_source})
  SOURCE_GROUP("Header Files" FILES ${folder_header})
  
  # create the project
  ADD_EXECUTABLE(your_project ${folder_source} ${folder_header})
   
    add include directories
  INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR})
  INCLUDE_DIRECTORIES(${OPENCV_INCLUDE_DIR})
   
  # link opencv libraries 
  TARGET_LINK_LIBRARIES(your_project ${OPENCV_LIBRARIES})
  • If CMake does not find any Find<name>.cmake file in the directories listed in MAKE_MODULE_PATH, it assumes the package was built with CMake and has a "<name>Config.cmake" file associated. CMake expects the user to specify the location of this file by filling a cache entry called <name>_DIR (this entry is created by CMake automatically). If the file is found, it is processed to load the settings of the package (an error or a warning is displayed otherwise).

For example, when YARP is built CMake creates a file called YARPConfig.cmake in the "build" directory selected by the user. When users want to create a project which uses YARP they do something like:

  SET(YARP_DIR "c:/yarp/build" CACHE LOCATION "where was yarp built?")
  
  or, if they defined YARP_ROOT:
  
  SET(YARP_DIR "$ENV{YARP_ROOT}" CACHE LOCATION "where was yarp built?")
  
  FIND_PACKAGE(YARP)

The macro FIND_PACKAGE automatically searches YARP_DIR for the file YARPConfig.CMake and executes it. YARPConfig.cmake creates the entries YARP_LIBRARIES, YARP_INCLUDES and YARP_DEFINES which are already processed so that projects added later will use them:

  LINK_LIBRARIES(${YARP_LIBRARIES})
  INCLUDE_DIRECTORIES(${YARP_INCLUDES})
  ADD_DEFINITIONS(${YARP_DEFINES})

Note: LINK_LIBRARIES is an obsolete CMake command, in the future should be replaced with TARGET_LINK_LIBRARIES

YARPConfig.cmake also append ${YARP_DIR}/conf to CMAKE_MODULE_PATH so that later all Find*.cmake provided with YARP can be accessed by CMake by calling FIND_PACKAGE command.