Internals of the generated find_package() infrastructure

CMake’s find_package is the preferred method for packages to communicate to CMake (and thereby to catkin) the libraries, include directories and such that packages should use.

There are a couple of modes of operation of find_package (see the CMake documentation), “module” mode and “config” mode. “module” mode is the one that uses cmake scripts named Find****.cmake. “config” mode is the preferred mode, and it works differently.

One reason we find the ‘config mode’ superior is that is supports multiple simultaneous installed versions of packages.

For a package T, ‘config mode’ consists of two cmake files, both of which are installed somewhere on cmake’s CMAKE_PREFIX_PATH. The first is ‘t-config-version.cmake’. We find the most succinct form to be like this:

set(PACKAGE_VERSION_EXACT False)
set(PACKAGE_VERSION_COMPATIBLE False)
if("${PACKAGE_FIND_VERSION}" STREQUAL "")
  set(PACKAGE_VERSION_COMPATIBLE TRUE)
  return()
endif()

if("${PACKAGE_FIND_VERSION}" VERSION_EQUAL "9.9.9")
  set(PACKAGE_VERSION_EXACT True)
  set(PACKAGE_VERSION_COMPATIBLE True)
endif()

if("${PACKAGE_FIND_VERSION}" VERSION_LESS "9.9.9")
  set(PACKAGE_VERSION_COMPATIBLE True)
endif()

where 9.9.9 is replaced by the numeric version of the package. The second file, t-config.cmake, tells the client what the assorted includes/libs are for the package by setting variables t_INCLUDE_DIRS, t_LIBRARIES and so forth. The user passes these values to cmake’s include_directories() and target_link_libraries().

Finding ROS by component (recommended method)

If you want to specify a dependency on several ROS components simultaneously, use find_package(ROS [XX.YY] REQUIRED COMPONENTS comp1 comp2), e.g.:

find_package(ROS REQUIRED COMPONENTS
             cpp_common rostime roscpp_traits
             roscpp_serialization sensor_msgs)
include_directories(${ROS_INCLUDE_DIRS})

add_executable(myexec ...)
target_link_libraries(${ROS_LIBRARIES})

See the cmake documentation for find_package() for more details. Your CMAKE_PREFIX_PATH will need to point to a ROS installation. The version string is as yet to be determined; “electric”, for instance, is not a valid version. We will have to establish some mapping between ROS versions and a numeric version of only numbers and dots.