Porting a CMake project to qiBuild

Requirements

This tutorial assumes that you already have a CMake-based project.

We will see how qiBuild can help you writing less code, while staying close to the “official” CMake recommendations when dealing with the Find<>.cmake or <>-config.cmake files.

In this tutorial, we will use a simple project called foobar.

It is pure CMake code, there is a foo library, and a bar executable linking with the foo library.

The sources of the pure CMake foobar project can be found here: foobar_pure_cmake.zip

Extract the archive in you qiBuild worktree, you should end up with something like:

.qi
|__ qibuild.xml
|__ foobar
    |__ CMakeLists.txt
    |__ libbar
        |__ CMakeLists.txt
        |__ bar
            |__ bar.h
            |__ bar.cpp
    |__ foo
        |__ CMakeLists.txt
        |__ main.cpp

A standard CMake project

The standard CMakeLists.txt files for such a project look like this:

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(foobar)

add_subdirectory(libbar)
add_subdirectory(foo)

libbar/CMakeLists.txt

include_directories(".")

add_library(bar
  bar/bar.hpp
  bar/bar.cpp)

install(TARGETS bar
  RUNTIME DESTINATION "lib"
  ARCHIVE DESTINATION "lib"
  LIBRARY DESTINATION "lib")

install(FILES bar/bar.h
  DESTINATION "include/bar")

foo/CMakeLists.txt

include_directories("${CMAKE_SOURCE_DIR}/libbar")

add_executable(foo main.cpp)
target_link_libraries(foo bar)

install(TARGETS foo DESTINATION "bin")

A few CMake limitations

  • You have to specify install rules for every target

  • If you move the bar library to an other directory, you will have to fix foo/CMakeLists.txt

  • You cannot use foobar as a subdirectory of a new project (because of the use of CMAKE_SOURCE_DIR

  • You have a standard layout when you install your targets:

    <prefix>
  |__ lib
      |__ libbar.a
  |__ bin
      |__ foo
  |__ include
      |__ bar
         |__ bar.hpp

But it has nothing to do with where targets are in your build directory. (foo is somewhere in build/foo/ and libbar.a in build/bar).

  • If you want to give a foobar SDK for someone working with Visual Studio, you will have to make sure libbar and foo contain a _d when there are build on debug (unless you are very careful, you cannot mix debug and release libraries on Visual Studio, so the _d is the safest way to do it)
  • If you want other people to use the bar library from an other project, you will have to configure a bar-config.cmake looking like:
find_path(BAR_INCLUDE_DIR bar/bar.hpp)
find_library(BAR_LIBRARY bar)

include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(bar
  DEFAULT_MSG
  BAR_INCLUDE_DIR
  BAR_LIBRARY)

mark_as_advanced(${BAR_INCLUDE_DIR} ${BAR_LIBRARY})

(and of course create the install rule for the bar-config.cmake)

  • Then, someone willing to use the bar library from an other project can do:
find_package(bar)

include_directories(${BAR_INCLUDE_DIRS})
add_executable(myexe ...)
target_link_libraries(myexe ${BAR_LIBRARIES})

This assumes that the person has installed the bar packaged somewhere CMake can find it. (For instance in /usr/local/share/cmake/bar-config.cmake), or that he sets -DBAR_DIR.

It the person also happens to have the foboar sources built somewhere, it cannot use them...

Neither libbar or bar.hpp can be found by CMake: bar.hpp is hidden somewhere in the sources of foobar, and libbar.a somewhere in the build directory of foobar, so it is impossible to use the carefully home-made bar-config.cmake, unless you install libbar to /usr/local/lib/libbar.a for instance.

qiBuild to the rescue!

The motivation for qiBuild is to help solve this CMake limitations with a clean, easy way, while staying the more compatible possible with other CMake projects.

Preparation

Add a call to find_package(qibuild) file at the root of the project and have it included right after the project() line.

cmake_minimum_required(VERSION 2.8)
project(foobar)
find_package(qibuild)

Note that you somehow have to find the qibuild-config.cmake find from you qibuild sources, if qibuild is not installed on your system.

You can do that by:

  • Using cmake -Dqibuild_DIR=/path/to/qibuild/cmake/qibuild,

or:

  • Create the qiproject.xml and use qibuild configure which will set the qibuild_DIR CMake variable for you.

Install rules

Replace the add_library by qi_create_lib, and remove the install rules to use qi_install_header instead:

include_directories(".")

qi_create_lib(bar
  bar/bar.hpp
  bar/bar.cpp)

qi_stage_lib(bar)

qi_install_header(bar/bar.hpp
  SUBFOLDER bar)

Using qi_create_lib and qi_install_header will have the following effects:

  • The install rules will been properly generated for the library
  • For the headers, you must choose a subfolder in which to put your headers. (otherwise, it is too easy to have conflicts, especially when you are generating a big SDK.) Unless you have a very good reason not to, please choose the same folder name to put you headers inside your source tree, and once your header is installed. (here, the bar argument of qi_install_header matches the location of bar.hpp: bar/bar.hpp).
  • A sdk directory will be created, with libbar in skd/lib

See also

Using the bar library

Add the following line in libbar/CMakeLists.txt:

qi_stage_lib(bar)

And replace code in foo/CMakeLists.txt to have

qi_use_lib(foo bar)

(no need to call include_directories or target_link_libraries anymore)

You should end up with

qi_create_bin(foo main.cpp)
qi_use_lib(foo bar)

So what happened?

Two versions of the foo-config.cmake file have been generated:

  • The first one is in build/cmake/sdk/bar-config.cmake : this one is supposed to be installed. You can see it is only using relative paths to find the library.
  • The second one is in build/sdk/cmake/bar-config.cmake : this one is supposed to be used inside your project: it contains absolute paths only.

So, since the layout in build/sdk is the same as the layout when the library is installed, and since the foo-config file has been automatically generated (along with the install rules), it makes no difference whether you want to find the bar library you have just built in the foobar project, using the bar library you have just built in a other project, or using the installed bar library.

Finding the bar-config.cmake in foobar/build/skd from an other project is as easy as:

list(APPEND CMAKE_FIND_ROOT_PATH "/path/to/foobar/build/sdk")

Finding the bar-config.cmake once bar has been installed in as easy as:

# No qiBuild required: the installed bar-config.cmake contains
# no qibuild-specific code:

find_package(bar)

include_directories(${BAR_INCLUDE_DIRS})
add_library(foo)
target_link_libraries(${BAR_LIBRARIES})

# Or, still using qibuild:
qi_use_lib(... bar)

Note

We always generate variables in the form <PREFIX>_INCLUDE_DIRS and <PREFIX>_LIBRARIES (all upper case, no version number, plural form)

Conclusion

This is what the final code looks like when you are done:

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(foobar)
find_package(qibuild)

add_subdirectory(libbar)
add_subdirectory(foo)

libbar/CMakeLists.txt

include_directories(".")

qi_create_lib(bar
  bar/bar.hpp
  bar/bar.cpp)

qi_stage_lib(bar)

qi_install_header(bar/bar.hpp
  SUBFOLDER bar)

foo/CMakeLists.txt

qi_create_bin(foo main.cpp)
qi_use_lib(foo bar)

Less code, so many features !

  • You have a nice layout in build/sdk
  • You can use the newly compiled bar library inside the foobar project, outside the foobar project, or using an installed foobar package with always the same line:
qi_use_lib(foo bar)
  • You did not have to write any install rule.
  • You did not have to write any bar-config.cmake.
  • You can build SDK packages for other people to use, even on Visual Studio, without handling all the annoying cross-platform stuff (for instance, on windows, the .dll must be generated next to the .exe otherwise the use has to set %PATH%, and so on...)
  • It is still pure, standard CMake code: you did not have to use the qibuild script.
  • Absolutely nothing has been generated in the source directory, build/sdk only contains the useful, re-distributable binaries (no .o here)

The final project can be found here: foobar_qibuild.zip

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