Policies/CMake Coding Style

This document describes the recommended coding style for CMake files in KDE, i.e. CMakeLists.txt files and *.cmake files.

General

To put in in one sentence: be as careful when writing the CMake files as when you are writing C++ code.

Indentation

Indent all code correctly, i.e. the body of

• if/else/endif
• foreach/endforeach
• while/endwhile
• macro/endmacro
• function/endfunction

Use spaces for indenting, 2, 3 or 4 spaces preferably. Use the same amount of spaces for indenting as is used in the rest of the file. Do not use tabs.

Upper/lower casing

Most important: use consistent upper- or lowercasing within one file !

In general, in KDE the all-lowercase style is preferred.

So, this is recommended:

add_executable(foo foo.c)

This is also acceptable:

ADD_EXECUTABLE(bar bar.c)

Mixed casing as shown below works too, but should not be done within KDE:

Add_Executable(hello hello.c)
aDd_ExEcUtAbLe(blub blub.c)

End commands

To make the code easier to read, use empty commands for endforeach(), endif(), endfunction(), endmacro() and endwhile(). Also, use empty else() commands.

For example, do this:

if(FOOVAR)
   some_command(...)
else()
   another_command(...)
endif()

and not this:

if(BARVAR)
   some_other_command(...)
endif(BARVAR)

Writing CMake Find-modules

(Not) Using pkg-config

You are free to use pkg-config in FindXXX.cmake modules, as long as the following conditions are met:

• the FindXXX.cmake must also work without pkg-config, as long as the package is either installed to one of the default locations (as /usr or /usr/local) or if CMAKE_PREFIX_PATH is set accordingly
• use only find_package(PkgConfig), don't use include(UsePkgConfig), this one is deprecated
• make sure the variables created by pkg_check_modules() are all prefixed with "PC_", so they don't mix up with other variables, e.g. set via find_path() etc.
• FindLibXml2.cmake as shipped with CMake 2.8.5 is a good example how pkg-config should be handled
• putting something like if(NOT WIN32) around the pkg-config stuff is not necessary (and should be removed if it is somewhere). If pkg-config is not found, e.g. on Windows, the macros simply do nothing.

Follow CMake's readme.txt

Follow the style guide from CMake when writing some FindFoo.cmake module: readme.txt

Use FindPackageHandleStandardArgs.cmake

For checking the results inside the Find-module, the macro find_package_handle_standard_args() (coming with CMake) should be used, using the new extended syntax, which supports also version checking.

Avoid Micro-Optimizations

Micro-optimizations like

if(FOO_LIBRARY AND FOO_INCLUDE_DIR)
   set(FOO_FOUND TRUE)
else()
   ... execute the whole find-logic
endif()

should be removed, the find-logic should be executed always. These shortcuts can cause problems e.g. when the same file is used from multiple directories but e.g. with different required versions or components etc.

Also manually quieting the module should not be done:

if ( SQLITE_INCLUDE_DIR AND SQLITE_LIBRARIES )
   set(Sqlite_FIND_QUIETLY TRUE)
endif ( SQLITE_INCLUDE_DIR AND SQLITE_LIBRARIES )

If find_package_handle_standard_args() is used, this is completely unnecessary, since find_package_handle_standard_args() only prints something if the result has changed compared to the previous run. So if, as in this example, sqlite has already been found, find_package_handle_standard_args() will only print this information during the first CMake run, but not on successive runs.

So, do not add such code. It is unnecessary and makes the modules look more complicated than necessary.

Writing FooConfig.cmake files

• See http://quickgit.kde.org/?p=kdeexamples.git&a=tree&hb=HEAD&f=buildsystem/HowToInstallALibrary for a fully commented example

• See also:
  • http://www.cmake.org/Wiki/CMake/Tutorials/Exporting_and_Importing_Targets
  • http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file
  • http://www.cmake.org/Wiki/CMake/Tutorials/Packaging