[cmake] What's the CMake syntax to set and use variables?

When writing CMake scripts there is a lot you need to know about the syntax and how to use variables in CMake.

The Syntax

Strings using set():

• set(MyString "Some Text")
• set(MyStringWithVar "Some other Text: ${MyString}")
• set(MyStringWithQuot "Some quote: \"${MyStringWithVar}\"")

Or with string():

• string(APPEND MyStringWithContent " ${MyString}")

Lists using set():

• set(MyList "a" "b" "c")
• set(MyList ${MyList} "d")

Or better with list():

• list(APPEND MyList "a" "b" "c")
• list(APPEND MyList "d")

Lists of File Names:

• set(MySourcesList "File.name" "File with Space.name")
• list(APPEND MySourcesList "File.name" "File with Space.name")
• add_excutable(MyExeTarget ${MySourcesList})

The Documentation

• CMake/Language Syntax
• CMake: Variables Lists Strings
• CMake: Useful Variables
• CMake set() Command
• CMake string()Command
• CMake list() Command
• Cmake: Generator Expressions

The Scope or "What value does my variable have?"

First there are the "Normal Variables" and things you need to know about their scope:

• Normal variables are visible to the CMakeLists.txt they are set in and everything called from there (add_subdirectory(), include(), macro() and function()).
• The add_subdirectory() and function() commands are special, because they open-up their own scope.
  • Meaning variables set(...) there are only visible there and they make a copy of all normal variables of the scope level they are called from (called parent scope).
  • So if you are in a sub-directory or a function you can modify an already existing variable in the parent scope with set(... PARENT_SCOPE)
  • You can make use of this e.g. in functions by passing the variable name as a function parameter. An example would be function(xyz _resultVar) is setting set(${_resultVar} 1 PARENT_SCOPE)
• On the other hand everything you set in include() or macro() scripts will modify variables directly in the scope of where they are called from.

Second there is the "Global Variables Cache". Things you need to know about the Cache:

• If no normal variable with the given name is defined in the current scope, CMake will look for a matching Cache entry.
• Cache values are stored in the CMakeCache.txt file in your binary output directory.

• The values in the Cache can be modified in CMake's GUI application before they are generated. Therefore they - in comparison to normal variables - have a type and a docstring. I normally don't use the GUI so I use set(... CACHE INTERNAL "") to set my global and persistant values.

  Please note that the INTERNAL cache variable type does imply FORCE

• In a CMake script you can only change existing Cache entries if you use the set(... CACHE ... FORCE) syntax. This behavior is made use of e.g. by CMake itself, because it normally does not force Cache entries itself and therefore you can pre-define it with another value.

• You can use the command line to set entries in the Cache with the syntax cmake -D var:type=value, just cmake -D var=value or with cmake -C CMakeInitialCache.cmake.
• You can unset entries in the Cache with unset(... CACHE).

The Cache is global and you can set them virtually anywhere in your CMake scripts. But I would recommend you think twice about where to use Cache variables (they are global and they are persistant). I normally prefer the set_property(GLOBAL PROPERTY ...) and set_property(GLOBAL APPEND PROPERTY ...) syntax to define my own non-persistant global variables.

Variable Pitfalls and "How to debug variable changes?"

To avoid pitfalls you should know the following about variables:

• Local variables do hide cached variables if both have the same name
• The find_... commands - if successful - do write their results as cached variables "so that no call will search again"
• Lists in CMake are just strings with semicolons delimiters and therefore the quotation-marks are important
  • set(MyVar a b c) is "a;b;c" and set(MyVar "a b c") is "a b c"
  • The recommendation is that you always use quotation marks with the one exception when you want to give a list as list
  • Generally prefer the list() command for handling lists
• The whole scope issue described above. Especially it's recommended to use functions() instead of macros() because you don't want your local variables to show up in the parent scope.
• A lot of variables used by CMake are set with the project() and enable_language() calls. So it could get important to set some variables before those commands are used.
• Environment variables may differ from where CMake generated the make environment and when the the make files are put to use.
  • A change in an environment variable does not re-trigger the generation process.
  • Especially a generated IDE environment may differ from your command line, so it's recommended to transfer your environment variables into something that is cached.

Sometimes only debugging variables helps. The following may help you:

• Simply use old printf debugging style by using the message() command. There also some ready to use modules shipped with CMake itself: CMakePrintHelpers.cmake, CMakePrintSystemInformation.cmake
• Look into CMakeCache.txt file in your binary output directory. This file is even generated if the actual generation of your make environment fails.
• Use variable_watch() to see where your variables are read/written/removed.
• Look into the directory properties CACHE_VARIABLES and VARIABLES
• Call cmake --trace ... to see the CMake's complete parsing process. That's sort of the last reserve, because it generates a lot of output.

Special Syntax

• Environment Variables
  • You can can read $ENV{...} and write set(ENV{...} ...) environment variables
• Generator Expressions
  • Generator expressions $<...> are only evaluated when CMake's generator writes the make environment (it comparison to normal variables that are replaced "in-place" by the parser)
  • Very handy e.g. in compiler/linker command lines and in multi-configuration environments
• References
  • With ${${...}} you can give variable names in a variable and reference its content.
  • Often used when giving a variable name as function/macro parameter.
• Constant Values (see if() command)
  • With if(MyVariable) you can directly check a variable for true/false (no need here for the enclosing ${...})
  • True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number.
  • False if the constant is 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty string, or ends in the suffix -NOTFOUND.
  • This syntax is often use for something like if(MSVC), but it can be confusing for someone who does not know this syntax shortcut.
• Recursive substitutions
  • You can construct variable names using variables. After CMake has substituted the variables, it will check again if the result is a variable itself. This is very powerful feature used in CMake itself e.g. as sort of a template set(CMAKE_${lang}_COMPILER ...)
  • But be aware this can give you a headache in if() commands. Here is an example where CMAKE_CXX_COMPILER_ID is "MSVC" and MSVC is "1":
    • if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC") is true, because it evaluates to if("1" STREQUAL "1")
    • if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") is false, because it evaluates to if("MSVC" STREQUAL "1")
    • So the best solution here would be - see above - to directly check for if(MSVC)
  • The good news is that this was fixed in CMake 3.1 with the introduction of policy CMP0054. I would recommend to always set cmake_policy(SET CMP0054 NEW) to "only interpret if() arguments as variables or keywords when unquoted."
• The option() command
  • Mainly just cached strings that only can be ON or OFF and they allow some special handling like e.g. dependencies
  • But be aware, don't mistake the option with the set command. The value given to option is really only the "initial value" (transferred once to the cache during the first configuration step) and is afterwards meant to be changed by the user through CMake's GUI.

References

• How is CMake used?
• cmake, lost in the concept of global variables (and PARENT_SCOPE or add_subdirectory alternatives)
• Looping over a string list
• How to store CMake build settings
• CMake compare to empty string with STREQUAL failed
• When should I quote variables?