Variables and the Cache

Local Variables

We will cover variables first. A local variable is set like this:

set(MY_VARIABLE "value")

The names of variables are usually all caps, and the value follows. You access a variable by using ${}, such as ${MY_VARIABLE}.[^1] CMake has the concept of scope; you can access the value of the variable after you set it as long as you are in the same scope. If you leave a function or a file in a sub directory, the variable will no longer be defined. You can set a variable in the scope immediately above your current one with PARENT_SCOPE at the end.

Lists are simply a series of values when you set them:

set(MY_LIST "one" "two")

which internally become ; separated values. So this is an identical statement:

set(MY_LIST "one;two")

The list( command has utilities for working with lists, and separate_arguments will turn a space separated string into a list (inplace). Note that an unquoted value in CMake is the same as a quoted one if there are no spaces in it; this allows you to skip the quotes most of the time when working with value that you know could not contain spaces.

When a variable is expanded using ${} syntax, all the same rules about spaces apply. Be especially careful with paths; paths may contain a space at any time and should always be quoted when they are a variable (never write ${MY_PATH}, always should be "${MY_PATH}").

Cache Variables

If you want to set a variable from the command line, CMake offers a variable cache. Some variables are already here, like CMAKE_BUILD_TYPE. The syntax for declaring a variable and setting it if it is not already set is:

set(MY_CACHE_VARIABLE "VALUE" CACHE STRING "Description")

This will not replace an existing value. This is so that you can set these on the command line and not have them overridden when the CMake file executes. If you want to use these variables as a make-shift global variable, then you can do:

set(MY_CACHE_VARIABLE "VALUE" CACHE STRING "" FORCE)
mark_as_advanced(MY_CACHE_VARIABLE)

The first line will cause the value to be set no matter what, and the second line will keep the variable from showing up in the list of variables if you run cmake -L .. or use a GUI. This is so common, you can also use the INTERNAL type to do the same thing (though technically it forces the STRING type, this won’t affect any CMake code that depends on the variable):

set(MY_CACHE_VARIABLE "VALUE" CACHE INTERNAL "")

Since BOOL is such a common variable type, you can set it more succinctly with the shortcut:

 option(MY_OPTION "This is settable from the command line" OFF)

For the BOOL datatype, there are several different wordings for ON and OFF.

See cmake-variables for a listing of known variables in CMake.

Environment variables

You can also set(ENV{variable_name} value) and get $ENV{variable_name} environment variables, though it is generally a very good idea to avoid them.

The Cache

The cache is actually just a text file, CMakeCache.txt, that gets created in the build directory when you run CMake. This is how CMake remembers anything you set, so you don’t have to re-list your options every time you rerun CMake.

Properties

The other way CMake stores information is in properties. This is like a variable, but it is attached to some other item, like a directory or a target. A global property can be a useful uncached global variable. Many target properties are initialized from a matching variable with CMAKE_ at the front. So setting CMAKE_CXX_STANDARD, for example, will mean that all new targets created will have CXX_STANDARD set to that when they are created. There are two ways to set properties:

set_property(TARGET TargetName
             PROPERTY CXX_STANDARD 11)
set_target_properties(TargetName PROPERTIES
                      CXX_STANDARD 11)

The first form is more general, and can set multiple targets/files/tests at once, and has useful options. The second is a shortcut for setting several properties on one target. And you can get properties similarly:

get_property(ResultVariable TARGET TargetName PROPERTY CXX_STANDARD)

See cmake-properties for a listing of all known properties. You can also make your own in some cases.[^2]

[^1]: if statements are a bit odd in that they can take the variable with or without the surrounding syntax; this is there for historical reasons: if predates the ${} syntax. [^2]: Interface targets, for example, may have limits on custom properties that are allowed.