CMake Primer

Introduction

The LLVM project and many of the core projects built on LLVM build using CMake. This document aims to provide a brief overview of CMake for developers modifying LLVM projects or building their own projects on top of LLVM.

The official CMake language references is available in the cmake-language manpage and cmake-language online documentation.

10,000 ft View

CMake is a tool that reads script files in its own language that describe how a software project builds. As CMake evaluates the scripts it constructs an internal representation of the software project. Once the scripts have been fully processed, if there are no errors, CMake will generate build files to actually build the project. CMake supports generating build files for a variety of command line build tools as well as for popular IDEs.

When a user runs CMake it performs a variety of checks similar to how autoconf worked historically. During the checks and the evaluation of the build description scripts CMake caches values into the CMakeCache. This is useful because it allows the build system to skip long-running checks during incremental development. CMake caching also has some drawbacks, but that will be discussed later.

Scripting Overview

CMake's scripting language has a very simple grammar. Every language construct is a command that matches the pattern _name_(_args_). Commands come in three primary types: language-defined (commands implemented in C++ in CMake), defined functions, and defined macros. The CMake distribution also contains a suite of CMake modules that contain definitions for useful functionality.

The example below is the full CMake build for building a C++ "Hello World" program. The example uses only CMake language-defined functions.

cmake_minimum_required(VERSION 3.15)
project(HelloWorld)
add_executable(HelloWorld HelloWorld.cpp)

The CMake language provides control flow constructs in the form of foreach loops and if blocks. To make the example above more complicated you could add an if block to define "APPLE" when targeting Apple platforms:

cmake_minimum_required(VERSION 3.15)
project(HelloWorld)
add_executable(HelloWorld HelloWorld.cpp)
if(APPLE)
  target_compile_definitions(HelloWorld PUBLIC APPLE)
endif()

Variables, Types, and Scope

Dereferencing

In CMake variables are "stringly" typed. All variables are represented as strings throughout evaluation. Wrapping a variable in ${} dereferences it and results in a literal substitution of the name for the value. CMake refers to this as "variable evaluation" in their documentation. Dereferences are performed before the command being called receives the arguments. This means dereferencing a list results in multiple separate arguments being passed to the command.

Variable dereferences can be nested and be used to model complex data. For example:

set(var_name var1)
set(${var_name} foo) # same as "set(var1 foo)"
set(${${var_name}}_var bar) # same as "set(foo_var bar)"

Dereferencing an unset variable results in an empty expansion. It is a common pattern in CMake to conditionally set variables knowing that it will be used in code paths that the variable isn't set. There are examples of this throughout the LLVM CMake build system.

An example of variable empty expansion is:

if(APPLE)
  set(extra_sources Apple.cpp)
endif()
add_executable(HelloWorld HelloWorld.cpp ${extra_sources})

In this example the extra_sources variable is only defined if you're targeting an Apple platform. For all other targets the extra_sources will be evaluated as empty before add_executable is given its arguments.

Lists

In CMake lists are semi-colon delimited strings, and it is strongly advised that you avoid using semi-colons in lists; it doesn't go smoothly. A few examples of defining lists:

# Creates a list with members a, b, c, and d
set(my_list a b c d)
set(my_list "a;b;c;d")

# Creates a string "a b c d"
set(my_string "a b c d")

Lists of Lists

One of the more complicated patterns in CMake is lists of lists. Because a list cannot contain an element with a semi-colon to construct a list of lists you make a list of variable names that refer to other lists. For example:

set(list_of_lists a b c)
set(a 1 2 3)
set(b 4 5 6)
set(c 7 8 9)

With this layout you can iterate through the list of lists printing each value with the following code:

foreach(list_name IN LISTS list_of_lists)
  foreach(value IN LISTS ${list_name})
    message(${value})
  endforeach()
endforeach()

You'll notice that the inner foreach loop's list is doubly dereferenced. This is because the first dereference turns list_name into the name of the sub-list (a, b, or c in the example), then the second dereference is to get the value of the list.

This pattern is used throughout CMake, the most common example is the compiler flags options, which CMake refers to using the following variable expansions: CMAKE_${LANGUAGE}_FLAGS and CMAKE_${LANGUAGE}_FLAGS_${CMAKE_BUILD_TYPE}.

Other Types

Variables that are cached or specified on the command line can have types associated with them. The variable's type is used by CMake's UI tool to display the right input field. A variable's type generally doesn't impact evaluation, however CMake does have special handling for some variables such as PATH. You can read more about the special handling in CMake's set documentation.

Scope

CMake inherently has a directory-based scoping. Setting a variable in a CMakeLists file, will set the variable for that file, and all subdirectories. Variables set in a CMake module that is included in a CMakeLists file will be set in the scope they are included from, and all subdirectories.

When a variable that is already set is set again in a subdirectory it overrides the value in that scope and any deeper subdirectories.

The CMake set command provides two scope-related options. PARENT_SCOPE sets a variable into the parent scope, and not the current scope. The CACHE option sets the variable in the CMakeCache, which results in it being set in all scopes. The CACHE option will not set a variable that already exists in the CACHE unless the FORCE option is specified.

In addition to directory-based scope, CMake functions also have their own scope. This means variables set inside functions do not bleed into the parent scope. This is not true of macros, and it is for this reason LLVM prefers functions over macros whenever reasonable.

Control Flow

CMake features the same basic control flow constructs you would expect in any scripting language, but there are a few quirks because, as with everything in CMake, control flow constructs are commands.

If, ElseIf, Else

In general CMake if blocks work the way you'd expect:

if(<condition>)
  message("do stuff")
elseif(<condition>)
  message("do other stuff")
else()
  message("do other other stuff")
endif()

The single most important thing to know about CMake's if blocks coming from a C background is that they do not have their own scope. Variables set inside conditional blocks persist after the endif().

Loops

The most common form of the CMake foreach block is:

foreach(var ...)
  message("do stuff")
endforeach()

The variable argument portion of the foreach block can contain dereferenced lists, values to iterate, or a mix of both:

foreach(var foo bar baz)
  message(${var})
endforeach()
# prints:
#  foo
#  bar
#  baz

set(my_list 1 2 3)
foreach(var ${my_list})
  message(${var})
endforeach()
# prints:
#  1
#  2
#  3

foreach(var ${my_list} out_of_bounds)
  message(${var})
endforeach()
# prints:
#  1
#  2
#  3
#  out_of_bounds

There is also a more modern CMake foreach syntax. The code below is equivalent to the code above:

foreach(var IN ITEMS foo bar baz)
  message(${var})
endforeach()
# prints:
#  foo
#  bar
#  baz

set(my_list 1 2 3)
foreach(var IN LISTS my_list)
  message(${var})
endforeach()
# prints:
#  1
#  2
#  3

foreach(var IN LISTS my_list ITEMS out_of_bounds)
  message(${var})
endforeach()
# prints:
#  1
#  2
#  3
#  out_of_bounds

Similar to the conditional statements, these generally behave how you would expect, and they do not have their own scope.

CMake also supports while loops, although they are not widely used in LLVM.

Modules, Functions and Macros

Modules

Modules are CMake's vehicle for enabling code reuse. CMake modules are just CMake script files. They can contain code to execute on include as well as definitions for commands.

In CMake macros and functions are universally referred to as commands, and they are the primary method of defining code that can be called multiple times.

In LLVM we have several CMake modules that are included as part of our distribution for developers who don't build our project from source. Those modules are the fundamental pieces needed to build LLVM-based projects with CMake. We also rely on modules as a way of organizing the build system's functionality for maintainability and re-use within LLVM projects.

Argument Handling

When defining a CMake command handling arguments is very useful. The examples in this section will all use the CMake function block, but this all applies to the macro block as well.

CMake commands can have named arguments that are required at every call site. In addition, all commands will implicitly accept a variable number of extra arguments (In C parlance, all commands are varargs functions). When a command is invoked with extra arguments (beyond the named ones) CMake will store the full list of arguments (both named and unnamed) in a list named ARGV, and the sublist of unnamed arguments in ARGN. Below is a trivial example of providing a wrapper function for CMake's built in function add_dependencies.

function(add_deps target)
  add_dependencies(${target} ${ARGN})
endfunction()

This example defines a new macro named add_deps which takes a required first argument, and just calls another function passing through the first argument and all trailing arguments.

CMake provides a module CMakeParseArguments which provides an implementation of advanced argument parsing. We use this all over LLVM, and it is recommended for any function that has complex argument-based behaviors or optional arguments. CMake's official documentation for the module is in the cmake-modules manpage, and is also available at the cmake-modules online documentation.

Functions Vs Macros

Functions and Macros look very similar in how they are used, but there is one fundamental difference between the two. Functions have their own scope, and macros don't. This means variables set in macros will bleed out into the calling scope. That makes macros suitable for defining very small bits of functionality only.

The other difference between CMake functions and macros is how arguments are passed. Arguments to macros are not set as variables, instead dereferences to the parameters are resolved across the macro before executing it. This can result in some unexpected behavior if using unreferenced variables. For example:

macro(print_list my_list)
  foreach(var IN LISTS my_list)
    message("${var}")
  endforeach()
endmacro()

set(my_list a b c d)
set(my_list_of_numbers 1 2 3 4)
print_list(my_list_of_numbers)
# prints:
# a
# b
# c
# d

Generally speaking this issue is uncommon because it requires using non-dereferenced variables with names that overlap in the parent scope, but it is important to be aware of because it can lead to subtle bugs.

LLVM Project Wrappers

LLVM projects provide lots of wrappers around critical CMake built-in commands. We use these wrappers to provide consistent behaviors across LLVM components and to reduce code duplication.

We generally (but not always) follow the convention that commands prefaced with llvm_ are intended to be used only as building blocks for other commands. Wrapper commands that are intended for direct use are generally named following with the project in the middle of the command name (i.e. add_llvm_executable is the wrapper for add_executable). The LLVM add_* wrapper functions are all defined in AddLLVM.cmake which is installed as part of the LLVM distribution. It can be included and used by any LLVM sub-project that requires LLVM.

Useful Built-in Commands

CMake has a bunch of useful built-in commands. This document isn't going to go into details about them because The CMake project has excellent documentation. To highlight a few useful functions see:

• add_custom_command
• add_custom_target
• file
• list
• math
• string

The full documentation for CMake commands is in the cmake-commands manpage and available on CMake's website