Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How little can I get away with in my CMakeLists?

Objectives

• Understand the deep implications of cmake_minimum_version

• Know how to set up a project

• Know how to make at least one target

Writing a CMakeLists

The following file is fine for the following examples:

/* simple.c or simple.cpp */
#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

This file can be compiled with C or C++.

Starting off

This is the simplest possible CMakeLists.txt:

cmake_minimum_required(VERSION 3.15)

project(MyProject)

add_executable(myexample simple.cpp)

Let’s look at the three lines:

1. The cmake_minimum_required command sets the policies so that the build is exactly like it would be on the listed version of CMake - in other words, CMake “dumbs itself down” to the version you request for any features that could produce a different build. This makes CMake almost perfectly backwards compatible.
2. You need to be working on a project, and it needs at least a name. CMake assumes a CXX (that’s C++) and C mixed project if you don’t give any LANGUAGES.
3. You need at least one library or executable to do anything interesting. The “thing” you make here is called a “target”, and the executable/library has the same name, by default, and it has to be unique in the project. You use add_executable for programs, and add_library for libraries.

Those commands have a few extra arguments that you can give:

cmake_minimum_required(VERSION 3.15...3.25)

project(MyProject
  VERSION
    1.0
  DESCRIPTION
    "Very nice project"
  LANGUAGES
    CXX
)

add_executable(myexample simple.cpp)

1. You can specify a range of versions - this will cause the policies to be set to the highest supported value in that range. As a general rule, set the highest version you’ve tested with here.
2. Projects can have versions, descriptions, and languages.
3. Whitespace doesn’t matter. Be clear/pretty, or use cmake-format.

git clone https://github.com/hsf-training/hsf-training-cmake-webpage.git
cd hsf-training-cmake-webpage/code/00-intro

# This is required in all CMakeLists Selecting a nice minimum version and range
cmake_minimum_required(VERSION 3.15...3.25)

# We can call the project anything we want Listing the language(s) avoids the C
# + CXX default
project(MyExample00 LANGUAGES C)

# We need an executable target
add_executable(simple_example simple.c)

More reading

• Based on Modern CMake basics

Key Points

• The cmake_minimum_version setting has deep implications

• You need a project line.

• You should prepare one or more targets to do anything interesting.