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CMAKE_README.md

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How to use CMake

This document gives an overview of the basics of cmake and how to use it to make libraries and executables for pod-embedded.

Cmake basics

CMake projects are specified using a CMakeFiles.txt in each library/exectutable directory.

Basic CMakeFiles.txt layout

A top level CMakeFiles.txt always contains a cmake_minimum_required and a project declaration. The rest is populated with add_subdirectory, calls, which include CMakeFiles.txts from subdirectories.

Variables

CMake varaibles are specified using set(VARIABLE_NAME value...), which sets the variable VARIABLE_NAME to any subsequent values (multiple values will make a list.)

Eg:

#set importantDirectory to /usr/local/bin
set(importantDirectory /usr/local/bin) 

#set libraries to list containing middleware, drivers, utils
set(libraries middleware 
              drivers 
              utils)

The fundametnal primitives when defining a cmake project are executables and libraries.

Making a library

To create a library that is imported by other executables, we use the following

#Define the project for the library
project(foobarlib VERSION 1.0
                  DESCRIPTION "The foobar library"
                  LANGUAGES CXX)

#Define the library target called foobar with all of its source directories
#Files specified with respect to CMakeLists.txt for the library
add_library(foobar src/somefile.c
                   src/bin.c
                   src/baz.c)

#Add any header files by specifying the include directory, in this case 
#called include
target_include_directories(foobar PUBLIC include)

#add any library dependencies
target_link_libraries(foobar PUBLIC someotherlibrary verynicedependency)
target_link_libraries(foobar PRIVATE thisonelibrary)

#export library to cmake file so it can be imported elsewhere
export(TARGETS foobar FILE FoobarConfig.cmake)

In this example we define a project called foobarlib in its own CMakeLists.txt and define a library called foobar with its respective source files. Then any header files are added by specifying an include directory with target_include_directories(...). Then we link any other libraries foobar depends on with target_link_libraries(...). Finally we export the library using export(...) so the library can be imported elsewhere in the project.

PUBLIC vs PRIVATE vs INTERFACE

Whenever a dependency is specified for a project, you must specify if that dependency is PUBLIC, PRIVATE or INTERFACE. This changes the availability of the dependencies you're adding to any project that depends on the library you're writing.

For example, if library foo depends on library bar publically, then any library that depends on foo will be linked with headers from bar.

The rule of thumb is:

  • If your source files depend on the library, make it PRIVATE.
  • If your header files depend on the library, make it INTERFACE
  • If both of the above are true, make it PUBLIC

Dependencies for executables should always be PRIVATE.

Making an executable

To make an executable, simply use

add_executable(binaryName, sourceFile1.c
                           sourceFile2.c
                           sourceFileN.c)

and specify includes and dependencies with target_include_directories(...) and target_link_libraries(...) in the same way as with libraries.