How to Create Linkable Zephyr Libraries Using CMake

A library is a collection of functions and data that can be compiled in to another program, rather than a program itself. Libraries are useful for sharing code between multiple projects. However, most Zephyr code examples online show you how to create a Zephyr application, but not how to create a library that uses Zephyr, that can then be linked to other Zephyr applications.

Using the INTERFACE Library Type

I’ve had luck using the CMake INTERFACE library type to create libraries using Zephyr. An INTERFACE does not build a library artifact (e.g. static library file) but rather provides a collection of source files, header files, compile rules e.t.c that other projects can “link against” (which just means incorporate --- since there is nothing to link).

This shows an example of the libraries CMakeLists.txt file.

my_library/CMakeLists.txt

cmake_minimum_required(VERSION 3.20.0)

# This is used by the C/C++ extension in VSCode to provide intellisense for the project.
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
project(MyLibrary CXX)

# Create the library target
add_library(MyLibrary INTERFACE)

# Explicitly set the library language to C++
set_target_properties(MyLibrary PROPERTIES LINKER_LANGUAGE CXX)

# Add sources to the library
target_sources(
    MyLibrary
    INTERFACE
    file_1.cpp
    file_2.cpp
    file_3.cpp
)

# Link against Zephyr interface library to get include paths and other settings
target_link_libraries(MyLibrary INTERFACE zephyr_interface)

# Make the library headers available to other targets
target_include_directories(MyLibrary INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})

This can then be consumed by another Zephyr project. For example, let’s pretend we have a Zephyr application called my_app that we want to link to the MyLibrary library. We will use FetchContent to download the library from GitHub.

my_app/CMakeLists.txt

cmake_minimum_required(VERSION 3.20.0)
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
FetchContent_Declare(MyLibrary
    GIT_REPOSITORY https://github.com/gbmhunter/MyLibrary.git
    GIT_TAG 1.0.0 # This can be a hash, tag or branch.
)
FetchContent_MakeAvailable(MyLibrary)
# Add the subdirectory just downloaded by fetch content
add_subdirectory(${MyLibrary_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR}/MyLibrary)

# Now link the library to the main Zephyr app
target_link_libraries(app INTERFACE MyLibrary)

I couldn’t get INTERFACE to work natively with FetchContent. For some reason add_subdirectory() worked fine when adding my library, but if I added the exact same library using FetchContent, no valid targets were created. My workaround was to insert the line add_subdirectory() after the FetchContent call. An alternative would be to add the library as a Git submodule and then use add_subdirectory() directly (no FetchContent needed in this case).