Build System

[中文]

This document explains the implementation of the ESP-IDF build system and the concept of “components”. Read this document if you want to know how to organize and build a new ESP-IDF project or component.

Overview

An ESP-IDF project can be seen as an amalgamation of a number of components. For example, for a webserver that shows the current humidity, there could be:

• The ESP-IDF base libraries (libc, ROM bindings, etc)

• The Wi-Fi drivers

• A TCP/IP stack

• The FreeRTOS operating system

• A webserver

• A driver for the humidity sensor

• Main code tying it all together

ESP-IDF makes these components explicit and configurable. To do that, when a project is compiled, the build system will look up all the components in the ESP-IDF directories, the project directories and (optionally) in additional custom component directories. It then allows the user to configure the ESP-IDF project using a text-based menu system to customize each component. After the components in the project are configured, the build system will compile the project.

Concepts

• A “project” is a directory that contains all the files and configuration to build a single “app” (executable), as well as additional supporting elements such as a partition table, data/filesystem partitions, and a bootloader.

• “Project configuration” is held in a single file called sdkconfig in the root directory of the project. This configuration file is modified via idf.py menuconfig to customise the configuration of the project. A single project contains exactly one project configuration.

• An “app” is an executable which is built by ESP-IDF. A single project will usually build two apps - a “project app” (the main executable, ie your custom firmware) and a “bootloader app” (the initial bootloader program which launches the project app).

• “components” are modular pieces of standalone code which are compiled into static libraries (.a files) and linked into an app. Some are provided by ESP-IDF itself, others may be sourced from other places.

• “Target” is the hardware for which an application is built. A full list of supported targets in your version if ESP-IDF can be seen by running idf.py –list-targets.

Some things are not part of the project:

• “ESP-IDF” is not part of the project. Instead it is standalone, and linked to the project via the IDF_PATH environment variable which holds the path of the esp-idf directory. This allows the IDF framework to be decoupled from your project.

• The toolchain for compilation is not part of the project. The toolchain should be installed in the system command line PATH.

Using the Build System

idf.py

The idf.py command-line tool provides a front-end for easily managing your project builds. It manages the following tools:

• CMake, which configures the project to be built

• A command-line build tool (either Ninja build or GNU Make)

• esptool.py for flashing the target.

The getting started guide contains a brief introduction to how to set up idf.py to configure, build, and flash projects.

idf.py should be run in an ESP-IDF “project” directory, i.e. one containing a CMakeLists.txt file. Older style projects with a Makefile will not work with idf.py.

Type idf.py --help for a list of commands. Here are a summary of the most useful ones:

• idf.py set-target <target> sets the target (chip) for which the project is built. See Selecting the Target.

• idf.py menuconfig runs the “menuconfig” tool to configure the project.

• idf.py build will build the project found in the current directory. This can involve multiple steps:

  • Create the build directory if needed. The sub-directory build is used to hold build output, although this can be changed with the -B option.

  • Run CMake as necessary to configure the project and generate build files for the main build tool.

  • Run the main build tool (Ninja or GNU Make). By default, the build tool is automatically detected but it can be explicitly set by passing the -G option to idf.py.

  Building is incremental so if no source files or configuration has changed since the last build, nothing will be done.

• idf.py clean will “clean” the project by deleting build output files from the build directory, forcing a “full rebuild” the next time the project is built. Cleaning doesn’t delete CMake configuration output and some other files.

• idf.py fullclean will delete the entire “build” directory contents. This includes all CMake configuration output. The next time the project is built, CMake will configure it from scratch. Note that this option recursively deletes all files in the build directory, so use with care. Project configuration is not deleted.

• idf.py flash will automatically build the project if necessary, and then flash it to the target. The -p and -b options can be used to set serial port name and flasher baud rate, respectively.

• idf.py monitor will display serial output from the target. The -p option can be used to set the serial port name. Type Ctrl-] to exit the monitor. See IDF Monitor for more details about using the monitor.

Multiple idf.py commands can be combined into one. For example, idf.py -p COM4 clean flash monitor will clean the source tree, then build the project and flash it to the target before running the serial monitor.

For commands that are not known to idf.py an attempt to execute them as a build system target will be made.

The command idf.py supports shell autocompletion for bash, zsh and fish shells.

In order to make shell autocompletion supported, please make sure you have at least Python 3.5 and click 7.1 or newer (see also).

To enable autocompletion for idf.py use the export command (see this). Autocompletion is initiated by pressing the TAB key. Type “idf.py -” and press the TAB key to autocomplete options.

The autocomplete support for PowerShell is planned in the future.

Note

The environment variables ESPPORT and ESPBAUD can be used to set default values for the -p and -b options, respectively. Providing these options on the command line overrides the default.

Advanced Commands

• idf.py app, idf.py bootloader, idf.py partition-table can be used to build only the app, bootloader, or partition table from the project as applicable.

• There are matching commands idf.py app-flash, etc. to flash only that single part of the project to the target.

• idf.py -p PORT erase-flash will use esptool.py to erase the target’s entire flash chip.

• idf.py size prints some size information about the app. size-components and size-files are similar commands which print more detailed per-component or per-source-file information, respectively. If you define variable -DOUTPUT_JSON=1 when running CMake (or idf.py), the output will be formatted as JSON not as human readable text. See idf.py-size for more information.

• idf.py reconfigure re-runs CMake even if it doesn’t seem to need re-running. This isn’t necessary during normal usage, but can be useful after adding/removing files from the source tree, or when modifying CMake cache variables. For example, idf.py -DNAME='VALUE' reconfigure can be used to set variable NAME in CMake cache to value VALUE.

• idf.py python-clean deletes generated Python byte code from the IDF directory which may cause issues when switching between IDF and Python versions. It is advised to run this target after switching versions of Python.

• idf.py docs will open direct link to documentation for project’s chip target and version in browser. To see all options use idf.py docs --help

The order of multiple idf.py commands on the same invocation is not important, they will automatically be executed in the correct order for everything to take effect (ie building before flashing, erasing before flashing, etc.).

idf.py options

To list all available root level options, run idf.py --help. To list options that are specific for a subcommand, run idf.py <command> --help, for example idf.py monitor --help. Here is a list of some useful options:

• -C <dir> allows overriding the project directory from the default current working directory.

• -B <dir> allows overriding the build directory from the default build subdirectory of the project directory.

• --ccache flag can be used to enable CCache when compiling source files, if the CCache tool is installed. This can dramatically reduce some build times.

Note that some older versions of CCache may exhibit bugs on some platforms, so if files are not rebuilt as expected then try disabling CCache and build again. CCache can be enabled by default by setting the IDF_CCACHE_ENABLE environment variable to a non-zero value.

• -v flag causes both idf.py and the build system to produce verbose build output. This can be useful for debugging build problems.

• --cmake-warn-uninitialized (or -w) will cause CMake to print uninitialized variable warnings inside the project directory (not for directories not found inside the project directory). This only controls CMake variable warnings inside CMake itself, not other types of build warnings. This option can also be set permanently by setting the IDF_CMAKE_WARN_UNINITIALIZED environment variable to a non-zero value.

Start a new project

Use the command idf.py create-project for starting a new project. Execute idf.py create-project --help for more information.

Example:

idf.py create-project --path my_projects my_new_project

This example will create a new project called my_new_project directly into the directory my_projects.

Using CMake Directly

idf.py is a wrapper around CMake for convenience. However, you can also invoke CMake directly if you prefer.

When idf.py does something, it prints each command that it runs for easy reference. For example, the idf.py build command is the same as running these commands in a bash shell (or similar commands for Windows Command Prompt):

mkdir -p build
cd build
cmake .. -G Ninja   # or 'Unix Makefiles'
ninja

In the above list, the cmake command configures the project and generates build files for use with the final build tool. In this case the final build tool is Ninja: running ninja actually builds the project.

It’s not necessary to run cmake more than once. After the first build, you only need to run ninja each time. ninja will automatically re-invoke cmake if the project needs reconfiguration.

If using CMake with ninja or make, there are also targets for more of the idf.py sub-commands - for example running make menuconfig or ninja menuconfig in the build directory will work the same as idf.py menuconfig.

Note

If you’re already familiar with CMake, you may find the ESP-IDF CMake-based build system unusual because it wraps a lot of CMake’s functionality to reduce boilerplate. See writing pure CMake components for some information about writing more “CMake style” components.

Flashing with ninja or make

It’s possible to build and flash directly from ninja or make by running a target like:

ninja flash

Or:

make app-flash

Available targets are: flash, app-flash (app only), bootloader-flash (bootloader only).

When flashing this way, optionally set the ESPPORT and ESPBAUD environment variables to specify the serial port and baud rate. You can set environment variables in your operating system or IDE project. Alternatively, set them directly on the command line:

ESPPORT=/dev/ttyUSB0 ninja flash

Note

Providing environment variables at the start of the command like this is Bash shell Syntax. It will work on Linux and macOS. It won’t work when using Windows Command Prompt, but it will work when using Bash-like shells on Windows.

Or:

make -j3 app-flash ESPPORT=COM4 ESPBAUD=2000000

Note

Providing variables at the end of the command line is make syntax, and works for make on all platforms.

Using CMake in an IDE

You can also use an IDE with CMake integration. The IDE will want to know the path to the project’s CMakeLists.txt file. IDEs with CMake integration often provide their own build tools (CMake calls these “generators”) to build the source files as part of the IDE.

When adding custom non-build steps like “flash” to the IDE, it is recommended to execute idf.py for these “special” commands.

For more detailed information about integrating ESP-IDF with CMake into an IDE, see Build System Metadata.

Setting up the Python Interpreter

ESP-IDF works well with all supported Python versions. It should work out-of-box even if you have a legacy system where the default python interpreter is still Python 2.7, however, it is advised to switch to Python 3 if possible.

idf.py and other Python scripts will run with the default Python interpreter, i.e. python. You can switch to a different one like python3 $IDF_PATH/tools/idf.py ..., or you can set up a shell alias or another script to simplify the command.

If using CMake directly, running cmake -D PYTHON=python3 ... will cause CMake to override the default Python interpreter.

If using an IDE with CMake, setting the PYTHON value as a CMake cache override in the IDE UI will override the default Python interpreter.

To manage the Python version more generally via the command line, check out the tools pyenv or virtualenv. These let you change the default Python version.

Possible issues

The user of idf.py may sometimes experience ImportError described below.

Traceback (most recent call last):
  File "/Users/user_name/e/esp-idf/tools/kconfig_new/confgen.py", line 27, in <module>
    import kconfiglib
ImportError: bad magic number in 'kconfiglib': b'\x03\xf3\r\n'

The exception is often caused by .pyc files generated by different Python versions. To solve the issue run the following command:

idf.py python-clean

Example Project

An example project directory tree might look like this:

- myProject/
             - CMakeLists.txt
             - sdkconfig
             - components/ - component1/ - CMakeLists.txt
                                         - Kconfig
                                         - src1.c
                           - component2/ - CMakeLists.txt
                                         - Kconfig
                                         - src1.c
                                         - include/ - component2.h
             - main/       - CMakeLists.txt
                           - src1.c
                           - src2.c

             - build/

This example “myProject” contains the following elements:

• A top-level project CMakeLists.txt file. This is the primary file which CMake uses to learn how to build the project; and may set project-wide CMake variables. It includes the file /tools/cmake/project.cmake which implements the rest of the build system. Finally, it sets the project name and defines the project.

• “sdkconfig” project configuration file. This file is created/updated when idf.py menuconfig runs, and holds configuration for all of the components in the project (including ESP-IDF itself). The “sdkconfig” file may or may not be added to the source control system of the project.

• Optional “components” directory contains components that are part of the project. A project does not have to contain custom components of this kind, but it can be useful for structuring reusable code or including third party components that aren’t part of ESP-IDF. Alternatively, EXTRA_COMPONENT_DIRS can be set in the top-level CMakeLists.txt to look for components in other places. See the renaming main section for more info. If you have a lot of source files in your project, we recommend grouping most into components instead of putting them all in “main”.

• “main” directory is a special component that contains source code for the project itself. “main” is a default name, the CMake variable COMPONENT_DIRS includes this component but you can modify this variable.

• “build” directory is where build output is created. This directory is created by idf.py if it doesn’t already exist. CMake configures the project and generates interim build files in this directory. Then, after the main build process is run, this directory will also contain interim object files and libraries as well as final binary output files. This directory is usually not added to source control or distributed with the project source code.

Component directories each contain a component CMakeLists.txt file. This file contains variable definitions to control the build process of the component, and its integration into the overall project. See Component CMakeLists Files for more details.

Each component may also include a Kconfig file defining the component configuration options that can be set via menuconfig. Some components may also include Kconfig.projbuild and project_include.cmake files, which are special files for overriding parts of the project.

Project CMakeLists File

Each project has a single top-level CMakeLists.txt file that contains build settings for the entire project. By default, the project CMakeLists can be quite minimal.

Minimal Example CMakeLists

Minimal project:

cmake_minimum_required(VERSION 3.5)
include($ENV{IDF_PATH}/tools/cmake/project.cmake)
project(myProject)

Mandatory Parts

The inclusion of these three lines, in the order shown above, is necessary for every project:

• cmake_minimum_required(VERSION 3.5) tells CMake the minimum version that is required to build the project. ESP-IDF is designed to work with CMake 3.5 or newer. This line must be the first line in the CMakeLists.txt file.

• include($ENV{IDF_PATH}/tools/cmake/project.cmake) pulls in the rest of the CMake functionality to configure the project, discover all the components, etc.

• project(myProject) creates the project itself, and specifies the project name. The project name is used for the final binary output files of the app - ie myProject.elf, myProject.bin. Only one project can be defined per CMakeLists file.

Optional Project Variables

These variables all have default values that can be overridden for custom behaviour. Look in /tools/cmake/project.cmake for all of the implementation details.

• COMPONENT_DIRS: Directories to search for components. Defaults to IDF_PATH/components, PROJECT_DIR/components, and EXTRA_COMPONENT_DIRS. Override this variable if you don’t want to search for components in these places.

• EXTRA_COMPONENT_DIRS: Optional list of additional directories to search for components. Paths can be relative to the project directory, or absolute.

• COMPONENTS: A list of component names to build into the project. Defaults to all components found in the COMPONENT_DIRS directories. Use this variable to “trim down” the project for faster build times. Note that any component which “requires” another component via the REQUIRES or PRIV_REQUIRES arguments on component registration will automatically have it added to this list, so the COMPONENTS list can be very short.

Any paths in these variables can be absolute paths, or set relative to the project directory.

To set these variables, use the cmake set command ie set(VARIABLE "VALUE"). The set() commands should be placed after the cmake_minimum(...) line but before the include(...) line.

Renaming main component

The build system provides special treatment to the main component. It is a component that gets automatically added to the build provided that it is in the expected location, PROJECT_DIR/main. All other components in the build are also added as its dependencies, saving the user from hunting down dependencies and providing a build that works right out of the box. Renaming the main component causes the loss of these behind-the-scenes heavy lifting, requiring the user to specify the location of the newly renamed component and manually specifying its dependencies. Specifically, the steps to renaming main are as follows:

1. Rename main directory.

2. Set EXTRA_COMPONENT_DIRS in the project CMakeLists.txt to include the renamed main directory.

3. Specify the dependencies in the renamed component’s CMakeLists.txt file via REQUIRES or PRIV_REQUIRES arguments on component registration.

Overriding default build specifications

The build sets some global build specifications (compile flags, definitions, etc.) that gets used in compiling all sources from all components.

For example, one of the default build specifications set is the compile option -Wextra. Suppose a user wants to use override this with -Wno-extra, it should be done after project():

cmake_minimum_required(VERSION 3.5)
include($ENV{IDF_PATH}/tools/cmake/project.cmake)
project(myProject)

idf_build_set_property(COMPILE_OPTIONS "-Wno-error" APPEND)

This ensures that the compile options set by the user won’t be overriden by the default build specifications, since the latter are set inside project().

Component CMakeLists Files

Each project contains one or more components. Components can be part of ESP-IDF, part of the project’s own components directory, or added from custom component directories (see above).

A component is any directory in the COMPONENT_DIRS list which contains a CMakeLists.txt file.

Searching for Components

The list of directories in COMPONENT_DIRS is searched for the project’s components. Directories in this list can either be components themselves (ie they contain a CMakeLists.txt file), or they can be top-level directories whose sub-directories are components.

When CMake runs to configure the project, it logs the components included in the build. This list can be useful for debugging the inclusion/exclusion of certain components.

Multiple components with the same name

When ESP-IDF is collecting all the components to compile, it will do this in the order specified by COMPONENT_DIRS; by default, this means ESP-IDF’s internal components first (IDF_PATH/components), then any components in directories specified in EXTRA_COMPONENT_DIRS, and finally the project’s components (PROJECT_DIR/components). If two or more of these directories contain component sub-directories with the same name, the component in the last place searched is used. This allows, for example, overriding ESP-IDF components with a modified version by copying that component from the ESP-IDF components directory to the project components directory and then modifying it there. If used in this way, the ESP-IDF directory itself can remain untouched.

Note

If a component is overridden in an existing project by moving it to a new location, the project will not automatically see the new component path. Run idf.py reconfigure (or delete the project build folder) and then build again.

Minimal Component CMakeLists

The minimal component CMakeLists.txt file simply registers the component to the build system using idf_component_register:

idf_component_register(SRCS "foo.c" "bar.c"
                       INCLUDE_DIRS "include"
                       REQUIRES mbedtls)

• SRCS is a list of source files (*.c, *.cpp, *.cc, *.S). These source files will be compiled into the component library.

• INCLUDE_DIRS is a list of directories to add to the global include search path for any component which requires this component, and also the main source files.

• REQUIRES is not actually required, but it is very often required to declare what other components this component will use. See Component Requirements.

A library with the name of the component will be built and linked into the final app.

Directories are usually specified relative to the CMakeLists.txt file itself, although they can be absolute.

There are other arguments that can be passed to idf_component_register. These arguments are discussed here.

See example component requirements and example component CMakeLists for more complete component CMakeLists.txt examples.

Create a new component

Use the command idf.py create-component for creating a new component. The new component will contain set of files necessary for building a component. You may include the component’s header file into your project and use its functionality. For more information execute idf.py create-component --help.

Example:

idf.py -C components create-component my_component

The example will create a new component in the subdirectory components under the current working directory. For more information about components follow the documentation page see above.

Preset Component Variables

The following component-specific variables are available for use inside component CMakeLists, but should not be modified:

• COMPONENT_DIR: The component directory. Evaluates to the absolute path of the directory containing CMakeLists.txt. The component path cannot contain spaces. This is the same as the CMAKE_CURRENT_SOURCE_DIR variable.

• COMPONENT_NAME: Name of the component. Same as the name of the component directory.

• COMPONENT_ALIAS: Alias of the library created internally by the build system for the component.

• COMPONENT_LIB: Name of the library created internally by the build system for the component.

The following variables are set at the project level, but available for use in component CMakeLists:

• CONFIG_*: Each value in the project configuration has a corresponding variable available in cmake. All names begin with CONFIG_. More information here.

• ESP_PLATFORM: Set to 1 when the CMake file is processed within ESP-IDF build system.

Build/Project Variables

The following are some project/build variables that are available as build properties and whose values can be queried using idf_build_get_property from the component CMakeLists.txt:

• PROJECT_NAME: Name of the project, as set in project CMakeLists.txt file.

• PROJECT_DIR: Absolute path of the project directory containing the project CMakeLists. Same as the CMAKE_SOURCE_DIR variable.

• COMPONENTS: Names of all components that are included in this build, formatted as a semicolon-delimited CMake list.

• IDF_VER: Git version of ESP-IDF (produced by git describe)

• IDF_VERSION_MAJOR, IDF_VERSION_MINOR, IDF_VERSION_PATCH: Components of ESP-IDF version, to be used in conditional expressions. Note that this information is less precise than that provided by IDF_VER variable. v4.0-dev-*, v4.0-beta1, v4.0-rc1 and v4.0 will all have the same values of IDF_VERSION_* variables, but different IDF_VER values.

• IDF_TARGET: Name of the target for which the project is being built.

• PROJECT_VER: Project version.

  • If CONFIG_APP_PROJECT_VER_FROM_CONFIG option is set, the value of CONFIG_APP_PROJECT_VER will be used.

  • Else, if PROJECT_VER variable is set in project CMakeLists.txt file, its value will be used.

  • Else, if the PROJECT_DIR/version.txt exists, its contents will be used as PROJECT_VER.

  • Else, if the project is located inside a Git repository, the output of git describe will be used.

  • Otherwise, PROJECT_VER will be “1”.

Other build properties are listed here.

Controlling Component Compilation

To pass compiler options when compiling source files belonging to a particular component, use the target_compile_options function:

target_compile_options(${COMPONENT_LIB} PRIVATE -Wno-unused-variable)

To apply the compilation flags to a single source file, use the CMake set_source_files_properties command:

set_source_files_properties(mysrc.c
    PROPERTIES COMPILE_FLAGS
    -Wno-unused-variable
)

This can be useful if there is upstream code that emits warnings.

When using these commands, place them after the call to idf_component_register in the component CMakeLists file.

Component Configuration

Each component can also have a Kconfig file, alongside CMakeLists.txt. This contains configuration settings to add to the configuration menu for this component.

These settings are found under the “Component Settings” menu when menuconfig is run.

To create a component Kconfig file, it is easiest to start with one of the Kconfig files distributed with ESP-IDF.

For an example, see Adding conditional configuration.

Preprocessor Definitions

The ESP-IDF build system adds the following C preprocessor definitions on the command line:

• ESP_PLATFORM : Can be used to detect that build happens within ESP-IDF.

• IDF_VER : Defined to a git version string. E.g. v2.0 for a tagged release or v1.0-275-g0efaa4f for an arbitrary commit.

Component Requirements

When compiling each component, the ESP-IDF build system recursively evaluates its dependencies. This means each component needs to declare the components that it depends on (“requires”).

When writing a component

idf_component_register(...
                       REQUIRES mbedtls
                       PRIV_REQUIRES console spiffs)

• REQUIRES should be set to all components whose header files are #included from the public header files of this component.

• PRIV_REQUIRES should be set to all components whose header files are #included from any source files in this component, unless already listed in REQUIRES. Also any component which is required to be linked in order for this component to function correctly.

• The values of REQUIRES and PRIV_REQUIRES should not depend on any configuration choices (CONFIG_xxx macros). This is because requirements are expanded before configuration is loaded. Other component variables (like include paths or source files) can depend on configuration choices.

• Not setting either or both REQUIRES variables is fine. If the component has no requirements except for the Common component requirements needed for RTOS, libc, etc.

If a components only supports some target chips (values of IDF_TARGET) then it can specify REQUIRED_IDF_TARGETS in the idf_component_register call to express these requirements. In this case the build system will generate an error if the component is included into the build, but does not support the selected target.

Note

In CMake terms, REQUIRES & PRIV_REQUIRES are approximate wrappers around the CMake functions target_link_libraries(... PUBLIC ...) and target_link_libraries(... PRIVATE ...).

Example of component requirements

Imagine there is a car component, which uses the engine component, which uses the spark_plug component:

- autoProject/
             - CMakeLists.txt
             - components/ - car/ - CMakeLists.txt
                                     - car.c
                                     - car.h
                           - engine/ - CMakeLists.txt
                                     - engine.c
                                     - include/ - engine.h
                           - spark_plug/  - CMakeLists.txt
                                          - plug.c
                                          - plug.h

Car component

The car.h header file is the public interface for the car component. This header includes engine.h directly because it uses some declarations from this header:

/* car.h */
#include "engine.h"

#ifdef ENGINE_IS_HYBRID
#define CAR_MODEL "Hybrid"
#endif

And car.c includes car.h as well:

/* car.c */
#include "car.h"

This means the car/CMakeLists.txt file needs to declare that car requires engine:

idf_component_register(SRCS "car.c"
                  INCLUDE_DIRS "."
                  REQUIRES engine)

• SRCS gives the list of source files in the car component.

• INCLUDE_DIRS gives the list of public include directories for this component. Because the public interface is car.h, the directory containing car.h is listed here.

• REQUIRES gives the list of components required by the public interface of this component. Because car.h is a public header and includes a header from engine, we include engine here. This makes sure that any other component which includes car.h will be able to recursively include the required engine.h also.

Engine component

The engine component also has a public header file include/engine.h, but this header is simpler:

/* engine.h */
#define ENGINE_IS_HYBRID

void engine_start(void);

The implementation is in engine.c:

/* engine.c */
#include "engine.h"
#include "spark_plug.h"

...

In this component, engine depends on spark_plug but this is a private dependency. spark_plug.h is needed to compile engine.c, but not needed to include engine.h.

This means that the engine/CMakeLists.txt file can use PRIV_REQUIRES:

idf_component_register(SRCS "engine.c"
                  INCLUDE_DIRS "include"
                  PRIV_REQUIRES spark_plug)

As a result, source files in the car component don’t need the spark_plug include directories added to their compiler search path. This can speed up compilation, and stops compiler command lines from becoming longer than necessary.

Spark Plug Component

The spark_plug component doesn’t depend on anything else. It has a public header file spark_plug.h, but this doesn’t include headers from any other components.

This means that the spark_plug/CMakeLists.txt file doesn’t need any REQUIRES or PRIV_REQUIRES clauses:

idf_component_register(SRCS "spark_plug.c"
                  INCLUDE_DIRS ".")

Source File Include Directories

Each component’s source file is compiled with these include path directories, as specified in the passed arguments to idf_component_register:

idf_component_register(..
                       INCLUDE_DIRS "include"
                       PRIV_INCLUDE_DIRS "other")

• The current component’s INCLUDE_DIRS and PRIV_INCLUDE_DIRS.

• The INCLUDE_DIRS belonging to all other components listed in the REQUIRES and PRIV_REQUIRES parameters (ie all the current component’s public and private dependencies).

• Recursively, all of the INCLUDE_DIRS of those components REQUIRES lists (ie all public dependencies of this component’s dependencies, recursively expanded).

Main component requirements

The component named main is special because it automatically requires all other components in the build. So it’s not necessary to pass REQUIRES or PRIV_REQUIRES to this component. See renaming main for a description of what needs to be changed if no longer using the main component.

Common component requirements

To avoid duplication, every component automatically requires some “common” IDF components even if they are not mentioned explicitly. Headers from these components can always be included.

The list of common components is: cxx, newlib, freertos, esp_hw_support, heap, log, lwip, soc, hal, esp_rom, esp_common, esp_system.

Including components in the build

• By default, every component is included in the build.

• If you set the COMPONENTS variable to a minimal list of components used directly by your project, then the build will expand to also include required components. The full list of components will be:

  • Components mentioned explicitly in COMPONENTS.

  • Those components’ requirements (evaluated recursively).

  • The “common” components that every component depends on.

• Setting COMPONENTS to the minimal list of required components can significantly reduce compile times.

Circular Dependencies

It’s possible for a project to contain Component A that requires (REQUIRES or PRIV_REQUIRES) Component B, and Component B that requires Component A. This is known as a dependency cycle or a circular dependency.

CMake will usually handle circular dependencies automatically by repeating the component library names twice on the linker command line. However this strategy doesn’t always work, and it’s possible the build will fail with a linker error about “Undefined reference to …”, referencing a symbol defined by one of the components inside the circular dependency. This is particularly likely if there is a large circular dependency, i.e. A->B->C->D->A.

The best solution is to restructure the components to remove the circular dependency. In most cases, a software architecture without circular dependencies has desirable properties of modularity and clean layering and will be more maintainable in the long term. However, removing circular dependencies is not always possible.

To bypass a linker error caused by a circular dependency, the simplest workaround is to increase the CMake LINK_INTERFACE_MULTIPLICITY property of one of the component libraries. This causes CMake to repeat this library and its dependencies more than two times on the linker command line.

For example:

set_property(TARGET ${COMPONENT_LIB} APPEND PROPERTY LINK_INTERFACE_MULTIPLICITY 3)

• This line should be placed after idf_component_register in the component CMakeLists.txt file.

• If possible, place this line in the component that creates the circular dependency by depending on a lot of other components. However, the line can be placed inside any component that is part of the cycle. Choosing the component that owns the source file shown in the linker error message, or the component that defines the symbol(s) mentioned in the linker error message, is a good place to start.

• Usually increasing the value to 3 (default is 2) is enough, but if this doesn’t work then try increasing the number further.

• Adding this option will make the linker command line longer, and the linking stage slower.

Advanced Workaround: Undefined Symbols

If only one or two symbols is causing a circular dependency, and all other dependencies are linear, then there is an alternative method to avoid linker errors: Specify the specific symbols required for the “reverse” dependency as undefined symbols at link time.

For example, if component A depends on component B but component B also needs to reference reverse_ops from component A (but nothing else), then you can add a line like the following to the component B CMakeLists.txt to resolve the cycle at link time:

# This symbol is provided by 'Component A' at link time
target_link_libraries(${COMPONENT_LIB} INTERFACE "-u reverse_ops")

• The -u argument means that the linker will always include this symbol in the link, regardless of dependency ordering.

• This line should be placed after idf_component_register in the component CMakeLists.txt file.

• If ‘Component B’ doesn’t need to access any headers of ‘Component A’, only link to a few symbol(s), then this line can be used instead of any REQUIRES from B to A. This further simplifies the component structure in the build system.

See the target_link_libraries documentation for more information about this CMake function.

Requirements in the build system implementation

• Very early in the CMake configuration process, the script expand_requirements.cmake is run. This script does a partial evaluation of all component CMakeLists.txt files and builds a graph of component requirements (this graph may have cycles). The graph is used to generate a file component_depends.cmake in the build directory.

• The main CMake process then includes this file and uses it to determine the list of components to include in the build (internal BUILD_COMPONENTS variable). The BUILD_COMPONENTS variable is sorted so dependencies are listed first, however as the component dependency graph has cycles this cannot be guaranteed for all components. The order should be deterministic given the same set of components and component dependencies.

• The value of BUILD_COMPONENTS is logged by CMake as “Component names: “

• Configuration is then evaluated for the components included in the build.

• Each component is included in the build normally and the CMakeLists.txt file is evaluated again to add the component libraries to the build.

Component Dependency Order

The order of components in the BUILD_COMPONENTS variable determines other orderings during the build:

• Order that project_include.cmake files are included into the project.

• Order that the list of header paths is generated for compilation (via -I argument). (Note that for a given component’s source files, only that component’s dependency’s header paths are passed to the compiler.)

Overriding Parts of the Project

project_include.cmake

For components that have build requirements which must be evaluated before any component CMakeLists files are evaluated, you can create a file called project_include.cmake in the component directory. This CMake file is included when project.cmake is evaluating the entire project.

project_include.cmake files are used inside ESP-IDF, for defining project-wide build features such as esptool.py command line arguments and the bootloader “special app”.

Unlike component CMakeLists.txt files, when including a project_include.cmake file the current source directory (CMAKE_CURRENT_SOURCE_DIR and working directory) is the project directory. Use the variable COMPONENT_DIR for the absolute directory of the component.

Note that project_include.cmake isn’t necessary for the most common component uses - such as adding include directories to the project, or LDFLAGS to the final linking step. These values can be customised via the CMakeLists.txt file itself. See Optional Project Variables for details.

project_include.cmake files are included in the order given in BUILD_COMPONENTS variable (as logged by CMake). This means that a component’s project_include.cmake file will be included after it’s all dependencies’ project_include.cmake files, unless both components are part of a dependency cycle. This is important if a project_include.cmake file relies on variables set by another component. See also above.

Take great care when setting variables or targets in a project_include.cmake file. As the values are included into the top-level project CMake pass, they can influence or break functionality across all components!

KConfig.projbuild

This is an equivalent to project_include.cmake for Component Configuration KConfig files. If you want to include configuration options at the top-level of menuconfig, rather than inside the “Component Configuration” sub-menu, then these can be defined in the KConfig.projbuild file alongside the CMakeLists.txt file.

Take care when adding configuration values in this file, as they will be included across the entire project configuration. Where possible, it’s generally better to create a KConfig file for Component Configuration.

project_include.cmake files are used inside ESP-IDF, for defining project-wide build features such as esptool.py command line arguments and the bootloader “special app”.

Configuration-Only Components

Special components which contain no source files, only Kconfig.projbuild and KConfig, can have a one-line CMakeLists.txt file which calls the function idf_component_register() with no arguments specified. This function will include the component in the project build, but no library will be built and no header files will be added to any include paths.

Debugging CMake

For full details about CMake and CMake commands, see the CMake v3.5 documentation.

Some tips for debugging the ESP-IDF CMake-based build system:

• When CMake runs, it prints quite a lot of diagnostic information including lists of components and component paths.

• Running cmake -DDEBUG=1 will produce more verbose diagnostic output from the IDF build system.

• Running cmake with the --trace or --trace-expand options will give a lot of information about control flow. See the cmake command line documentation.

When included from a project CMakeLists file, the project.cmake file defines some utility modules and global variables and then sets IDF_PATH if it was not set in the system environment.

It also defines an overridden custom version of the built-in CMake project function. This function is overridden to add all of the ESP-IDF specific project functionality.

Warning On Undefined Variables

By default, idf.py passes the --warn-uninitialized flag to CMake so it will print a warning if an undefined variable is referenced in the build. This can be very useful to find buggy CMake files.

If you don’t want this behaviour, it can be disabled by passing --no-warnings to idf.py.

Browse the /tools/cmake/project.cmake file and supporting functions in /tools/cmake/ for more details.

Example Component CMakeLists

Because the build environment tries to set reasonable defaults that will work most of the time, component CMakeLists.txt can be very small or even empty (see Minimal Component CMakeLists). However, overriding component variables is usually required for some functionality.

Here are some more advanced examples of component CMakeLists files.

Adding conditional configuration

The configuration system can be used to conditionally compile some files depending on the options selected in the project configuration.

Kconfig:

config FOO_ENABLE_BAR
    bool "Enable the BAR feature."
    help
        This enables the BAR feature of the FOO component.

CMakeLists.txt:

 set(srcs "foo.c" "more_foo.c")

 if(CONFIG_FOO_ENABLE_BAR)
     list(APPEND srcs "bar.c")
 endif()

idf_component_register(SRCS "${srcs}"
                     ...)

This example makes use of the CMake if function and list APPEND function.

This can also be used to select or stub out an implementation, as such:

Kconfig:

config ENABLE_LCD_OUTPUT
    bool "Enable LCD output."
    help
        Select this if your board has a LCD.

config ENABLE_LCD_CONSOLE
    bool "Output console text to LCD"
    depends on ENABLE_LCD_OUTPUT
    help
        Select this to output debugging output to the lcd

config ENABLE_LCD_PLOT
    bool "Output temperature plots to LCD"
    depends on ENABLE_LCD_OUTPUT
    help
        Select this to output temperature plots

CMakeLists.txt:

if(CONFIG_ENABLE_LCD_OUTPUT)
   set(srcs lcd-real.c lcd-spi.c)
else()
   set(srcs lcd-dummy.c)
endif()

# We need font if either console or plot is enabled
if(CONFIG_ENABLE_LCD_CONSOLE OR CONFIG_ENABLE_LCD_PLOT)
   list(APPEND srcs "font.c")
endif()

idf_component_register(SRCS "${srcs}"
                    ...)

Conditions which depend on the target

The current target is available to CMake files via IDF_TARGET variable.

In addition to that, if target xyz is used (IDF_TARGET=xyz), then Kconfig variable CONFIG_IDF_TARGET_XYZ will be set.

Note that component dependencies may depend on IDF_TARGET variable, but not on Kconfig variables. Also one can not use Kconfig variables in include statements in CMake files, but IDF_TARGET can be used in such context.

Source Code Generation

Some components will have a situation where a source file isn’t supplied with the component itself but has to be generated from another file. Say our component has a header file that consists of the converted binary data of a BMP file, converted using a hypothetical tool called bmp2h. The header file is then included in as C source file called graphics_lib.c:

add_custom_command(OUTPUT logo.h
     COMMAND bmp2h -i ${COMPONENT_DIR}/logo.bmp -o log.h
     DEPENDS ${COMPONENT_DIR}/logo.bmp
     VERBATIM)

add_custom_target(logo DEPENDS logo.h)
add_dependencies(${COMPONENT_LIB} logo)

set_property(DIRECTORY "${COMPONENT_DIR}" APPEND PROPERTY
     ADDITIONAL_MAKE_CLEAN_FILES logo.h)

This answer is adapted from the CMake FAQ entry, which contains some other examples that will also work with ESP-IDF builds.

In this example, logo.h will be generated in the current directory (the build directory) while logo.bmp comes with the component and resides under the component path. Because logo.h is a generated file, it should be cleaned when the project is cleaned. For this reason it is added to the ADDITIONAL_MAKE_CLEAN_FILES property.

Note

If generating files as part of the project CMakeLists.txt file, not a component CMakeLists.txt, then use build property PROJECT_DIR instead of ${COMPONENT_DIR} and ${PROJECT_NAME}.elf instead of ${COMPONENT_LIB}.)

If a a source file from another component included logo.h, then add_dependencies would need to be called to add a dependency between the two components, to ensure that the component source files were always compiled in the correct order.

Embedding Binary Data

Sometimes you have a file with some binary or text data that you’d like to make available to your component - but you don’t want to reformat the file as C source.

You can specify argument EMBED_FILES in the component registration, giving space-delimited names of the files to embed:

idf_component_register(...
                       EMBED_FILES server_root_cert.der)

Or if the file is a string, you can use the variable EMBED_TXTFILES. This will embed the contents of the text file as a null-terminated string:

idf_component_register(...
                       EMBED_TXTFILES server_root_cert.pem)

The file’s contents will be added to the .rodata section in flash, and are available via symbol names as follows:

extern const uint8_t server_root_cert_pem_start[] asm("_binary_server_root_cert_pem_start");
extern const uint8_t server_root_cert_pem_end[]   asm("_binary_server_root_cert_pem_end");

The names are generated from the full name of the file, as given in EMBED_FILES. Characters /, ., etc. are replaced with underscores. The _binary prefix in the symbol name is added by objcopy and is the same for both text and binary files.

To embed a file into a project, rather than a component, you can call the function target_add_binary_data like this:

target_add_binary_data(myproject.elf "main/data.bin" TEXT)

Place this line after the project() line in your project CMakeLists.txt file. Replace myproject.elf with your project name. The final argument can be TEXT to embed a null-terminated string, or BINARY to embed the content as-is.

For an example of using this technique, see the “main” component of the file_serving example protocols/http_server/file_serving/main/CMakeLists.txt - two files are loaded at build time and linked into the firmware.

It is also possible embed a generated file:

add_custom_command(OUTPUT my_processed_file.bin
                  COMMAND my_process_file_cmd my_unprocessed_file.bin)
target_add_binary_data(my_target "my_processed_file.bin" BINARY)

In the example above, my_processed_file.bin is generated from my_unprocessed_file.bin through some command my_process_file_cmd, then embedded into the target.

To specify a dependence on a target, use the DEPENDS argument:

add_custom_target(my_process COMMAND ...)
target_add_binary_data(my_target "my_embed_file.bin" BINARY DEPENDS my_process)

The DEPENDS argument to target_add_binary_data ensures that the target executes first.

Code and Data Placements

ESP-IDF has a feature called linker script generation that enables components to define where its code and data will be placed in memory through linker fragment files. These files are processed by the build system, and is used to augment the linker script used for linking app binary. See Linker Script Generation for a quick start guide as well as a detailed discussion of the mechanism.

Fully Overriding The Component Build Process

Obviously, there are cases where all these recipes are insufficient for a certain component, for example when the component is basically a wrapper around another third-party component not originally intended to be compiled under this build system. In that case, it’s possible to forego the ESP-IDF build system entirely by using a CMake feature called ExternalProject. Example component CMakeLists:

# External build process for quirc, runs in source dir and
# produces libquirc.a
externalproject_add(quirc_build
    PREFIX ${COMPONENT_DIR}
    SOURCE_DIR ${COMPONENT_DIR}/quirc
    CONFIGURE_COMMAND ""
    BUILD_IN_SOURCE 1
    BUILD_COMMAND make CC=${CMAKE_C_COMPILER} libquirc.a
    INSTALL_COMMAND ""
    )

 # Add libquirc.a to the build process
 add_library(quirc STATIC IMPORTED GLOBAL)
 add_dependencies(quirc quirc_build)

 set_target_properties(quirc PROPERTIES IMPORTED_LOCATION
      ${COMPONENT_DIR}/quirc/libquirc.a)
 set_target_properties(quirc PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
      ${COMPONENT_DIR}/quirc/lib)

 set_directory_properties( PROPERTIES ADDITIONAL_MAKE_CLEAN_FILES
      "${COMPONENT_DIR}/quirc/libquirc.a")

(The above CMakeLists.txt can be used to create a component named quirc that builds the quirc project using its own Makefile.)

• externalproject_add defines an external build system.

  • SOURCE_DIR, CONFIGURE_COMMAND, BUILD_COMMAND and INSTALL_COMMAND should always be set. CONFIGURE_COMMAND can be set to an empty string if the build system has no “configure” step. INSTALL_COMMAND will generally be empty for ESP-IDF builds.

  • Setting BUILD_IN_SOURCE means the build directory is the same as the source directory. Otherwise you can set BUILD_DIR.

  • Consult the ExternalProject documentation for more details about externalproject_add()

• The second set of commands adds a library target, which points to the “imported” library file built by the external system. Some properties need to be set in order to add include directories and tell CMake where this file is.

• Finally, the generated library is added to ADDITIONAL_MAKE_CLEAN_FILES. This means make clean will delete this library. (Note that the other object files from the build won’t be deleted.)

ExternalProject dependencies, clean builds

CMake has some unusual behaviour around external project builds:

• ADDITIONAL_MAKE_CLEAN_FILES only works when “make” is used as the build system. If Ninja or an IDE build system is used, it won’t delete these files when cleaning.

• However, the ExternalProject configure & build commands will always be re-run after a clean is run.

• Therefore, there are two alternative recommended ways to configure the external build command:

  1. Have the external BUILD_COMMAND run a full clean compile of all sources. The build command will be run if any of the dependencies passed to externalproject_add with DEPENDS have changed, or if this is a clean build (ie any of idf.py clean, ninja clean, or make clean was run.)

  2. Have the external BUILD_COMMAND be an incremental build command. Pass the parameter BUILD_ALWAYS 1 to externalproject_add. This means the external project will be built each time a build is run, regardless of dependencies. This is only recommended if the external project has correct incremental build behaviour, and doesn’t take too long to run.

The best of these approaches for building an external project will depend on the project itself, its build system, and whether you anticipate needing to frequently recompile the project.

Custom sdkconfig defaults

For example projects or other projects where you don’t want to specify a full sdkconfig configuration, but you do want to override some key values from the ESP-IDF defaults, it is possible to create a file sdkconfig.defaults in the project directory. This file will be used when creating a new config from scratch, or when any new config value hasn’t yet been set in the sdkconfig file.

To override the name of this file or to specify multiple files, set the SDKCONFIG_DEFAULTS environment variable or set SDKCONFIG_DEFAULTS in top-level CMakeLists.txt. If specifying multiple files, use semicolon as the list separator. File names not specified as full paths are resolved relative to current project.

Some of the IDF examples include a sdkconfig.ci file. This is part of the continuous integration (CI) test framework and is ignored by the normal build process.

Target-dependent sdkconfig defaults

In addition to sdkconfig.defaults file, build system will also load defaults from sdkconfig.defaults.TARGET_NAME file, where TARGET_NAME is the value of IDF_TARGET. For example, for esp32 target, default settings will be taken from sdkconfig.defaults first, and then from sdkconfig.defaults.esp32.

If SDKCONFIG_DEFAULTS is used to override the name of defaults file/files, the name of target-specific defaults file will be derived from SDKCONFIG_DEFAULTS value/values using the rule above.

Flash arguments

There are some scenarios that we want to flash the target board without IDF. For this case we want to save the built binaries, esptool.py and esptool write_flash arguments. It’s simple to write a script to save binaries and esptool.py.

After running a project build, the build directory contains binary output files (.bin files) for the project and also the following flashing data files:

• flash_project_args contains arguments to flash the entire project (app, bootloader, partition table, PHY data if this is configured).

• flash_app_args contains arguments to flash only the app.

• flash_bootloader_args contains arguments to flash only the bootloader.

You can pass any of these flasher argument files to esptool.py as follows:

python esptool.py --chip esp32 write_flash @build/flash_project_args

Alternatively, it is possible to manually copy the parameters from the argument file and pass them on the command line.

The build directory also contains a generated file flasher_args.json which contains project flash information, in JSON format. This file is used by idf.py and can also be used by other tools which need information about the project build.

Building the Bootloader

The bootloader is built by default as part of idf.py build, or can be built standalone via idf.py bootloader.

The bootloader is a special “subproject” inside /components/bootloader/subproject. It has its own project CMakeLists.txt file and builds separate .ELF and .BIN files to the main project. However it shares its configuration and build directory with the main project.

The subproject is inserted as an external project from the top-level project, by the file /components/bootloader/project_include.cmake. The main build process runs CMake for the subproject, which includes discovering components (a subset of the main components) and generating a bootloader-specific config (derived from the main sdkconfig).

Selecting the Target

ESP-IDF supports multiple targets (chips). A full list of supported targets in your version if ESP-IDF can be seen by running idf.py –list-targets.

To select the target before building the project, use idf.py set-target <target> command, for example:

idf.py set-target esp32s2

Important

idf.py set-target will clear the build directory and re-generate the sdkconfig file from scratch. The old sdkconfig file will be saved as sdkconfig.old.

Note

The behavior of idf.py set-target command is equivalent to:

1. clearing the build directory (idf.py fullclean)

2. removing the sdkconfig file (mv sdkconfig sdkconfig.old)

3. configuring the project with the new target (idf.py -DIDF_TARGET=esp32 reconfigure)

It is also possible to pass the desired IDF_TARGET as an environment variable (e.g. export IDF_TARGET=esp32s2) or as a CMake variable (e.g. -DIDF_TARGET=esp32s2 argument to CMake or idf.py). Setting the environment variable is a convenient method if you mostly work with one type of the chip.

To specify the _default_ value of IDF_TARGET for a given project, add CONFIG_IDF_TARGET value to sdkconfig.defaults. For example, CONFIG_IDF_TARGET="esp32s2". This value will be used if IDF_TARGET is not specified by other method: using an environment variable, CMake variable, or idf.py set-target command.

If the target has not been set by any of these methods, the build system will default to esp32 target.

Writing Pure CMake Components

The ESP-IDF build system “wraps” CMake with the concept of “components”, and helper functions to automatically integrate these components into a project build.

However, underneath the concept of “components” is a full CMake build system. It is also possible to make a component which is pure CMake.

Here is an example minimal “pure CMake” component CMakeLists file for a component named json:

add_library(json STATIC
cJSON/cJSON.c
cJSON/cJSON_Utils.c)

target_include_directories(json PUBLIC cJSON)

• This is actually an equivalent declaration to the IDF json component /components/json/CMakeLists.txt.

• This file is quite simple as there are not a lot of source files. For components with a large number of files, the globbing behaviour of ESP-IDF’s component logic can make the component CMakeLists style simpler.)

• Any time a component adds a library target with the component name, the ESP-IDF build system will automatically add this to the build, expose public include directories, etc. If a component wants to add a library target with a different name, dependencies will need to be added manually via CMake commands.

Using Third-Party CMake Projects with Components

CMake is used for a lot of open-source C and C++ projects — code that users can tap into for their applications. One of the benefits of having a CMake build system is the ability to import these third-party projects, sometimes even without modification! This allows for users to be able to get functionality that may not yet be provided by a component, or use another library for the same functionality.

Importing a library might look like this for a hypothetical library foo to be used in the main component:

# Register the component
idf_component_register(...)

# Set values of hypothetical variables that control the build of `foo`
set(FOO_BUILD_STATIC OFF)
set(FOO_BUILD_TESTS OFF)

# Create and import the library targets
add_subdirectory(foo)

# Publicly link `foo` to `main` component
target_link_libraries(main PUBLIC foo)

For an actual example, take a look at build_system/cmake/import_lib. Take note that what needs to be done in order to import the library may vary. It is recommended to read up on the library’s documentation for instructions on how to import it from other projects. Studying the library’s CMakeLists.txt and build structure can also be helpful.

It is also possible to wrap a third-party library to be used as a component in this manner. For example, the mbedtls component is a wrapper for Espressif’s fork of mbedtls. See its component CMakeLists.txt .

The CMake variable ESP_PLATFORM is set to 1 whenever the ESP-IDF build system is being used. Tests such as if (ESP_PLATFORM) can be used in generic CMake code if special IDF-specific logic is required.

Using ESP-IDF components from external libraries

The above example assumes that the external library foo (or tinyxml in the case of the import_lib example) doesn’t need to use any ESP-IDF APIs apart from common APIs such as libc, libstdc++, etc. If the external library needs to use APIs provided by other ESP-IDF components, this needs to be specified in the external CMakeLists.txt file by adding a dependency on the library target idf::<componentname>.

For example, in the foo/CMakeLists.txt file:

add_library(foo bar.c fizz.cpp buzz.cpp)

if(ESP_PLATFORM)
  # On ESP-IDF, bar.c needs to include esp_spi_flash.h from the spi_flash component
  target_link_libraries(foo PRIVATE idf::spi_flash)
endif()

Using Prebuilt Libraries with Components

Another possibility is that you have a prebuilt static library (.a file), built by some other build process.

The ESP-IDF build system provides a utility function add_prebuilt_library for users to be able to easily import and use prebuilt libraries:

add_prebuilt_library(target_name lib_path [REQUIRES req1 req2 ...] [PRIV_REQUIRES req1 req2 ...])

where:

• target_name- name that can be used to reference the imported library, such as when linking to other targets

• lib_path- path to prebuilt library; may be an absolute or relative path to the component directory

Optional arguments REQUIRES and PRIV_REQUIRES specify dependency on other components. These have the same meaning as the arguments for idf_component_register.

Take note that the prebuilt library must have been compiled for the same target as the consuming project. Configuration relevant to the prebuilt library must also match. If not paid attention to, these two factors may contribute to subtle bugs in the app.

For an example, take a look at build_system/cmake/import_prebuilt.

Using ESP-IDF in Custom CMake Projects

ESP-IDF provides a template CMake project for easily creating an application. However, in some instances the user might already have an existing CMake project or may want to create a custom one. In these cases it is desirable to be able to consume IDF components as libraries to be linked to the user’s targets (libraries/ executables).

It is possible to do so by using the build system APIs provided by tools/cmake/idf.cmake. For example:

cmake_minimum_required(VERSION 3.5)
project(my_custom_app C)

# Include CMake file that provides ESP-IDF CMake build system APIs.
include($ENV{IDF_PATH}/tools/cmake/idf.cmake)

# Include ESP-IDF components in the build, may be thought as an equivalent of
# add_subdirectory() but with some additional processing and magic for ESP-IDF build
# specific build processes.
idf_build_process(esp32)

# Create the project executable and plainly link the newlib component to it using
# its alias, idf::newlib.
add_executable(${CMAKE_PROJECT_NAME}.elf main.c)
target_link_libraries(${CMAKE_PROJECT_NAME}.elf idf::newlib)

# Let the build system know what the project executable is to attach more targets, dependencies, etc.
idf_build_executable(${CMAKE_PROJECT_NAME}.elf)

The example in build_system/cmake/idf_as_lib demonstrates the creation of an application equivalent to hello world application using a custom CMake project.

ESP-IDF CMake Build System API

idf-build-commands

idf_build_get_property(var property [GENERATOR_EXPRESSION])

Retrieve a build property property and store it in var accessible from the current scope. Specifying GENERATOR_EXPRESSION will retrieve the generator expression string for that property, instead of the actual value, which can be used with CMake commands that support generator expressions.

idf_build_set_property(property val [APPEND])

Set a build property property with value val. Specifying APPEND will append the specified value to the current value of the property. If the property does not previously exist or it is currently empty, the specified value becomes the first element/member instead.

idf_build_component(component_dir)

Present a directory component_dir that contains a component to the build system. Relative paths are converted to absolute paths with respect to current directory. All calls to this command must be performed before idf_build_process.

This command does not guarantee that the component will be processed during build (see the COMPONENTS argument description for idf_build_process)

idf_build_process(target
                  [PROJECT_DIR project_dir]
                  [PROJECT_VER project_ver]
                  [PROJECT_NAME project_name]
                  [SDKCONFIG sdkconfig]
                  [SDKCONFIG_DEFAULTS sdkconfig_defaults]
                  [BUILD_DIR build_dir]
                  [COMPONENTS component1 component2 ...])

Performs the bulk of the behind-the-scenes magic for including ESP-IDF components such as component configuration, libraries creation, dependency expansion and resolution. Among these functions, perhaps the most important from a user’s perspective is the libraries creation by calling each component’s idf_component_register. This command creates the libraries for each component, which are accessible using aliases in the form idf::component_name. These aliases can be used to link the components to the user’s own targets, either libraries or executables.

The call requires the target chip to be specified with target argument. Optional arguments for the call include:

• PROJECT_DIR - directory of the project; defaults to CMAKE_SOURCE_DIR

• PROJECT_NAME - name of the project; defaults to CMAKE_PROJECT_NAME

• PROJECT_VER - version/revision of the project; defaults to “1”

• SDKCONFIG - output path of generated sdkconfig file; defaults to PROJECT_DIR/sdkconfig or CMAKE_SOURCE_DIR/sdkconfig depending if PROJECT_DIR is set

• SDKCONFIG_DEFAULTS - list of files containing default config to use in the build (list must contain full paths); defaults to empty. For each value filename in the list, the config from file filename.target, if it exists, is also loaded.

• BUILD_DIR - directory to place ESP-IDF build-related artifacts, such as generated binaries, text files, components; defaults to CMAKE_BINARY_DIR

• COMPONENTS - select components to process among the components known by the build system (added via idf_build_component). This argument is used to trim the build. Other components are automatically added if they are required in the dependency chain, i.e. the public and private requirements of the components in this list are automatically added, and in turn the public and private requirements of those requirements, so on and so forth. If not specified, all components known to the build system are processed.

idf_build_executable(executable)

Specify the executable executable for ESP-IDF build. This attaches additional targets such as dependencies related to flashing, generating additional binary files, etc. Should be called after idf_build_process.

idf_build_get_config(var config [GENERATOR_EXPRESSION])

Get the value of the specified config. Much like build properties, specifying GENERATOR_EXPRESSION will retrieve the generator expression string for that config, instead of the actual value, which can be used with CMake commands that support generator expressions. Actual config values are only known after call to idf_build_process, however.

idf-build-properties

These are properties that describe the build. Values of build properties can be retrieved by using the build command idf_build_get_property. For example, to get the Python interpreter used for the build:

idf_build_get_property(python PYTHON)
message(STATUS "The Python intepreter is: ${python}")

• BUILD_DIR - build directory; set from idf_build_process BUILD_DIR argument

• BUILD_COMPONENTS - list of components included in the build; set by idf_build_process

• BUILD_COMPONENT_ALIASES - list of library alias of components included in the build; set by idf_build_process

• C_COMPILE_OPTIONS - compile options applied to all components’ C source files

• COMPILE_OPTIONS - compile options applied to all components’ source files, regardless of it being C or C++

• COMPILE_DEFINITIONS - compile definitions applied to all component source files

• CXX_COMPILE_OPTIONS - compile options applied to all components’ C++ source files

• EXECUTABLE - project executable; set by call to idf_build_executable

• EXECUTABLE_NAME - name of project executable without extension; set by call to idf_build_executable

• EXECUTABLE_DIR - path containing the output executable

• IDF_PATH - ESP-IDF path; set from IDF_PATH environment variable, if not, inferred from the location of idf.cmake

• IDF_TARGET - target chip for the build; set from the required target argument for idf_build_process

• IDF_VER - ESP-IDF version; set from either a version file or the Git revision of the IDF_PATH repository

• INCLUDE_DIRECTORIES - include directories for all component source files

• KCONFIGS - list of Kconfig files found in components in build; set by idf_build_process

• KCONFIG_PROJBUILDS - list of Kconfig.projbuild files found in components in build; set by idf_build_process

• PROJECT_NAME - name of the project; set from idf_build_process PROJECT_NAME argument

• PROJECT_DIR - directory of the project; set from idf_build_process PROJECT_DIR argument

• PROJECT_VER - version of the project; set from idf_build_process PROJECT_VER argument

• PYTHON - Python interpreter used for the build; set from PYTHON environment variable if available, if not “python” is used

• SDKCONFIG - full path to output config file; set from idf_build_process SDKCONFIG argument

• SDKCONFIG_DEFAULTS - list of files containing default config to use in the build; set from idf_build_process SDKCONFIG_DEFAULTS argument

• SDKCONFIG_HEADER - full path to C/C++ header file containing component configuration; set by idf_build_process

• SDKCONFIG_CMAKE - full path to CMake file containing component configuration; set by idf_build_process

• SDKCONFIG_JSON - full path to JSON file containing component configuration; set by idf_build_process

• SDKCONFIG_JSON_MENUS - full path to JSON file containing config menus; set by idf_build_process

idf-component-commands

idf_component_get_property(var component property [GENERATOR_EXPRESSION])

Retrieve a specified component’s component property, property and store it in var accessible from the current scope. Specifying GENERATOR_EXPRESSION will retrieve the generator expression string for that property, instead of the actual value, which can be used with CMake commands that support generator expressions.

idf_component_set_property(component property val [APPEND])

Set a specified component’s component property, property with value val. Specifying APPEND will append the specified value to the current value of the property. If the property does not previously exist or it is currently empty, the specified value becomes the first element/member instead.

idf_component_register([[SRCS src1 src2 ...] | [[SRC_DIRS dir1 dir2 ...] [EXCLUDE_SRCS src1 src2 ...]]
                       [INCLUDE_DIRS dir1 dir2 ...]
                       [PRIV_INCLUDE_DIRS dir1 dir2 ...]
                       [REQUIRES component1 component2 ...]
                       [PRIV_REQUIRES component1 component2 ...]
                       [LDFRAGMENTS ldfragment1 ldfragment2 ...]
                       [REQUIRED_IDF_TARGETS target1 target2 ...]
                       [EMBED_FILES file1 file2 ...]
                       [EMBED_TXTFILES file1 file2 ...]
                       [KCONFIG kconfig]
                       [KCONFIG_PROJBUILD kconfig_projbuild])

Register a component to the build system. Much like the project() CMake command, this should be called from the component’s CMakeLists.txt directly (not through a function or macro) and is recommended to be called before any other command. Here are some guidelines on what commands can not be called before idf_component_register:

• commands that are not valid in CMake script mode

• custom commands defined in project_include.cmake

• build system API commands except idf_build_get_property; although consider whether the property may not have been set yet

Commands that set and operate on variables are generally okay to call before idf_component_register.

The arguments for idf_component_register include:

• SRCS - component source files used for creating a static library for the component; if not specified, component is a treated as a config-only component and an interface library is created instead.

• SRC_DIRS, EXCLUDE_SRCS - used to glob source files (.c, .cpp, .S) by specifying directories, instead of specifying source files manually via SRCS. Note that this is subject to the limitations of globbing in CMake. Source files specified in EXCLUDE_SRCS are removed from the globbed files.

• INCLUDE_DIRS - paths, relative to the component directory, which will be added to the include search path for all other components which require the current component

• PRIV_INCLUDE_DIRS - directory paths, must be relative to the component directory, which will be added to the include search path for this component’s source files only

• REQUIRES - public component requirements for the component

• PRIV_REQUIRES - private component requirements for the component; ignored on config-only components

• LDFRAGMENTS - component linker fragment files

• REQUIRED_IDF_TARGETS - specify the only target the component supports

• KCONFIG - override the default Kconfig file

• KCONFIG_PROJBUILD - override the default Kconfig.projbuild file

The following are used for embedding data into the component, and is considered as source files when determining if a component is config-only. This means that even if the component does not specify source files, a static library is still created internally for the component if it specifies either:

• EMBED_FILES - binary files to be embedded in the component

• EMBED_TXTFILES - text files to be embedded in the component

idf-component-properties

These are properties that describe a component. Values of component properties can be retrieved by using the build command idf_component_get_property. For example, to get the directory of the freertos component:

idf_component_get_property(dir freertos COMPONENT_DIR)
message(STATUS "The 'freertos' component directory is: ${dir}")

• COMPONENT_ALIAS - alias for COMPONENT_LIB used for linking the component to external targets; set by idf_build_component and alias library itself is created by idf_component_register

• COMPONENT_DIR - component directory; set by idf_build_component

• COMPONENT_OVERRIDEN_DIR - contains the directory of the original component if this component overrides another component

• COMPONENT_LIB - name for created component static/interface library; set by idf_build_component and library itself is created by idf_component_register

• COMPONENT_NAME - name of the component; set by idf_build_component based on the component directory name

• COMPONENT_TYPE - type of the component, whether LIBRARY or CONFIG_ONLY. A component is of type LIBRARY if it specifies source files or embeds a file

• EMBED_FILES - list of files to embed in component; set from idf_component_register EMBED_FILES argument

• EMBED_TXTFILES - list of text files to embed in component; set from idf_component_register EMBED_TXTFILES argument

• INCLUDE_DIRS - list of component include directories; set from idf_component_register INCLUDE_DIRS argument

• KCONFIG - component Kconfig file; set by idf_build_component

• KCONFIG_PROJBUILD - component Kconfig.projbuild; set by idf_build_component

• LDFRAGMENTS - list of component linker fragment files; set from idf_component_register LDFRAGMENTS argument

• PRIV_INCLUDE_DIRS - list of component private include directories; set from idf_component_register PRIV_INCLUDE_DIRS on components of type LIBRARY

• PRIV_REQUIRES - list of private component dependentices; set from idf_component_register PRIV_REQUIRES argument

• REQUIRED_IDF_TARGETS - list of targets the component supports; set from idf_component_register EMBED_TXTFILES argument

• REQUIRES - list of public component dependencies; set from idf_component_register REQUIRES argument

• SRCS - list of component source files; set from SRCS or SRC_DIRS/EXCLUDE_SRCS argument of idf_component_register

File Globbing & Incremental Builds

The preferred way to include source files in an ESP-IDF component is to list them manually via SRCS argument to idf_component_register:

idf_component_register(SRCS library/a.c library/b.c platform/platform.c
                       ...)

This preference reflects the CMake best practice of manually listing source files. This could, however, be inconvenient when there are lots of source files to add to the build. The ESP-IDF build system provides an alternative way for specifying source files using SRC_DIRS:

idf_component_register(SRC_DIRS library platform
                       ...)

This uses globbing behind the scenes to find source files in the specified directories. Be aware, however, that if a new source file is added and this method is used, then CMake won’t know to automatically re-run and this file won’t be added to the build.

The trade-off is acceptable when you’re adding the file yourself, because you can trigger a clean build or run idf.py reconfigure to manually re-run CMake. However, the problem gets harder when you share your project with others who may check out a new version using a source control tool like Git…

For components which are part of ESP-IDF, we use a third party Git CMake integration module (/tools/cmake/third_party/GetGitRevisionDescription.cmake) which automatically re-runs CMake any time the repository commit changes. This means if you check out a new ESP-IDF version, CMake will automatically rerun.

For project components (not part of ESP-IDF), there are a few different options:

• If keeping your project file in Git, ESP-IDF will automatically track the Git revision and re-run CMake if the revision changes.

• If some components are kept in a third git repository (not the project repository or ESP-IDF repository), you can add a call to the git_describe function in a component CMakeLists file in order to automatically trigger re-runs of CMake when the Git revision changes.

• If not using Git, remember to manually run idf.py reconfigure whenever a source file may change.

• To avoid this problem entirely, use SRCS argument to idf_component_register to list all source files in project components.

The best option will depend on your particular project and its users.

Build System Metadata

For integration into IDEs and other build systems, when CMake runs the build process generates a number of metadata files in the build/ directory. To regenerate these files, run cmake or idf.py reconfigure (or any other idf.py build command).

• compile_commands.json is a standard format JSON file which describes every source file which is compiled in the project. A CMake feature generates this file, and many IDEs know how to parse it.

• project_description.json contains some general information about the ESP-IDF project, configured paths, etc.

• flasher_args.json contains esptool.py arguments to flash the project’s binary files. There are also flash_*_args files which can be used directly with esptool.py. See Flash arguments.

• CMakeCache.txt is the CMake cache file which contains other information about the CMake process, toolchain, etc.

• config/sdkconfig.json is a JSON-formatted version of the project configuration values.

• config/kconfig_menus.json is a JSON-formatted version of the menus shown in menuconfig, for use in external IDE UIs.

JSON Configuration Server

A tool called confserver.py is provided to allow IDEs to easily integrate with the configuration system logic. confserver.py is designed to run in the background and interact with a calling process by reading and writing JSON over process stdin & stdout.

You can run confserver.py from a project via idf.py confserver or ninja confserver, or a similar target triggered from a different build generator.

For more information about confserver.py, see tools/kconfig_new/README.md.

Build System Internals

Build Scripts

The listfiles for the ESP-IDF build system reside in /tools/cmake. The modules which implement core build system functionality are as follows:

• build.cmake - Build related commands i.e. build initialization, retrieving/setting build properties, build processing.

• component.cmake - Component related commands i.e. adding components, retrieving/setting component properties, registering components.

• kconfig.cmake - Generation of configuration files (sdkconfig, sdkconfig.h, sdkconfig.cmake, etc.) from Kconfig files.

• ldgen.cmake - Generation of final linker script from linker fragment files.

• target.cmake - Setting build target and toolchain file.

• utilities.cmake - Miscellaneous helper commands.

Aside from these files, there are two other important CMake scripts in /tools/cmake:

• idf.cmake - Sets up the build and includes the core modules listed above. Included in CMake projects in order to access ESP-IDF build system functionality.

• project.cmake - Includes idf.cmake and provides a custom project() command that takes care of all the heavy lifting of building an executable. Included in the top-level CMakeLists.txt of standard ESP-IDF projects.

The rest of the files in /tools/cmake are support or third-party scripts used in the build process.

Build Process

This section describes the standard ESP-IDF application build process. The build process can be broken down roughly into four phases:

ESP-IDF Build System Process

Initialization

This phase sets up necessary parameters for the build.

• Upon inclusion of idf.cmake in project.cmake, the following steps are performed:

  • Set IDF_PATH from environment variable or inferred from path to project.cmake included in the top-level CMakeLists.txt.

  • Add /tools/cmake to CMAKE_MODULE_PATH and include core modules plus the various helper/third-party scripts.

  • Set build tools/executables such as default Python interpreter.

  • Get ESP-IDF git revision and store as IDF_VER.

  • Set global build specifications i.e. compile options, compile definitions, include directories for all components in the build.

  • Add components in components to the build.

• The initial part of the custom project() command performs the following steps:

  • Set IDF_TARGET from environment variable or CMake cache and the corresponding CMAKE_TOOLCHAIN_FILE to be used.

  • Add components in EXTRA_COMPONENTS_DIRS to the build.

  • Prepare arguments for calling command idf_build_process() from variables such as COMPONENTS/EXCLUDE_COMPONENTS, SDKCONFIG, SDKCONFIG_DEFAULTS.

The call to idf_build_process() command marks the end of this phase.

Enumeration

This phase builds a final list of components to be processed in the build, and is performed in the first half of idf_build_process().

• Retrieve each component’s public and private requirements. A child process is created which executes each component’s CMakeLists.txt in script mode. The values of idf_component_register REQUIRES and PRIV_REQUIRES argument is returned to the parent build process. This is called early expansion. The variable CMAKE_BUILD_EARLY_EXPANSION is defined during this step.

• Recursively include components based on public and private requirements.

Processing

This phase processes the components in the build, and is the second half of idf_build_process().

• Load project configuration from sdkconfig file and generate an sdkconfig.cmake and sdkconfig.h header. These define configuration variables/macros that are accessible from the build scripts and C/C++ source/header files, respectively.

• Include each component’s project_include.cmake.

• Add each component as a subdirectory, processing its CMakeLists.txt. The component CMakeLists.txt calls the registration command, idf_component_register which adds source files, include directories, creates component library, links dependencies, etc.

Finalization

This phase is everything after idf_build_process().

• Create executable and link the component libraries to it.

• Generate project metadata files such as project_description.json and display relevant information about the project built.

Browse /tools/cmake/project.cmake for more details.

Migrating from ESP-IDF GNU Make System

Some aspects of the CMake-based ESP-IDF build system are very similar to the older GNU Make-based system. The developer needs to provide values the include directories, source files etc. There is a syntactical difference, however, as the developer needs to pass these as arguments to the registration command, idf_component_register.

Automatic Conversion Tool

An automatic project conversion tool is available in /tools/cmake/convert_to_cmake.py. Run this command line tool with the path to a project like this:

$IDF_PATH/tools/cmake/convert_to_cmake.py /path/to/project_dir

The project directory must contain a Makefile, and GNU Make (make) must be installed and available on the PATH.

The tool will convert the project Makefile and any component component.mk files to their equivalent CMakeLists.txt files.

It does so by running make to expand the ESP-IDF build system variables which are set by the build, and then producing equivalent CMakelists files to set the same variables.

Important

When the conversion tool converts a component.mk file, it doesn’t determine what other components that component depends on. This information needs to be added manually by editing the new component CMakeLists.txt file and adding REQUIRES and/or PRIV_REQUIRES clauses. Otherwise, source files in the component will fail to compile as headers from other components are not found. See Component Requirements.

The conversion tool is not capable of dealing with complex Makefile logic or unusual targets. These will need to be converted by hand.

No Longer Available in CMake

Some features are significantly different or removed in the CMake-based system. The following variables no longer exist in the CMake-based build system:

• COMPONENT_BUILD_DIR: Use CMAKE_CURRENT_BINARY_DIR instead.

• COMPONENT_LIBRARY: Defaulted to $(COMPONENT_NAME).a, but the library name could be overriden by the component. The name of the component library can no longer be overriden by the component.

• CC, LD, AR, OBJCOPY: Full paths to each tool from the gcc xtensa cross-toolchain. Use CMAKE_C_COMPILER, CMAKE_C_LINK_EXECUTABLE, CMAKE_OBJCOPY, etc instead. Full list here.

• HOSTCC, HOSTLD, HOSTAR: Full names of each tool from the host native toolchain. These are no longer provided, external projects should detect any required host toolchain manually.

• COMPONENT_ADD_LDFLAGS: Used to override linker flags. Use the CMake target_link_libraries command instead.

• COMPONENT_ADD_LINKER_DEPS: List of files that linking should depend on. target_link_libraries will usually infer these dependencies automatically. For linker scripts, use the provided custom CMake function target_linker_scripts.

• COMPONENT_SUBMODULES: No longer used, the build system will automatically enumerate all submodules in the ESP-IDF repository.

• COMPONENT_EXTRA_INCLUDES: Used to be an alternative to COMPONENT_PRIV_INCLUDEDIRS for absolute paths. Use PRIV_INCLUDE_DIRS argument to idf_component_register for all cases now (can be relative or absolute).

• COMPONENT_OBJS: Previously, component sources could be specified as a list of object files. Now they can be specified as a list of source files via SRCS argument to idf_component_register.

• COMPONENT_OBJEXCLUDE: Has been replaced with EXCLUDE_SRCS argument to idf_component_register. Specify source files (as absolute paths or relative to component directory), instead.

• COMPONENT_EXTRA_CLEAN: Set property ADDITIONAL_MAKE_CLEAN_FILES instead but note CMake has some restrictions around this functionality.

• COMPONENT_OWNBUILDTARGET & COMPONENT_OWNCLEANTARGET: Use CMake ExternalProject instead. See Fully Overriding The Component Build Process for full details.

• COMPONENT_CONFIG_ONLY: Call idf_component_register without any arguments instead. See Configuration-Only Components.

• CFLAGS, CPPFLAGS, CXXFLAGS: Use equivalent CMake commands instead. See Controlling Component Compilation.

No Default Values

Unlike in the legacy Make-based build system, the following have no default values:

• Source directories (COMPONENT_SRCDIRS variable in Make, SRC_DIRS argument to idf_component_register in CMake)

• Include directories (COMPONENT_ADD_INCLUDEDIRS variable in Make, INCLUDE_DIRS argument to idf_component_register in CMake)

No Longer Necessary

• In the legacy Make-based build system, it is required to also set COMPONENT_SRCDIRS if COMPONENT_SRCS is set. In CMake, the equivalent is not necessary i.e. specifying SRC_DIRS to idf_component_register if SRCS is also specified (in fact, SRCS is ignored if SRC_DIRS is specified).

Flashing from make

make flash and similar targets still work to build and flash. However, project sdkconfig no longer specifies serial port and baud rate. Environment variables can be used to override these. See Flashing with ninja or make for more details.

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