esp-idf/docs/build_system.rst

Build System
************

This document explains the Espressif IoT Development Framework build system and the
concept of "components"

Read this document if you want to know how to organise a new ESP-IDF project.

We recommend using the esp-idf-template_ project as a starting point for your project.

Using the Build System
======================

The esp-idf README file contains a description of how to use the build system to build your project.

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 ESP32 base libraries (libc, rom bindings etc)
- The WiFi 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 environment 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 a text-based
menu system to customize each component. After the components in the
project are configured, the build process 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 output 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 ``make 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.

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, or the path to the toolchain can be set as part of the compiler prefix in the project configuration.


Example Project
---------------

An example project directory tree might look like this::

    - myProject/
                 - Makefile
                 - sdkconfig
                 - components/ - component1/ - component.mk
                                             - Kconfig
                                             - src1.c
                               - component2/ - component.mk
                                             - Kconfig
                                             - src1.c
                                             - include/ - component2.h
                 - main/       - src1.c
                               - src2.c
                               - component.mk

                 - build/

This example "myProject" contains the following elements:

- A top-level project Makefile. This Makefile set the ``PROJECT_NAME`` variable and (optionally) defines
  other project-wide make variables. It includes the core ``$(IDF_PATH)/make/project.mk`` makefile which
  implements the rest of the ESP-IDF build system.

- "sdkconfig" project configuration file. This file is created/updated when "make 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.

- "main" directory is a special "pseudo-component" that contains source code for the project itself. "main" is a default name, the Makefile variable ``SRCDIRS`` defaults to this but can be set to look for pseudo-components in other directories.

- "build" directory is where build output is created. After the make process is run, this directory will 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 contain a component makefile - ``component.mk``. This may contain variable definitions
to control the build process of the component, and its integration into the overall project. See `Component Makefiles` for more details.

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

Project Makefiles
-----------------

Each project has a single Makefile that contains build settings for the entire project. By default, the project Makefile can be quite minimal.

Minimal Example Makefile
^^^^^^^^^^^^^^^^^^^^^^^^

::

   PROJECT_NAME := myProject
   
   include $(IDF_PATH)/make/project.mk


Mandatory Project Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^

- ``PROJECT_NAME``: Name of the project. Binary output files will use this name - ie myProject.bin, myProject.elf.

Optional Project Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^

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

- ``PROJECT_PATH``: Top-level project directory. Defaults to the directory containing the Makefile. Many other project variables are based on this variable. The project path cannot contain spaces.
- ``BUILD_DIR_BASE``: The build directory for all objects/libraries/binaries. Defaults to ``$(PROJECT_PATH)/build``.
- ``COMPONENT_DIRS``: Directories to search for components. Defaults to `$(IDF_PATH)/components`, `$(PROJECT_PATH)/components` and ``EXTRA_COMPONENT_DIRS``. Override this variable if you don't want to search for components in the esp-idf & project ``components`` directories.
- ``EXTRA_COMPONENT_DIRS``: Optional list of additional directories to search for components. Components themselves are in sub-directories of these directories, this is a top-level directory containing the component directories.
- ``COMPONENTS``: A list of component names to build into the project. Defaults to all components found in the COMPONENT_DIRS directories.
- ``SRCDIRS``: Directories under the main project directory which contain project-specific "pseudo-components". Defaults to 'main'. The difference between specifying a directory here and specifying it under ``EXTRA_COMPONENT_DIRS`` is that a directory in ``SRCDIRS`` is a component itself (contains a file "component.mk"), whereas a directory in ``EXTRA_COMPONENT_DIRS`` contains component directories which contain a file "component.mk". See the `Example Project` for a concrete case of this.


Component Makefiles
-------------------

Each project contains one or more components, which can either be part of esp-idf or added from other component directories.

A component is any sub-directory that contains a `component.mk` file.[#f1]_.

Minimal Component Makefile
^^^^^^^^^^^^^^^^^^^^^^^^^^

The minimal ``component.mk`` file is an empty file(!). If the file is empty, the default component behaviour is set:

- All source files in the same directory as the makefile (``*.c``, ``*.cpp``, ``*.S``) will be compiled into the component library
- A sub-directory "include" will be added to the global include search path for all other components.
- The component library will be linked into the project app.

See `example component makefiles` for more complete component makefile examples.

Note that there is a different between an empty ``component.mk`` file (which invokes default component build behaviour) and no ``component.mk`` file (which means no default component build behaviour will occur.) It is possible for a component to have no `component.mk` file, if it only contains other files which influence the project configuration or build process.

.. component variables:

Preset Component Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^

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

- ``COMPONENT_PATH``: The component directory. Evaluates to the absolute path of the directory containing ``component.mk``. The component path cannot contain spaces.
- ``COMPONENT_NAME``: Name of the component. Defaults to the name of the component directory.
- ``COMPONENT_BUILD_DIR``: The component build directory. Evaluates to the absolute path of a directory inside `$(BUILD_DIR_BASE)` where this component's source files are to be built. This is also the Current Working Directory any time the component is being built, so relative paths in make targets, etc. will be relative to this directory.
- ``COMPONENT_LIBRARY``: Name of the static library file (relative to the component build directory) that will be built for this component. Defaults to ``$(COMPONENT_NAME).a``.

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

- ``PROJECT_NAME``: Name of the project, as set in project Makefile
- ``PROJECT_PATH``: Absolute path of the project directory containing the project Makefile.
- ``COMPONENTS``: Name of all components that are included in this build.
- ``CONFIG_*``: Each value in the project configuration has a corresponding variable available in make. All names begin with ``CONFIG_``.
- ``CC``, ``LD``, ``AR``, ``OBJCOPY``: Full paths to each tool from the gcc xtensa cross-toolchain.
- ``HOSTCC``, ``HOSTLD``, ``HOSTAR``: Full names of each tool from the host native toolchain.

If you modify any of these variables inside ``component.mk`` then this will not prevent other components from building but it may make your component hard to build and/or debug.

Optional Project-Wide Component Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following variables can be set inside ``component.mk`` to control build settings across the entire project:

- ``COMPONENT_ADD_INCLUDEDIRS``: Paths, relative to the component
  directory, which will be added to the include search path for
  all components in the project. Defaults to ``include`` if not overridden. If an include directory is only needed to compile
  this specific component, add it to ``COMPONENT_PRIV_INCLUDEDIRS`` instead.
- ``COMPONENT_ADD_LDFLAGS``: Add linker arguments to the LDFLAGS for
  the app executable. Defaults to ``-l$(COMPONENT_NAME)``.  If
  adding pre-compiled libraries to this directory, add them as
  absolute paths - ie $(COMPONENT_PATH)/libwhatever.a
- ``COMPONENT_DEPENDS``: Optional list of component names that should
  be compiled before this component. This is not necessary for
  link-time dependencies, because all component include directories
  are available at all times. It is necessary if one component
  generates an include file which you then want to include in another
  component. Most components do not need to set this variable.
- ``COMPONENT_ADD_LINKER_DEPS``: Optional list of component-relative paths
  to files which should trigger a re-link of the ELF file if they change.
  Typically used for linker script files and binary libraries. Most components do
  not need to set this variable.

The following variable only works for components that are part of esp-idf itself:

- ``COMPONENT_SUBMODULES``: Optional list of git submodule paths
  (relative to COMPONENT_PATH) used by the component. These will be
  checked (and initialised if necessary) by the build process. This
  variable is ignored if the component is outside the IDF_PATH
  directory.


Optional Component-Specific Variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The following variables can be set inside ``component.mk`` to control the build of that component:

- ``COMPONENT_PRIV_INCLUDEDIRS``: 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.
- ``COMPONENT_EXTRA_INCLUDES``: Any extra include paths used when
  compiling the component's source files. These will be prefixed with
  '-I' and passed as-is to the compiler.  Similar to the
  ``COMPONENT_PRIV_INCLUDEDIRS`` variable, except these paths are not
  expanded relative to the component directory.
- ``COMPONENT_SRCDIRS``: Directory paths, must be relative to the
  component directory, which will be searched for source files (``*.cpp``,
  ``*.c``, ``*.S``). Defaults to '.', ie the component directory
  itself. Override this to specify a different list of directories
  which contain source files.
- ``COMPONENT_OBJS``: Object files to compile. Default value is a .o
  file for each source file that is found in ``COMPONENT_SRCDIRS``.
  Overriding this list allows you to exclude source files in
  ``COMPONENT_SRCDIRS`` that would otherwise be compiled. See
  `Specifying source files`
- ``COMPONENT_EXTRA_CLEAN``: Paths, relative to the component build
  directory, of any files that are generated using custom make rules
  in the component.mk file and which need to be removed as part of
  ``make clean``. See `Source Code Generation` for an example.
- ``COMPONENT_OWNBUILDTARGET`` & `COMPONENT_OWNCLEANTARGET`: These
  targets allow you to fully override the default build behaviour for
  the component. See `Fully Overriding The Component Makefile` for
  more details.
- ``CFLAGS``: Flags passed to the C compiler. A default set of
  ``CFLAGS`` is defined based on project settings. Component-specific
  additions can be made via ``CFLAGS +=``. It is also possible
  (although not recommended) to override this variable completely for
  a component.
- ``CPPFLAGS``: Flags passed to the C preprocessor (used for .c, .cpp
  and .S files). A default set of ``CPPFLAGS`` is defined based on
  project settings. Component-specific additions can be made via
  ``CPPFLAGS +=``. It is also possible (although not recommended) to
  override this variable completely for a component.
- ``CXXFLAGS``: Flags passed to the C++ compiler. A default set of
  ``CXXFLAGS`` is defined based on project
  settings. Component-specific additions can be made via ``CXXFLAGS
  +=``. It is also possible (although not recommended) to override
  this variable completely for a component.

To apply compilation flags to a single source file, you can add a variable override as a target, ie::

  apps/dhcpserver.o: CFLAGS += -Wno-unused-variable

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

Component Configuration
-----------------------

Each component can also have a Kconfig file, alongside ``component.mk``. This contains contains
configuration settings to add to the "make menuconfig" 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`.

Build Process Internals
-----------------------

Top Level: Project Makefile
^^^^^^^^^^^^^^^^^^^^^^^^^^^

- "make" is always run from the project directory and the project makefile, typically named Makefile.
- The project makefile sets ``PROJECT_NAME`` and optionally customises other `optional project variables`
- The project makefile includes ``$(IDF_PATH)/make/project.mk`` which contains the project-level Make logic.
- ``project.mk`` fills in default project-level make variables and includes make variables from the project configuration. If the generated makefile containing project configuration is out of date, then it is regenerated (via targets in ``project_config.mk``) and then the make process restarts from the top.
- ``project.mk`` builds a list of components to build, based on the default component directories or a custom list of components set in `optional project variables`.
- Each component can set some `optional project-wide component variables`. These are included via generated makefiles named ``component_project_vars.mk`` - there is one per component. These generated makefiles are included into ``project.mk``. If any are missing or out of date, they are regenerated (via a recursive make call to the component makefile) and then the make process restarts from the top.
- `Makefile.projbuild` files from components are included into the make process, to add extra targets or configuration. 
- By default, the project makefile also generates top-level build & clean targets for each component and sets up `app` and `clean` targets to invoke all of these sub-targets.
- In order to compile each component, a recursive make is performed for the component makefile.

To better understand the project make process, have a read through the ``project.mk`` file itself.

Second Level: Component Makefiles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- Each call to a component makefile goes via the ``$(IDF_PATH)/make/component_wrapper.mk`` wrapper makefile.
- The ``component_wrapper.mk`` is called with the current directory set to the component build directory, and the ``COMPONENT_MAKEFILE`` variable is set to the absolute path to ``component.mk``.
- ``component_wrapper.mk`` sets default values for all `component variables`, then includes the `component.mk` file which can override or modify these.
- If ``COMPONENT_OWNBUILDTARGET`` and ``COMPONENT_OWNCLEANTARGET`` are not defined, default build and clean targets are created for the component's source files and the prerequisite ``COMPONENT_LIBRARY`` static library file.
- The ``component_project_vars.mk`` file has its own target in ``component_wrapper.mk``, which is evaluated from ``project.mk`` if this file needs to be rebuilt due to changes in the component makefile or the project configuration.

To better understand the component make process, have a read through the ``component_wrapper.mk`` file and some of the ``component.mk`` files included with esp-idf.

Debugging The Make Process
--------------------------

Some tips for debugging the esp-idf build system:

- Appending ``V=1`` to the make arguments (or setting it as an environment variable) will cause make to echo all commands executed, and also each directory as it is entered for a sub-make.
- Running ``make -w`` will cause make to echo each directory as it is entered for a sub-make - same as ``V=1`` but without also echoing all commands.
- Running ``make --trace`` (possibly in addition to one of the above arguments) will print out every target as it is built, and the dependency which caused it to be built.
- Running ``make -p`` prints a (very verbose) summary of every generated target in each makefile.

For more debugging tips and general make information, see the `GNU Make Manual`.

Overriding Parts of the Project
-------------------------------

Makefile.projbuild
^^^^^^^^^^^^^^^^^^

For components that have build requirements that must be evaluated in the top-level
project make pass, you can create a file called ``Makefile.projbuild`` in the
component directory. This makefile is included when ``project.mk`` is evaluated.

For example, if your component needs to add to CFLAGS for the entire
project (not just for its own source files) then you can set
``CFLAGS +=`` in Makefile.projbuild.

``Makefile.projbuild`` files are used heavily inside esp-idf, for defining project-wide build features such as ``esptool.py`` command line arguments and the ``bootloader`` "special app".

Note that ``Makefile.projbuild`` 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 ``component.mk`` file itself. See `Optional Project-Wide Component Variables` for details.

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

KConfig.projbuild
^^^^^^^^^^^^^^^^^

This is an equivalent to `Makefile.projbuild` 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 ``component.mk`` 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`.


Example Component Makefiles
---------------------------

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

Here are some more advanced examples of ``component.mk`` makefiles:


Adding source directories
^^^^^^^^^^^^^^^^^^^^^^^^^

By default, sub-directories are ignored. If your project has sources in sub-directories
instead of in the root of the component then you can tell that to the build
system by setting ``COMPONENT_SRCDIRS``::

    COMPONENT_SRCDIRS := src1 src2

This will compile all source files in the src1/ and src2/ sub-directories
instead.

Specifying source files
^^^^^^^^^^^^^^^^^^^^^^^

The standard component.mk logic adds all .S and .c files in the source
directories as sources to be compiled unconditionally. It is possible
to circumvent that logic and hard-code the objects to be compiled by
manually setting the ``COMPONENT_OBJS`` variable to the name of the
objects that need to be generated::

    COMPONENT_OBJS := file1.o file2.o thing/filea.o thing/fileb.o anotherthing/main.o
    COMPONENT_SRCDIRS := . thing anotherthing

Note that ``COMPONENT_SRCDIRS`` must be set as well.

Adding conditional configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The configuration system can be used to conditionally compile some files
depending on the options selected in ``make menuconfig``:

``Kconfig``::

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

``component.mk``::

    COMPONENT_OBJS := foo_a.o foo_b.o

    ifdef CONFIG_FOO_BAR
    COMPONENT_OBJS += foo_bar.o foo_bar_interface.o
    endif

See the `GNU Make Manual` for conditional syntax that can be used use in makefiles.


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::

    COMPONENT_EXTRA_CLEAN := logo.h

    graphics_lib.o: logo.h

    logo.h: $(COMPONENT_PATH)/logo.bmp
        bmp2h -i $^ -o $@


In this example, graphics_lib.o and 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 needs to be cleaned when make clean is called which
why it is added to the COMPONENT_EXTRA_CLEAN variable.

Cosmetic Improvements
^^^^^^^^^^^^^^^^^^^^^

Because logo.h is a generated file, it needs to be cleaned when make
clean is called which why it is added to the COMPONENT_EXTRA_CLEAN
variable.

Adding logo.h to the ``graphics_lib.o`` dependencies causes it to be
generated before ``graphics_lib.c`` is compiled.

If a a source file in another component included ``logo.h``, then this
component's name would have to be added to the other component's
``COMPONENT_DEPENDS`` list to ensure that the components were built
in-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 set a variable COMPONENT_EMBED_FILES in component.mk, giving the names of the files to embed in this way::

  COMPONENT_EMBED_FILES := server_root_cert.der

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

  COMPONENT_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 COMPONENT_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.

For an example of using this technique, see examples/04_https_request - the certificate file contents are loaded from the text .pem file at compile time.


Fully Overriding The Component Makefile
---------------------------------------

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 setting COMPONENT_OWNBUILDTARGET and
possibly  COMPONENT_OWNCLEANTARGET and defining your own targets named ``build`` and ``clean`` in ``component.mk``
target. The build target can do anything as long as it creates
$(COMPONENT_LIBRARY) for the project make process to link into the app binary.

(Actually, even this is not strictly necessary - if the COMPONENT_ADD_LDFLAGS variable
is set then the component can instruct the linker to link other binaries instead.)


.. _esp-idf-template: https://github.com/espressif/esp-idf-template
.. _GNU Make Manual: https://www.gnu.org/software/make/manual/make.html
.. _[_f1]: Actually, some components in esp-idf are "pure configuration" components that don't have a component.mk file, only a Makefile.projbuild and/or Kconfig.projbuild file. However, these components are unusual and most components have a component.mk file.