Build System (Legacy GNU Make)

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Note

Since ESP-IDF V4.0, the default build system is based on CMake. This documentation is for the legacy build system based on GNU Make. Support for this build system may be removed in future major releases.

This document explains the legacy GNU Make Espressif IoT Development Framework build system and the concept of “components”

Read this document if you want to know how to organise an ESP-IDF project using GNU Make build system.

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 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 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 sets 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 COMPONENT_DIRS includes this component but you can modify this variable (or set EXTRA_COMPONENT_DIRS) to look for components in other places.

  • “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, $(PROJECT_PATH)/main 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.

  • COMPONENTS: A list of component names to build into the project. Defaults to all components found in the COMPONENT_DIRS directories.

  • EXCLUDE_COMPONENTS: Optional list of component names to exclude during the build process. Note that this decreases build time, but not binary size.

  • TEST_EXCLUDE_COMPONENTS: Optional list of component names to exclude during the build process of unit tests.

Any paths in these Makefile variables should be absolute paths. You can convert relative paths using $(PROJECT_PATH)/xxx, $(IDF_PATH)/xxx, or use the Make function $(abspath xxx).

These variables should all be set before the line include $(IDF_PATH)/make/project.mk in the Makefile.

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 directory that contains a component.mk 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 component.mk file), or they can be top-level directories whose subdirectories are components.

Running the make list-components target dumps many of these variables and can help debug the discovery of component directories.

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 the idf components first, the project components second and optionally the components in EXTRA_COMPONENT_DIRS last. If two or more of these directories contain component subdirectories 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 simply copying the component from the esp-idf component directory to the project component tree and then modifying it there. If used in this way, the esp-idf directory itself can remain untouched.

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, *.cc, *.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 difference 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.

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.

  • IDF_VER: ESP-IDF version, retrieved from either $(IDF_PATH)/version.txt file (if present) else using git command git describe. Recommended format here is single liner that specifies major IDF release version, e.g. v2.0 for a tagged release or v2.0-275-g0efaa4f for an arbitrary commit. Application can make use of this by calling esp_get_idf_version().

  • 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 ESP_IDF_VERSION_* variables, but different IDF_VER values.

  • 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 Makefile file, its value will be used.

    • Else, if the $PROJECT_PATH/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”.

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.

  • COMPONENT_CONFIG_ONLY: If set, this flag indicates that the component produces no built output at all (ie COMPONENT_LIBRARY is not built), and most other component variables are ignored. This flag is used for IDF internal components which contain only KConfig.projbuild and/or Makefile.projbuild files to configure the project, but no source files.

  • 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.

  • COMPONENT_ADD_LDFRAGMENTS: Paths to linker fragment files for the linker script generation functionality. See Linker Script Generation.

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.

Preprocessor Definitions

ESP-IDF build systems 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 — ESP-IDF version, see Preset Component Variables for more details.

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.

  • This component wrapper includes all component Makefile.componentbuild files, making any recipes, variables etc in these files available to every component.

  • 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.

Running Make Non-Interactively

When running make in a situation where you don’t want interactive prompts (for example: inside an IDE or an automated build system) append BATCH_BUILD=1 to the make arguments (or set it as an environment variable).

Setting BATCH_BUILD implies the following:

  • Verbose output (same as V=1, see below). If you don’t want verbose output, also set V=0.

  • If the project configuration is missing new configuration items (from new components or esp-idf updates) then the project use the default values, instead of prompting the user for each item.

  • If the build system needs to invoke menuconfig, an error is printed and the build fails.

Advanced Make Targets

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

  • make erase_flash and make erase_otadata will use esptool.py to erase the entire flash chip and the OTA selection setting from the flash chip, respectively.

  • make size prints some size information about the app. make size-components and make size-files are similar targets which print more detailed per-component or per-source-file information, respectively.

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.

Warning On Undefined Variables

By default, the build process will print a warning if an undefined variable is referenced (like $(DOES_NOT_EXIST)). This can be useful to find errors in variable names.

If you don’t want this behaviour, it can be disabled in menuconfig’s top level menu under SDK tool configuration.

Note that this option doesn’t trigger a warning if ifdef or ifndef are used in Makefiles.

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.

Makefile.componentbuild

For components that e.g. include tools to generate source files from other files, it is necessary to be able to add recipes, macros or variable definitions into the component build process of every components. This is done by having a Makefile.componentbuild in a component directory. This file gets included in component_wrapper.mk, before the component.mk of the component is included. As with the Makefile.projbuild, take care with these files: as they’re included in each component build, a Makefile.componentbuild error may only show up when compiling an entirely different component.

Configuration-Only Components

Some special components which contain no source files, only Kconfig.projbuild and Makefile.projbuild, may set the flag COMPONENT_CONFIG_ONLY in the component.mk file. If this flag is set, most other component variables are ignored and no build step is run for the component.

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. For this, ESP-IDF has the compile_only_if and compile_only_if_not macros:

Kconfig:

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

component.mk:

$(call compile_only_if,$(CONFIG_FOO_ENABLE_BAR),bar.o)

As can be seen in the example, the compile_only_if macro takes a condition and a list of object files as parameters. If the condition is true (in this case: if the BAR feature is enabled in menuconfig) the object files (in this case: bar.o) will always be compiled. The opposite goes as well: if the condition is not true, bar.o will never be compiled. compile_only_if_not does the opposite: compile if the condition is false, not compile if the condition is true.

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

component.mk:

# If LCD is enabled, compile interface to it, otherwise compile dummy interface
$(call compile_only_if,$(CONFIG_ENABLE_LCD_OUTPUT),lcd-real.o lcd-spi.o)
$(call compile_only_if_not,$(CONFIG_ENABLE_LCD_OUTPUT),lcd-dummy.o)

#We need font if either console or plot is enabled
$(call compile_only_if,$(or $(CONFIG_ENABLE_LCD_CONSOLE),$(CONFIG_ENABLE_LCD_PLOT)), font.o)

Note the use of the Make ‘or’ function to include the font file. Other substitution functions, like ‘and’ and ‘if’ will also work here. Variables that do not come from menuconfig can also be used: ESP-IDF uses the default Make policy of judging a variable which is empty or contains only whitespace to be false while a variable with any non-whitespace in it is true.

(Note: Older versions of this document advised conditionally adding object file names to COMPONENT_OBJS. While this still is possible, this will only work when all object files for a component are named explicitely, and will not clean up deselected object files in a make clean pass.)

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

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 the “main” component of the file_serving example protocols/http_server/file_serving/main/component.mk - two files are loaded at build time and linked into the firmware.

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 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 overridden then the component can instruct the linker to link other binaries instead.)

Note

When using an external build process with PSRAM, remember to add -mfix-esp32-psram-cache-issue to the C compiler arguments. See CONFIG_SPIRAM_CACHE_WORKAROUND for details of this flag.

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 running make defconfig, or creating a new config from scratch.

To override the name of this file, set the SDKCONFIG_DEFAULTS environment variable.

Save flash arguments

There’re 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. We can use command make print_flash_cmd, it will print the flash arguments:

--flash_mode dio --flash_freq 40m --flash_size detect 0x1000 bootloader/bootloader.bin 0x10000 example_app.bin 0x8000 partition_table_unit_test_app.bin

Then use flash arguments as the arguemnts for esptool write_flash arguments:

python esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 921600 --before default_reset --after hard_reset write_flash -z --flash_mode dio --flash_freq 40m --flash_size detect 0x1000 bootloader/bootloader.bin 0x10000 example_app.bin 0x8000 partition_table_unit_test_app.bin

Building the Bootloader

The bootloader is built by default as part of “make all”, or can be built standalone via “make bootloader-clean”. There is also “make bootloader-list-components” to see the components included in the bootloader build.

The component in IDF components/bootloader is special, as the second stage bootloader is a separate .ELF and .BIN file to the main project. However it shares its configuration and build directory with the main project.

This is accomplished by adding a subproject under components/bootloader/subproject. This subproject has its own Makefile, but it expects to be called from the project’s own Makefile via some glue in the components/bootloader/Makefile.projectbuild file. See these files for more details.

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