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+Build System
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+------------
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+
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+This document explains the Espressif IoT Development Framework build system and the
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+concept of "components"
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+
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+Read this document if you want to know how to organise a new ESP-IDF project.
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+
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+We recommend using the esp-idf-template_ project as a starting point for your project.
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+
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+Overview
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+========
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+
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+An ESP-IDF project can be seen as an almagation of a number of components.
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+For example, for a webserver that shows the current humidity, we would
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+have:
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+
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+- The ESP32 base libraries (libc, rom bindings etc)
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+- The WiFi drivers
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+- A TCP/IP stack
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+- The FreeRTOS operating system
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+- A webserver
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+- A driver for an humidity sensor
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+- Main code tying it all together
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+
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+ESP-IDF makes these components explicit and configurable. To do that, when a project
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+is compiled, the build environment will look up all the components in the
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+ESP-IDF directories, the project directories and optionally custom other component
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+directories. It then allows the user to configure compile-time options using
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+a friendly text-based menu system to customize the ESP-IDF as well as other components
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+to the requirements of the project. After the components are customized, the
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+build process will compile everything into an output file, which can then be uploaded
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+into a board in a way that can also be defined by components.
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+
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+A project in this sense is defined as a directory under which all the files required
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+to build it live, excluding the ESP-IDF files and the toolchain. A simple project
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+tree might look like this::
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+
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+ - myProject/ - build/
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+ - components/ - component1/ - component.mk
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+ - Kconfig
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+ - src1.c
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+ - component2/ - component.mk
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+ - Kconfig
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+ - src1.c
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+ - main/ - src1.c
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+ - src2.c
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+ - Makefile
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+
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+As we can see, a project consists of a components/ subdirectory containing its
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+components as well as one or more directories containing the project-specific
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+sources; by default a single directory called 'main' is assumed. The project
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+directory will also have a Makefile where the projects name as well as optionally
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+other options are defined. After compilation, the project directory will contain
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+a 'build'-directory containing all of the objects, libraries and other generated
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+files as well as the final binary.
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+
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+Components also have a custom makefile - ``component.mk``. This contains various definititions
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+influencing the build process of the component as well as the project it's used
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+in. Components may also include a Kconfig file defining the compile-time options that are
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+settable by means of the menu system.
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+
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+Project makefile variables that can be set by the programmer::
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+
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+ PROJECT_NAME: Mandatory. Name for the project
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+ BUILD_DIR_BASE: Set the directory where all objects/libraries/binaries end up in.
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+ Defaults to $(PROJECT_PATH)/build
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+ COMPONENT_DIRS: Search path for components. Defaults to the component/ directories
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+ in the ESP-IDF path and the project path.
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+ COMPONENTS: A list of component names. Defaults to all the component found in the
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+ COMPONENT_DIRS directory
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+ EXTRA_COMPONENT_DIRS: Defaults to unset. Use this to add directories to the default
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+ COMPONENT_DIRS.
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+ SRCDIRS: Directories under the project dir containing project-specific sources.
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+ Defaults to 'main'. These are treated as 'lite' components: they do not have
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+ include directories that are passed to the compilation pass of all components and
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+ they do not have a Kconfig option.
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+
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+Component makefile variables that can be set by the programmer::
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+
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+ COMPONENT_ADD_INCLUDEDIRS: Relative path to include directories to be added to
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+ the entire project
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+ COMPONENT_PRIV_INCLUDEDIRS: Relative path to include directories that are only used
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+ when compiling this specific component
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+ COMPONENT_DEPENDS: Names of any components that need to be compiled before this component.
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+ COMPONENT_ADD_LDFLAGS: Ld flags to add for this project. Defaults to -l$(COMPONENT_NAME).
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+ Add libraries etc in the current directory as $(abspath libwhatever.a)
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+ COMPONENT_EXTRA_INCLUDES: Any extra include paths. These will be prefixed with '-I' and
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+ passed to the compiler; please put absolute paths here.
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+ COMPONENT_SRCDIRS: Relative directories to look in for sources. Defaults to '.', the current
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+ directory (the root of the component) only. Use this to specify any subdirectories. Note
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+ that specifying this overwrites the default action of compiling everything in the
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+ components root dir; to keep this behaviour please also add '.' as a directory in this
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+ list.
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+ COMPONENT_OBJS: Object files to compile. Defaults to the .o variants of all .c and .S files
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+ that are found in COMPONENT_SRCDIRS.
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+ COMPONENT_EXTRA_CLEAN: Files that are generated using rules in the components Makefile
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+ that also need to be cleaned
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+ COMPONENT_BUILDRECIPE: Recipe to build the component. Optional. Defaults to building all
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+ COMPONENT_OBJS and linking them into lib(componentname).a
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+ COMPONENT_CLEANRECIPE: Recipe to clean the component. Optional. Defaults to removing
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+ all built objects and libraries.
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+ COMPONENT_BUILD_DIR: Equals the cwd of the component build, which is the build dir
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+ of the component (where all the .o etc files should be created).
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+
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+These variables are already set early on in the Makefile and the values in it will
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+be usable in component or project Makefiles::
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+
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+ CC, LD, AR, OBJCOPY: Xtensa gcc tools
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+ HOSTCC, HOSTLD etc: Host gcc tools
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+ LDFLAGS, CFLAGS: Set to usable values as defined in ESP-IDF Makefile
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+ PROJECT_NAME: Name of the project, as set in project makefile
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+ PROJECT_PATH: Path to the root of the project folder
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+ COMPONENTS: Name of the components to be included
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+ CONFIG_*: All values set by 'make menuconfig' have corresponding Makefile variables.
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+
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+For components, there also are these defines::
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+
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+ COMPONENT_PATH: Absolute path to the root of the source tree of the component we're
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+ compiling
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+ COMPONENT_LIBRARY: The full path to the static library the components compilation pass
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+ is supposed to generate
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+
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+Make Process
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+------------
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+
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+The Make process is always invoked from the project directory by the
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+user; invoking it anywhere else gives an error. This is what happens if
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+we build a binary:
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+
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+The Makefile first determines how it was included. It figures out
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+various paths as well as the components available to it. It will also
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+collect the ldflags and includes that the components specify they need.
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+It does this by running a dummy make on the components with a "get_variable"
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+target that will output these values.
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+
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+The Makefile will then create targets to build the lib*.a libraries of
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+all components and make the elf target depend on this. The main Makefile
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+invokes Make on the componen.mk of each components inside a sub-mke: this way
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+the components have full freedom to do whatever is necessary to build
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+the library without influencing other components. By default, the
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+component.mk includes the utility makefile $(IDF_PATH)/make/component_common.mk.
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+This provides default targets and configurations that will work
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+out-of-the-box for most projects.
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+
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+KConfig
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+-------
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+
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+Each component can also have a Kconfig file, alongside the component.mk, that contains
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+details to add to "menuconfig" for this component.
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+
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+Makefile.projbuild
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+------------------
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+
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+For components that have parts that need to be run when building of the
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+project is done, you can create a file called Makefile.projbuild in the
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+component root directory. This file will be included in the main
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+Makefile.
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+
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+
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+KConfig.projbuild
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+-----------------
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+
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+There's an equivalent to Makefile.projbuild for KConfig: if you want to include
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+options at the top-level, not inside the 'components' submenu then create a Kconfig.projbuild and
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+it will be included in the main menu of menuconfig.
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+
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+Take good care when (re)defining stuff here: because it's included with all the other
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+.projbuild files, it's possible to overwrite variables or re-declare targets defined in
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+the ESP-IDF makefile/Kconfig and other .projbuild files. It's generally better to just
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+create a KConfig file, if you can.
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+
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+
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+Writing Component Makefiles
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+---------------------------
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+
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+A component consists of a directory which doubles as the name for the
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+component: a component named 'httpd' lives in a directory called 'httpd'
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+Because components usually live under the project directory (although
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+they can also reside in an other folder), the path to this may be
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+something like /home/myuser/projects/myprojects/components/httpd .
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+
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+One of the things that most components will have is a component.mk makefile,
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+containing instructions on how to build the component. Because the
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+build environment tries to set reasonable defaults that will work most
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+of the time, component.mk can be very small.
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+
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+Simplest component.mk
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+=====================
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+
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+At the minimum, component.mk will just include the ESP-IDF component "common" makefile,
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+which adds common component functionality::
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+
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+This will take all the .c and .S files in the component root and compile
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+them into object files, finally linking them into a library.
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+
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+
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+Adding source directories
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+=========================
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+
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+By default, subdirectories are ignored. If your project has sources in subdirectories
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+instead of in the root of the component then you can tell that to the build
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+system by setting COMPONENT_SRCDIRS::
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+
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+ COMPONENT_SRCDIRS := src1 src2
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+This will compile all source files in the src1/ and src2/ subdirectories
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+instead.
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+
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+Specifying source files
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+=======================
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+
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+The standard component.mk logic adds all .S and .c files in the source
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+directories as sources to be compiled unconditionally. It is possible
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+to circumvent that logic and hardcode the objects to be compiled by
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+manually setting the COMPONENT_OBJS variable to the name of the
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+objects that need to be generated::
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+
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+ COMPONENT_OBJS := file1.o file2.o thing/filea.o thing/fileb.o anotherthing/main.o
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+
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+Adding conditional configuration
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+================================
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+
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+The configuration system can be used to conditionally compile some files
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+dependending on the options selected in ``make menuconfig``:
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+
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+Kconfig::
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+ config FOO_ENABLE_BAR
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+ bool "Enable the BAR feature."
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+ help
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+ This enables the BAR feature of the FOO component.
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+
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+Makefile::
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+ COMPONENT_OBJS := foo_a.o foo_b.o $(if $(CONFIG_FOO_ENABLE_BAR),foo_bar.o foo_bar_interface.o)
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+
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+Source Code Generation
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+======================
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+
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+Some components will have a situation where a source file isn't supplied
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+with the component itself but has to be generated from another file. Say
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+our component has a header file that consists of the converted binary
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+data of a BMP file, converted using a hypothetical tool called bmp2h. The
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+header file is then included in as C source file called graphics_lib.c::
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+
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+ COMPONENT_EXTRA_CLEAN := logo.h
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+
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+ graphics_lib.o: logo.h
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+
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+ logo.h: $(COMPONENT_PATH)/logo.bmp
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+ bmp2h -i $^ -o $@
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+
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+In this example, graphics_lib.o and logo.h will be generated in the
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+current directory (the build directory) while logo.bmp comes with the
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+component and resides under the component path. Because logo.h is a
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+generated file, it needs to be cleaned when make clean is called which
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+why it is added to the COMPONENT_EXTRA_CLEAN variable.
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+
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+Cosmetic Improvements
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+=====================
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+
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+The above example will work just fine, but there's one last cosmetic
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+improvement that can be done. The make system tries to make the make
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+process somewhat easier on the eyes by hiding the commands (unless you
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+run make with the V=1 switch) and this does not do that yet. Here's an
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+improved version that will output in the same style as the rest of the
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+make process::
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+
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+ COMPONENT_EXTRA_CLEAN := test_tjpgd_logo.h
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+
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+ graphics_lib.o: logo.h
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+
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+ logo.h: $(COMPONENT_PATH)/logo.bmp
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+ $(summary) BMP2H $@
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+ $(Q) bmp2h -i $^ -o $@
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+
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+ include $(IDF_PATH)/make/component_common.mk
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+
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+Fully Overriding The Component Makefile
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+---------------------------------------
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+
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+Obviously, there are cases where all these recipes are insufficient for a
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+certain component, for example when the component is basically a wrapper
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+around another third-party component not originally intended to be
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+compiled under this build system. In that case, it's possible to forego
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+the build system entirely by setting COMPONENT_OWNBUILDTARGET and
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+possibly COMPONENT_OWNCLEANTARGET and defining your own build- and clean
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+target. The build target can do anything as long as it creates
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+$(COMPONENT_LIBRARY) for the main file to link into the project binary,
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+and even that is not necessary: if the COMPONENT_ADD_LDFLAGS variable
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+is set, the component can instruct the linker to do anything else as well.
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+
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+
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+.. _esp-idf-template: https://github.com/espressif/esp-idf-template
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Angus Gratton