What Classes Do

A .bbclass file is a reusable module of BitBake code. When a recipe calls inherit cmake, it pulls in the entire text of cmake.bbclass as if it had been written at the bottom of the recipe — providing pre-built do_configure, do_compile, and do_install task definitions tuned for CMake projects.

Classes solve the "2000 recipes all need the same 50-line configure task" problem. They can:

• Define or override task functions
• Add new tasks with addtask
• Set default variable values
• Register BitBake event handlers
• Export functions to shell

EXPORT_FUNCTIONS

When a class defines a task that a recipe might want to override while still calling the class version, EXPORT_FUNCTIONS is used:

# In cmake.bbclass:
cmake_do_configure() {
    cmake -DCMAKE_TOOLCHAIN_FILE=... ${EXTRA_OECMAKE} ${S}
}
EXPORT_FUNCTIONS do_configure

After EXPORT_FUNCTIONS do_configure, the class version is stored as cmake_do_configure. A recipe can override do_configure and still call cmake_do_configure:

# In a recipe:
inherit cmake

do_configure:prepend() {
    export MY_ENV_VAR="value"   # runs before cmake's do_configure
}
# cmake_do_configure still runs; :prepend just adds a preamble

Build System Classes

Class	inherit Keyword	What it provides
autotools	autotools	./configure && make && make install DESTDIR=${D}
cmake	cmake	cmake -B build && cmake --build && cmake --install
meson	meson	meson setup && ninja && ninja install
cargo	cargo	Rust crates via cargo build
go	go	Go modules
setuptools3	setuptools3	Python packages via setup.py
python3-poetry-core	python3-poetry-core	Python packages with Poetry

Packaging and Integration Classes

Class	Purpose
pkgconfig	Generates .pc files; sets PKG_CONFIG_PATH for DEPENDS resolution
update-rc.d	Installs SysV init scripts into /etc/rcX.d/
systemd	Installs and enables systemd units; uses SYSTEMD_SERVICE variable
useradd	Creates user accounts on target via USERADD_PARAM
mime	Registers MIME types with update-mime-database
gettext	Handles i18n and locale file packaging

Image Assembly Classes

Class	Purpose
image	Core rootfs assembly: do_rootfs, do_image
core-image	Extends image; adds IMAGE_FEATURES → package group mapping
image-buildinfo	Writes build metadata (date, machine, layers) into the rootfs
populate_sdk	Generates standard SDK (.sh installer)
populate_sdk_ext	Generates extensible SDK (eSDK) with embedded devtool

Utility Classes

Class	Purpose
native	Rebase recipe to build for the host machine
nativesdk	Rebase recipe to build inside the SDK environment
cross	Used internally by cross-compiler recipes (gcc, binutils)
devshell	Provides do_devshell task — opens shell with recipe's full build env
externalsrc	Build from a local directory instead of fetching (EXTERNALSRC)
cve-check	Scans recipe CPE identifiers against the NVD CVE database
buildstats	Records per-task build time statistics

image.bbclass Internals

# (simplified from meta/classes-recipe/image.bbclass)
python do_rootfs() {
    # 1. Create an empty rootfs staging directory
    # 2. Install all packages in IMAGE_INSTALL using the selected package manager
    #    (rpm/ipk/deb depending on PACKAGE_CLASSES)
    # 3. Run ROOTFS_POSTPROCESS_COMMAND hooks
    #    (locale generation, ldconfig, runinterp, etc.)
    # 4. Remove dev files if "dev-pkgs" not in IMAGE_FEATURES
    # 5. Run IMAGE_POSTPROCESS_COMMAND hooks
}

python do_image() {
    # For each type in IMAGE_FSTYPES:
    #   Call the corresponding image type handler
    #   e.g., image_types_wic.bbclass for "wic"
    #         image_types.bbclass for "ext4", "squashfs", "tar.gz"
}

IMAGE_FEATURES Reference

IMAGE_FEATURES is processed by core-image.bbclass to translate feature names into package group additions:

IMAGE_FEATURES += "ssh-server-openssh debug-tweaks tools-debug"

Feature	What it installs	Use case
ssh-server-openssh	OpenSSH sshd	Remote login
ssh-server-dropbear	Dropbear SSH	Smaller SSH footprint
debug-tweaks	Empty root password, allowed passwordless sudo	Development only
tools-debug	gdb, strace, valgrind	Runtime debugging
tools-sdk	gcc, make, pkg-config on target	On-device compilation
dev-pkgs	All -dev packages for installed libs	Linking on target
read-only-rootfs	Configures rootfs for read-only mount	Factory/production
package-management	Includes package manager (rpm/opkg/dpkg)	OTA updates

Writing a Custom Class

# meta-myproduct/classes/deploy-strip.bbclass

# Append to do_install for every recipe that inherits this class
do_install:append() {
    # Strip debug symbols from shared libraries to reduce image size
    find ${D} -name "*.so*" -type f \
        -exec ${STRIP} --strip-unneeded {} \;
}

# Define and register a new task
python do_validate_licenses() {
    lic = d.getVar('LICENSE')
    if 'GPL-3' in lic:
        bb.warn("Recipe %s uses GPL-3: verify license policy" % d.getVar('PN'))
}
addtask validate_licenses after do_configure before do_compile

Contributors: Vasu Grover

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