# Buildroot

Buildroot usage and documentation by Thomas Petazzoni. Contributions from Karsten Kruse, Ned Ludd, Martin Herren and others.

- About Buildroot
- Obtaining Buildroot
- Using Buildroot
- Customizing the generated target filesystem
- Customizing the Busybox configuration
- Customizing the uClibc configuration
- Customizing the Linux kernel configuration
- Understanding how to rebuild packages
- How Buildroot works
- Using the uClibc toolchain outside Buildroot
- Use an external toolchain
- Location of downloaded packages
- Adding new packages to Buildroot
- Creating your own board support
- Resources

## About Buildroot

Buildroot is a set of Makefiles and patches that allows you to easily generate a cross-compilation toolchain, a root filesystem and a Linux kernel image for your target. Buildroot can be used for one, two or all of these options, independently.

Buildroot is useful mainly for people working with embedded systems. Embedded systems often use processors that are not the regular x86 processors everyone is used to having in his PC. They can be PowerPC processors, MIPS processors, ARM processors, etc.

A compilation toolchain is the set of tools that allows you to compile code for your system. It consists of a compiler (in our case, `gcc`), binary utils like assembler and linker (in our case, `binutils`) and a C standard library (for example GNU Libc, uClibc or dietlibc). The system installed on your development station certainly already has a compilation toolchain that you can use to compile an application that runs on your system. If you're using a PC, your compilation toolchain runs on an x86 processor and generates code for an x86 processor. Under most Linux systems, the compilation toolchain uses the GNU libc (glibc) as the C standard library. This compilation toolchain is called the "host compilation toolchain". The machine on which it is running, and on which you're working, is called the "host system". The compilation toolchain is provided by your distribution, and Buildroot has nothing to do with it (other than using it to build a cross-compilation toolchain and other tools that are run on the development host).

As said above, the compilation toolchain that comes with your system runs on and generates code for the processor in your host system. As your embedded system has a different processor, you need a cross-compilation toolchain — a compilation toolchain that runs on your host system but generates code for your target system (and target processor). For example, if your host system uses x86 and your target system uses ARM, the regular compilation toolchain on your host runs on x86 and generates code for x86, while the cross-compilation toolchain runs on x86 and generates code for ARM.

Even if your embedded system uses an x86 processor, you might be interested in Buildroot for two reasons:

- The compilation toolchain on your host certainly uses the GNU Libc which is a complete but huge C standard library. Instead of using GNU Libc on your target system, you can use uClibc which is a tiny C standard library. If you want to use this C library, then you need a compilation toolchain to generate binaries linked with it. Buildroot can do that for you.
- Buildroot automates the building of a root filesystem with all needed tools like busybox. That makes it much easier than doing it by hand.

You might wonder why such a tool is needed when you can compile `gcc`, `binutils`, `uClibc` and all the other tools by hand. Of course doing so is possible. But, dealing with all of the configure options and problems of every `gcc` or `binutils` version is very time-consuming and uninteresting. Buildroot automates this process through the use of Makefiles and has a collection of patches for each `gcc` and `binutils` version to make them work on most architectures.

Moreover, Buildroot provides an infrastructure for reproducing the build process of your kernel, cross-toolchain, and embedded root filesystem. Being able to reproduce the build process will be useful when a component needs to be patched or updated or when another person is supposed to take over the project.

## Obtaining Buildroot

Buildroot releases are made approximately every 3 months. Direct Git access and daily snapshots are also available if you want more bleeding edge.

Releases are available at http://buildroot.net/downloads/.

The latest snapshot is always available at http://buildroot.net/downloads/snapshots/buildroot-snapshot.tar.bz2, and previous snapshots are also available at http://buildroot.net/downloads/snapshots/.

To download Buildroot using Git you can simply follow the rules described on the "Accessing Git" page (http://buildroot.net/git.html) of the Buildroot website (http://buildroot.net). For the impatient, here's a quick recipe:

     $ git clone git://git.buildroot.net/buildroot

## Using Buildroot

Buildroot has a nice configuration tool similar to the one you can find in the Linux kernel (http://www.kernel.org/) or in Busybox (http://www.busybox.org/). Note that you can (and should) build everything as a normal user. There is no need to be root to configure and use Buildroot. The first step is to run the configuration assistant:

     $ make menuconfig

to run the curses-based configurator, or

     $ make xconfig

to run the Qt3-based configurator.

Both of these "make" commands will need to build a configuration utility, so you may need to install "development" packages for relevent libraries used by the configuration utilities. On Debian-like systems, the `libncurses5-dev` package is required to use the menuconfig interface, and the `libqt3-mt-dev` is required to use the xconfig interface.

For each menu entry in the configuration tool, you can find associated help that describes the purpose of the entry.

Once everything is configured, the configuration tool generates a `.config` file that contains the description of your configuration. It will be used by the Makefiles to do what's needed.

Let's go:

     $ make

This command will generally perform the following steps:

- Download source files (as required)
- Configure cross-compile toolchain
- Build/install cross-compile toolchain
- Build/install selected target packages
- Build a kernel image
- Create a root filesystem in selected formats

Some of the above steps might not be performed if they are not selected in the Buildroot configuration.

Buildroot output is stored in a single directory, `output/`. This directory contains several subdirectories:

- `images/` where all the images (kernel image, bootloader and root filesystem images) are stored.
- `build/` where all the components except for the cross-compilation toolchain are built (this includes tools needed to run Buildroot on the host and packages compiled for the target). The `build/` directory contains one subdirectory for each of these components.
- `staging/` which contains a hierarchy similar to a root filesystem hierarchy. This directory contains the installation of the cross-compilation toolchain and all the userspace packages selected for the target. However, this directory is not intended to be the root filesystem for the target: it contains a lot of development files, unstripped binaries and libraries that make it far too big for an embedded system. These development files are used to compile libraries and applications for the target that depend on other libraries.
- `target/` which contains almost the root filesystem for the target: everything needed is present except the device files in `/dev/` (Buildroot can't create them because Buildroot doesn't run as root and does not want to run as root). Therefore, this directory should not be used on your target. Instead, you should use one of the images built in the `images/` directory. If you need an extracted image of the root filesystem for booting over NFS, then use the tarball image generated in `images/` and extract it as root.
  Compared to `staging/`, `target/` contains only the files and libraries needed to run the selected target applications: the development files (headers, etc.) are not present.
- `host/` contains the installation of tools compiled for the host that are needed for the proper execution of Buildroot except for the cross-compilation toolchain which is installed under `staging/`.
- `toolchain/` contains the build directories for the various components of the cross-compilation toolchain.

### Offline builds

If you intend to do an offline build and just want to download all sources that you previously selected in the configurator (menuconfig or xconfig), then issue:

     $ make source

You can now disconnect or copy the content of your `dl` directory to the build-host.

### Building out-of-tree

Buildroot supports building out of tree with a syntax similar to the Linux kernel. To use it, add O=<directory> to the make command line:

     $ make O=/tmp/build

All the output files will be located under `/tmp/build`.

### Environment variables

Buildroot also honors some environment variables when they are passed to `make`:

- `HOSTCXX`, the host C++ compiler to use
- `HOSTCC`, the host C compiler to use
- `UCLIBC_CONFIG_FILE=<path/to/.config>`, path to the uClibc configuration file to use to compile uClibc if an internal toolchain is being built
- `BUSYBOX_CONFIG_FILE=<path/to/.config>`, path to the Busybox configuration file
- `LINUX26_KCONFIG=<path/to/.config>`, path to the Linux kernel configuration file
- `BUILDROOT_COPYTO`, an additional location to which the binary images of the root filesystem, kernel, etc. built by Buildroot are copied
- `BUILDROOT_DL_DIR` to override the directory in which Buildroot stores/retrieves downloaded files

An example that uses config files located in the toplevel directory and in your $HOME:

    $ make UCLIBC_CONFIG_FILE=uClibc.config BUSYBOX_CONFIG_FILE=$HOME/bb.config

If you want to use a compiler other than the default `gcc` or `g++` for building helper-binaries on your host, then do

    $ make HOSTCXX=g++-4.3-HEAD HOSTCC=gcc-4.3-HEAD

If you want the result of your build to be copied to another directory like /tftpboot for downloading to a board using tftp, then you can use BUILDROOT_COPYTO to specify your location

Typically, this is set in your ~/.bashrc file

    $ export BUILDROOT_COPYTO=/tftpboot

## Customizing the generated target filesystem

There are a few ways to customize the resulting target filesystem:

- Customize the target filesystem directly and rebuild the image. The target filesystem is available under `output/target/`. You can simply make your changes here and run make afterwards — this will rebuild the target filesystem image. This method allows you to do anything to the target filesystem, but if you decide to completely rebuild your toolchain and tools, these changes will be lost.
- Customize the target filesystem skeleton available under `target/generic/target_skeleton/`. You can customize configuration files or other stuff here. However, the full file hierarchy is not yet present because it's created during the compilation process. Therefore, you can't do everything on this target filesystem skeleton, but changes to it do remain even if you completely rebuild the cross-compilation toolchain and the tools.
  You can also customize the `target/generic/device_table.txt` file which is used by the tools that generate the target filesystem image to properly set permissions and create device nodes.
  These customizations are deployed into `output/target/` just before the actual image is made. Simply rebuilding the image by running make should propagate any new changes to the image.
- Add support for your own target in Buildroot so that you have your own target skeleton (see this section for details).
- In the Buildroot configuration, you can specify the path to a post-build script that gets called after Buildroot builds all the selected software but before the the rootfs packages are assembled. The destination root filesystem folder is given as the first argument to this script, and this script can then be used to copy programs, static data or any other needed file to your target filesystem.
  You should, however, use this feature with care. Whenever you find that a certain package generates wrong or unneeded files, you should fix that package rather than work around it with a post-build cleanup script.
- A special package, customize, stored in `package/customize` can be used. You can put all the files that you want to see in the final target root filesystem in `package/customize/source` and then enable this special package in the configuration system.

## Customizing the Busybox configuration

Busybox is very configurable, and you may want to customize it. You can follow these simple steps to do so. This method isn't optimal, but it's simple and it works:

1. Do an initial compilation of Buildroot with busybox without trying to customize it.
2. Invoke `make busybox-menuconfig`. The nice configuration tool appears, and you can customize everything.
3. Run the compilation of Buildroot again.

Otherwise, you can simply change the `package/busybox/busybox-<version>.config` file if you know the options you want to change without using the configuration tool.

If you want to use an existing config file for busybox, then see section environment variables.

## Customizing the uClibc configuration

Just like BusyBox, uClibc offers a lot of configuration options. They allow you to select various functionalities depending on your needs and limitations.

The easiest way to modify the configuration of uClibc is to follow these steps:

1. Do an initial compilation of Buildroot without trying to customize uClibc.
2. Invoke `make uclibc-menuconfig`. The nice configuration assistant, similar to the one used in the Linux kernel or Buildroot, appears. Make your configuration changes as appropriate.
3. Copy the `.config` file to `toolchain/uClibc/uClibc.config` or `toolchain/uClibc/uClibc.config-locale`. The former is used if you haven't selected locale support in Buildroot configuration, and the latter is used if you have selected locale support.
4. Run the compilation of Buildroot again.

Otherwise, you can simply change `toolchain/uClibc/uClibc.config` or `toolchain/uClibc/uClibc.config-locale` without running the configuration assistant.

If you want to use an existing config file for uclibc, then see section environment variables.

## Customizing the Linux kernel configuration

The Linux kernel configuration can be customized just like BusyBox and uClibc using `make linux26-menuconfig`. Make sure you have enabled the kernel build in `make menuconfig` first. Once done, run `make` to (re)build everything.

If you want to use an existing config file for Linux, then see section environment variables.

## Understanding how to rebuild packages

One of the most common questions asked by Buildroot users is how to rebuild a given package or how to remove a package without rebuilding everything from scratch.

Removing a package is currently unsupported by Buildroot without rebuilding from scratch. This is because Buildroot doesn't keep track of which package installs what files in the `output/staging` and `output/target` directories. However, implementing clean package removal is on the TODO-list of Buildroot developers.

The easiest way to rebuild a single package from scratch is to remove its build directory in `output/build`. Buildroot will then re-extract, re-configure, re-compile and re-install this package from scratch.

However, if you don't want to rebuild the package completely from scratch, a better understanding of the Buildroot internals is needed. Internally, to keep track of which steps have been done and which steps remain to be done, Buildroot maintains stamp files (empty files that just tell whether this or that action has been done). The problem is that these stamp files are not uniformely named and handled by the different packages, so some understanding of the particular package is needed.

For packages relying on Buildroot packages infrastructures (see this section for details), the following stamp files are relevent:

- `output/build/packagename-version/.stamp_configured`. If removed, Buildroot will trigger the recompilation of the package from the configuration step (execution of `./configure`).
- `output/build/packagename-version/.stamp_built`. If removed, Buildroot will trigger the recompilation of the package from the compilation step (execution of `make`).

For other packages, an analysis of the specific package.mk file is needed. For example, the zlib Makefile used to look like this (before it was converted to the generic package infrastructure):

    $(ZLIB_DIR)/.configured: $(ZLIB_DIR)/.patched
            (cd $(ZLIB_DIR); rm -rf config.cache; \
                            [...]
            )
            touch $@

    $(ZLIB_DIR)/libz.a: $(ZLIB_DIR)/.configured
            $(MAKE) -C $(ZLIB_DIR) all libz.a
            touch -c $@

If you want to trigger the reconfiguration, you need to remove `output/build/zlib-version/.configured`. If you want to trigger only the recompilation, you need to remove `output/build/zlib-version/libz.a`.

Note that most packages, if not all, will progressively be ported over the generic or the autotools infrastructure, making it much easier to rebuild individual packages.

## How Buildroot works

As mentioned above, Buildroot is basically a set of Makefiles that downloads, configures and compiles software with the correct options. It also includes patches for various software packages — mainly the ones involved in the cross-compilation tool chain (`gcc`, `binutils` and `uClibc`).

There is basically one Makefile per software package, and they are named with the `.mk` extension. Makefiles are split into three main sections:

- toolchain (in the `toolchain/` directory) contains the Makefiles and associated files for all software related to the cross-compilation toolchain: `binutils`, `ccache`, `gcc`, `gdb`, `kernel-headers` and `uClibc`.
- package (in the `package/` directory) contains the Makefiles and associated files for all user-space tools that Buildroot can compile and add to the target root filesystem. There is one sub-directory per tool.
- target (in the `target` directory) contains the Makefiles and associated files for software related to the generation of the target root filesystem image. Four types of filesystems are supported: ext2, jffs2, cramfs and squashfs. For each of them there is a sub-directory with the required files. There is also a `default/` directory that contains the target filesystem skeleton.

Each directory contains at least 2 files:

- `something.mk` is the Makefile that downloads, configures, compiles and installs the package `something`.
- `Config.in` is a part of the configuration tool description file. It describes the options related to the package.

The main Makefile performs the following steps (once the configuration is done):

1. Create all the output directories: `staging`, `target`, `build`, `stamps`, etc. in the output directory (`output/` by default, another value can be specified using `O=`)
2. Generate all the targets listed in the `BASE_TARGETS` variable. When an internal toolchain is used, this means generating the cross-compilation toolchain. When an external toolchain is used, this means checking the features of the external toolchain and importing it into the Buildroot environment.
3. Generate all the targets listed in the `TARGETS` variable. This variable is filled by all the individual components' Makefiles. Generating these targets will trigger the compilation of the userspace packages (libraries, programs), the kernel, the bootloader and the generation of the root filesystem images, depending on the configuration.

## Creating your own board support

Creating your own board support in Buildroot allows you to have a convenient place to store your project's target filesystem skeleton and configuration files for Buildroot, Busybox, uClibc, and the kernel.

Follow these steps to integrate your board in Buildroot:

1. Create a new directory in `target/device/` named after your company or organization
2. Add a line `source "target/device/yourcompany/Config.in"` in `target/device/Config.in` so that your board appears in the configuration system
3. In `target/device/yourcompany/`, create a directory for your project. This way, you'll be able to store several of your company's projects inside Buildroot.
4. Create a `target/device/yourcompany/Config.in` file that looks like the following:

       menuconfig BR2_TARGET_COMPANY
              bool "Company projects"

       if BR2_TARGET_COMPANY

       config BR2_TARGET_COMPANY_PROJECT_FOOBAR
              bool "Support for Company project Foobar"
              help
                This option enables support for Company project Foobar

       endif

   Of course, you should customize the different values to match your company/organization and your project. This file will create a menu entry that contains the different projects of your company/organization.
5. Create a `target/device/yourcompany/Makefile.in` file that looks like the following:

       ifeq ($(BR2_TARGET_COMPANY_PROJECT_FOOBAR),y)
       include target/device/yourcompany/project-foobar/Makefile.in
       endif

6. Create the `target/device/yourcompany/project-foobar/Makefile.in` file. It is recommended that you define a `BOARD_PATH` variable set to `target/device/yourcompany/project-foobar` as it will simplify further definitions. Then, the file might define one or several of the following variables:
   - `TARGET_SKELETON` to a directory that contains the target skeleton for your project. If this variable is defined, this target skeleton will be used instead of the default one. If defined, the convention is to define it to `$(BOARD_PATH)/target_skeleton` so that the target skeleton is stored in the board specific directory.
   - `TARGET_DEVICE_TABLE` to a file that contains the target device table — the list of device files (in `/dev/`) to be created by the root filesystem build procedure. If this variable is defined, the given device table will be used instead of the default one. If defined, the convention is to define it to `$(BOARD_PATH)/target_device_table.txt`. See `target/generic/device_table.txt` for an example file.
7. In the `target/device/yourcompany/project-foobar/` directory you can store configuration files for the kernel, Busybox or uClibc. You can furthermore create one or more preconfigured configuration files, referencing those files. These config files are named `something_defconfig` and are stored in the toplevel `configs/` directory. Your users will then be able to run `make something_defconfig` and get the right configuration for your project

## Using the generated toolchain outside Buildroot

You may want to compile for your target your own programs or other software that are not packaged in Buildroot. In order to do this you can use the toolchain that was generated by Buildroot.

The toolchain generated by Buildroot is located by default in `output/staging/`. The simplest way to use it is to add `output/staging/usr/bin/` to your PATH environnement variable and then to use `ARCH-linux-gcc`, `ARCH-linux-objdump`, `ARCH-linux-ld`, etc.

Important: do not try to move a gcc-3.x toolchain to another directory — it won't work because there are some hardcoded paths in the gcc-3.x configuration. If you are using a current gcc-4.x, it is possible to relocate the toolchain — but then `--sysroot` must be passed every time the compiler is called to tell where the libraries and header files are.

It is also possible to generate the Buildroot toolchain in a directory other than `output/staging` by using the `Build options -> Toolchain and header file location` options. This could be useful if the toolchain must be shared with other users.

## Location of downloaded packages

It might be useful to know that the various tarballs that are downloaded by the Makefiles are all stored in the `DL_DIR` which by default is the `dl` directory. It's useful, for example, if you want to keep a complete version of Buildroot which is know to be working with the associated tarballs. This will allow you to regenerate the toolchain and the target filesystem with exactly the same versions.

If you maintain several Buildroot trees, it might be better to have a shared download location. This can be accessed by creating a symbolic link from the `dl` directory to the shared download location:

    ln -s <shared download location> dl

Another way of accessing a shared download location is to create the `BUILDROOT_DL_DIR` environment variable. If this is set, then the value of DL_DIR in the project is overridden. The following line should be added to `"~/.bashrc"`.

    export BUILDROOT_DL_DIR <shared download location>

## Using an external toolchain

It might be useful not to use the toolchain generated by Buildroot, for example if you already have a toolchain that is known to work for your specific CPU, or if the toolchain generation feature of Buildroot is not sufficiently flexible for you (for example if you need to generate a system with glibc instead of uClibc). Buildroot supports using an external toolchain.

To enable the use of an external toolchain, go in the `Toolchain` menu, and :

- Select the `External binary toolchain` toolchain type
- Adjust the `External toolchain path` appropriately. It should be set to a path where a bin/ directory contains your cross-compiling tools
- Adjust the `External toolchain prefix` so that the prefix, suffixed with `-gcc` or `-ld` will correspond to your cross-compiling tools

If you are using an external toolchain based on uClibc, the `Core C library from the external toolchain` and `Libraries to copy from the external toolchain` options should already have correct values. However, if your external toolchain is based on glibc, you'll have to change these values according to your cross-compiling toolchain.

To generate external toolchains, we recommend using Crosstool-NG. It allows generating toolchains based on uClibc, glibc and eglibc for a wide range of architectures and has good community support.

## Adding new packages to Buildroot

This section covers how new packages (userspace libraries or applications) can be integrated into Buildroot. It also allows to understand how existing packages are integrated, which is needed to fix issues or tune their configuration.

- Package directory
- `Config.in` file
- The `.mk` file
  - Makefile for generic packages : tutorial
  - Makefile for generic packages : reference
  - Makefile for autotools-based packages : tutorial
  - Makefile for autotools-based packages : reference
  - Manual Makefile : tutorial

### Package directory

First of all, create a directory under the `package` directory for your software, for example `foo`.

Some packages have been grouped by topic in a sub-directory: `multimedia`, `java`, `databases`, `editors`, `x11r7`, `games`. If your package fits in one of these categories, then create your package directory in these.

### `Config.in` file

Then, create a file named `Config.in`. This file will contain the option descriptions related to our `libfoo` software that will be used and displayed in the configuration tool. It should basically contain :

    config BR2_PACKAGE_LIBFOO
            bool "libfoo"
            help
    	  This is a comment that explains what libfoo is.

    	  http://foosoftware.org/libfoo/

Of course, you can add other options to configure particular things in your software. You can look at examples in other packages. The syntax of the Config.in file is the same as the one for the kernel Kconfig file. The documentation for this syntax is available at http://lxr.free-electrons.com/source/Documentation/kbuild/kconfig-language.txt

Finally you have to add your new `libfoo/Config.in` to `package/Config.in` (or in a category subdirectory if you decided to put your package in one of the existing categories). The files included there are sorted alphabetically per category and are NOT supposed to contain anything but the bare name of the package.

    source "package/libfoo/Config.in"

### The `.mk` file

Finally, here's the hardest part. Create a file named `foo.mk`. It describes how the package should be downloaded, configured, built, installed, etc.

Depending on the package type, the `.mk` file must be written in a different way, using different infrastructures:

- Makefiles for generic packages (not using autotools), based on an infrastructure similar to the one used for autotools-based packages, but which requires a little more work from the developer : specify what should be done at for the configuration, compilation, installation and cleanup of the package. This infrastructure must be used for all packages that do not use the autotools as their build system. In the future, other specialized infrastructures might be written for other build systems.
  We cover them through a tutorial and a reference.
- Makefiles for autotools-based (autoconf, automake, etc.) softwares. We provide a dedicated infrastructure for such packages, since autotools is a very common build system. This infrastructure must be used for new packages that rely on the autotools as their build system.
  We cover them through a tutorial and a reference.
- Manual Makefiles. These are currently obsolete and no new manual Makefiles should be added. However, since there are still many of them in the tree and because the , we keep them documented in a tutorial.

#### Makefile for generic packages : tutorial

    01: #############################################################
    02: #
    03: # libfoo
    04: #
    05: #############################################################
    06: LIBFOO_VERSION:=1.0
    07: LIBFOO_SOURCE:=libfoo-$(LIBFOO_VERSION).tar.gz
    08: LIBFOO_SITE:=http://www.foosoftware.org/download
    09: LIBFOO_INSTALL_STAGING=YES
    10: LIBFOO_DEPENDENCIES = host-libaaa libbbb
    11:
    12: define LIBFOO_BUILD_CMDS
    13:         $(MAKE) CC=$(TARGET_CC) LD=$(TARGET_LD) -C $(@D) all
    14: endef
    15:
    16: define LIBFOO_INSTALL_STAGING_CMDS
    17:         $(INSTALL) -D $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
    18:         $(INSTALL) -D $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
    19:         cp -dpf $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
    20: endef
    21:
    22: define LIBFOO_INSTALL_TARGET_CMDS
    23:         cp -dpf $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
    24:         -$(STRIPCMP) $(STRIP_STRIP_UNNEEDED) $(TARGET_DIR)/isr/lib/libfoo.so*
    25: endef
    26:
    27: $(eval $(call GENTARGETS,package,libfoo))

The Makefile begins on line 6 to 8 by metadata informations: the version of the package (`LIBFOO_VERSION`), the name of the tarball containing the package (`LIBFOO_SOURCE`) and the Internet location at which the tarball can be downloaded (`LIBFOO_SITE`). All variables must start with the same prefix, `LIBFOO_` in this case. This prefix is always the uppercased version of the package name (see below to understand where the package name is defined).

On line 9, we specify that this package wants to install something to the staging space. This is often needed for libraries since they must install header files and other development files in the staging space. This will ensure that the commands listed in the `LIBFOO_INSTALL_STAGING_CMDS` variable will be executed.

On line 10, we specify the list of dependencies this package relies on. These dependencies are listed in terms of lower-case package names, which can be packages for the target (without the `host-` prefix) or packages for the host (with the `host-`) prefix). Buildroot will ensure that all these packages are built and installed before the current package starts its configuration.

The rest of the Makefile defines what should be done at the different steps of the package configuration, compilation and installation. `LIBFOO_BUILD_CMDS` tells what steps should be performed to build the package. `LIBFOO_INSTALL_STAGING_CMDS` tells what steps should be performed to install the package in the staging space. `LIBFOO_INSTALL_TARGET_CMDS` tells what steps should be performed to install the package in the target space.

All these steps rely on the `$(@D)` variable, which contains the directory where the source code of the package has been extracted.

Finally, on line 27, we call the `GENTARGETS` which generates, according to the variables defined previously, all the Makefile code necessary to make your package working.

#### Makefile for generic packages : reference

The `GENTARGETS` macro takes three arguments:

- The first argument is the package directory prefix. If your package is in `package/libfoo`, then the directory prefix is `package`. If your package is in `package/editors/foo`, then the directory prefix must be `package/editors`.
- The second argument is the lower-cased package name. It must match the prefix of the variables in the `.mk` file and must match the configuration option name in the `Config.in` file. For example, if the package name is `libfoo`, so the variables in the `.mk` must start with `LIBFOO_` and the configuration option in the `Config.in` file must be `BR2_PACKAGE_LIBFOO`.
- The third argument is optional. It can be used to tell if the package if a target package (cross-compiled for the target) or a host package (natively compiled for the host). If unspecified, it is assumed that it is a target package. See below for details.

For a given package, in a single `.mk` file, it is possible to call GENTARGETS twice, once to create the rules to generate a target package and once to create the rules to generate a host package:

    $(eval $(call GENTARGETS,package,libfoo))
    $(eval $(call GENTARGETS,package,libfoo,host))

This might be useful if the compilation of the target package requires some tools to be installed on the host. If the package name is `libfoo`, then the name of the package for the target is also `libfoo`, while the name of the package for the host is `host-libfoo`. These names should be used in the DEPENDENCIES variables of other packages if they depend on `libfoo` or `host-libfoo`.

The call to the `GENTARGETS` macro must be at the end of the `.mk` file, after all variable definitions.

For the target package, the `GENTARGETS` uses the variables defined by the .mk file and prefixed by the uppercased package name: `LIBFOO_*`. For target package, it uses the `HOST_LIBFOO_*`. For some variables, if the `HOST_LIBFOO_` prefixed variable doesn't exist, the package infrastructure uses the corresponding variable prefixed by `LIBFOO_`. This is done for variables that are likely to have the same value for both the target and host packages. See below for details.

The list of variables that can be set in a `.mk` file to give metadata informations is (assuming the package name is `libfoo`) :

- `LIBFOO_VERSION`, mandatory, must contain the version of the package. Note that if `HOST_LIBFOO_VERSION` doesn't exist, it is assumed to be the same as `LIBFOO_VERSION`.
  Example: `LIBFOO_VERSION=0.1.2`
- `LIBFOO_SOURCE` may contain the name of the tarball of the package. If `HOST_LIBFOO_SOURCE` is not specified, it defaults to `LIBFOO_VERSION`. If none are specified, then the value is assumed to be `packagename-$(LIBFOO_VERSION).tar.gz`.
  Example: `LIBFOO_SOURCE = foobar-$(LIBFOO_VERSION).tar.bz2`
- `LIBFOO_PATCH` may contain the name of a patch, that will be downloaded from the same location as the tarball indicated in `LIBFOO_SOURCE`. If `HOST_LIBFOO_PATCH` is not specified, it defaults to `LIBFOO_PATCH`. Also note that another mechanism is available to patch a package: all files of the form `packagename-packageversion-description.patch` present in the package directory inside Buildroot will be applied to the package after extraction.
- `LIBFOO_SITE` may contain the Internet location of the tarball of the package. If `HOST_LIBFOO_SITE` is not specified, it defaults to `LIBFOO_SITE`. If none are specified, then the location is assumed to be `http://$$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/packagename`.
  Example: `LIBFOO_SITE=http://www.foosoftware.org/libfoo`.
- `LIBFOO_DEPENDENCIES` lists the dependencies (in terms of package name) that are required for the current target package to compile. These dependencies are guaranteed to be compiled and installed before the configuration of the current package starts. In a similar way, `HOST_LIBFOO_DEPENDENCIES` lists the dependency for the current host package.
- `LIBFOO_INSTALL_STAGING` can be set to `YES` or `NO` (default). If set to `YES`, then the commands in the `LIBFOO_INSTALL_STAGING_CMDS` variables are executed to install the package into the staging directory.
- `LIBFOO_INSTALL_TARGET` can be set to `YES` (default) or `NO`. If set to `YES`, then the commands in the `LIBFOO_INSTALL_TARGET_CMDS` variables are executed to install the package into the target directory.

The recommended way to define these variables is to use the following syntax:

    LIBFOO_VERSION=2.32

Now, the variables that define what should be performed at the different steps of the build process.

- `LIBFOO_CONFIGURE_CMDS`, used to list the actions to be performed to configure the package before its compilation
- `LIBFOO_BUILD_CMDS`, used to list the actions to be performed to compile the package
- `HOST_LIBFOO_INSTALL_CMDS`, used to list the actions to be performed to install the package, when the package is a host package. The package must install its files to the directory given by `$(HOST_DIR)`. All files, including development files such as headers should be installed, since other packages might be compiled on top of this package.
- `LIBFOO_INSTALL_TARGET_CMDS`, used to list the actions to be performed to install the package to the target directory, when the package is a target package. The package must install its files to the directory given by `$(TARGET_DIR)`. Only the files required for execution of the package should be installed. Header files and documentation should not be installed.
- `LIBFOO_INSTALL_STAGING_CMDS`, used to list the actions to be performed to install the package to the staging directory, when the package is a target package. The package must install its files to the directory given by `$(STAGING_DIR)`. All development files should be installed, since they might be needed to compile other packages.
- `LIBFOO_CLEAN_CMDS`, used to list the actions to perform to clean up the build directory of the package.
- `LIBFOO_UNINSTALL_TARGET_CMDS`, used to list the actions to uninstall the package from the target directory `$(TARGET_DIR)`
- `LIBFOO_UNINSTALL_STAGING_CMDS`
, used to list the actions to uninstall the package from the staging directory `$(STAGING_DIR)`.

The preferred way to define these variables is:

    define LIBFOO_CONFIGURE_CMDS
     action 1
     action 2
     action 3
    endef

In the action definitions, you can use the following variables:

- `$(@D)`, which contains the directory in which the package source code has been uncompressed.
- `$(TARGET_CC)`, `$(TARGET_LD)`, etc. to get the target cross-compilation utilities
- `$(TARGET_CROSS)` to get the cross-compilation toolchain prefix
- Of course the `$(HOST_DIR)`, `$(STAGING_DIR)` and `$(TARGET_DIR)` variables to install the packages properly.

The last feature of the generic infrastructure is the ability to add hook more actions after existing steps. These hooks aren't really useful for generic packages, since the `.mk` file already has full control over the actions performed in each step of the package construction. The hooks are more useful for packages using the autotools infrastructure described below. But since they are provided by the generic infrastructure, they are documented here.

The following hook points are available:

- `LIBFOO_POST_PATCH_HOOKS`
- `LIBFOO_POST_CONFIGURE_HOOKS`
- `LIBFOO_POST_BUILD_HOOKS`
- `LIBFOO_POST_INSTALL_HOOKS` (for host packages only)
- `LIBFOO_POST_INSTALL_STAGING_HOOKS` (for target packages only)
- `LIBFOO_POST_INSTALL_TARGET_HOOKS` (for target packages only)

This variables are lists of variable names containing actions to be performed at this hook point. This allows several hooks to be registered at a given hook point. Here is an example:

    define LIBFOO_POST_PATCH_FIXUP
      action1
      action2
    endef

    LIBFOO_POST_PATCH_HOOKS += LIBFOO_POST_PATCH_FIXUP

#### Makefile for autotools-based packages : tutorial

First, let's see how to write a `.mk` file for an autotools-based package, with an example :

    01: #############################################################
    02: #
    03: # foo
    04: #
    05: #############################################################
    06:
    07: FOO_VERSION:=1.0
    08: FOO_SOURCE:=foo-$(FOO_VERSION).tar.gz
    09: FOO_SITE:=http://www.foosoftware.org/downloads
    10: FOO_INSTALL_STAGING = YES
    11: FOO_INSTALL_TARGET = YES
    12: FOO_CONF_OPT =  --enable-shared
    13: FOO_DEPENDENCIES = libglib2 host-pkg-config
    14:
    15: $(eval $(call AUTOTARGETS,package,foo))

On line 7, we declare the version of the package. On line 8 and 9, we declare the name of the tarball and the location of the tarball on the Web. Buildroot will automatically download the tarball from this location.

On line 10, we tell Buildroot to install the package to the staging directory. The staging directory, located in `output/staging/` is the directory where all the packages are installed, including their development files, etc. By default, packages are not installed to the staging directory, since usually, only libraries need to be installed in the staging directory: their development files are needed to compile other libraries or applications depending on them. Also by default, when staging installation is enabled, packages are installed in this location using the `make install` command.

On line 11, we tell Buildroot to also install the package to the target directory. This directory contains what will become the root filesystem running on the target. Usually, we try not to install the documentation and to install stripped versions of the binary. By default, target installation is enabled, so in fact, this line is not strictly necessary. Also by default, packages are installed in this location using the `make install-strip` command.

On line 12, we tell Buildroot to pass a custom configure option, that will be passed to the `./configure` script before configuring and building the package.

On line 13, we declare our dependencies, so that they are built before the build process of our package starts.

Finally, on line line 14, we invoke the `AUTOTARGETS` macro that generates all the Makefile rules that actually allows the package to be built.

#### Makefile for autotools packages : reference

The main macro of the autotools package infrastructure is `AUTOTARGETS`. It has the same number of arguments and the same semantic as the `GENTARGETS` macro, which is the main macro of the generic package infrastructure. For autotools packages, the ability to have target and host packages is also available (and is actually widely used).

Just like the generic infrastructure, the autotools infrastructure works by defining a number of variables before calling the `AUTOTARGETS` macro.

First, all the package meta-information variables that exist in the generic infrastructure also exist in the autotools infrastructure: `LIBFOO_VERSION`, `LIBFOO_SOURCE`, `LIBFOO_PATCH`, `LIBFOO_SITE`, `LIBFOO_SUBDIR`, `LIBFOO_DEPENDENCIES`, `LIBFOO_INSTALL_STAGING`, `LIBFOO_INSTALL_TARGET`.

A few additional variables, specific to the autotools infrastructure, can also be defined. Many of them are only useful in very specific cases, typical packages will therefore only use a few of them.

- `LIBFOO_SUBDIR` may contain the name of a subdirectory inside the package that contains the configure script. This is useful, if for example, the main configure script is not at the root of the tree extracted by the tarball. If `HOST_LIBFOO_SUBDIR` is not specified, it defaults to `LIBFOO_SUBDIR`.
- `LIBFOO_CONF_ENV`, to specify additional environment variables to pass to the configure script. By default, empty.
- `LIBFOO_CONF_OPT`, to specify additional configure options to pass to the configure script. By default, empty.
- `LIBFOO_MAKE`, to specify an alternate `make` command. This is typically useful when parallel make it enabled in the configuration (using `BR2_JLEVEL`) but that this feature should be disabled for the given package, for one reason or another. By default, set to `$(MAKE)`. If parallel building is not supported by the package, then it should do `LIBFOO_MAKE=$(MAKE1)`.
- `LIBFOO_MAKE_ENV`, to specify additional environment variables to pass to make in the build step. These are passed before the `make` command. By default, empty.
- `LIBFOO_MAKE_OPT`, to specify additional variables to pass to make in the build step. These are passed after the `make` command. By default, empty.
- `LIBFOO_AUTORECONF`, tells whether the package should be autoreconfigured or not (i.e, if the configure script and Makefile.in files should be re-generated by re-running autoconf, automake, libtool, etc.). Valid values are `YES` and `NO`. By default, the value is `NO`
- `LIBFOO_AUTORECONF_OPT` to specify additional options passed to the autoreconf program if `LIBFOO_AUTORECONF=YES`. By default, empty.
- `LIBFOO_LIBTOOL_PATCH` tells whether the Buildroot patch to fix libtool cross-compilation issues should be applied or not. Valid values are `YES` and `NO`. By default, the value is `YES`
- `LIBFOO_USE_CONFIG_CACHE` tells whether the configure script should really on a cache file that caches test results from previous configure script. Usually, this variable should be left to its default value. Only for specific packages having issues with the configure cache can set this variable to the `NO` value (but this is more a work-around than a really fix)
- `LIBFOO_INSTALL_STAGING_OPT` contains the make options used to install the package to the staging directory. By default, the value is `DESTDIR=$$(STAGING_DIR) install`, which is correct for most autotools packages. It is still possible to override it.
- `LIBFOO_INSTALL_TARGET_OPT` contains the make options used to install the package to the target directory. By default, the value is `DESTDIR=$$(TARGET_DIR) install-strip` if `BR2_ENABLE_DEBUG` is not set, and `DESTDIR=$$(TARGET_DIR) install-exec` if `BR2_ENABLE_DEBUG` is set. These default values are correct for most autotools packages, but it is still possible to override them if needed.
- `LIBFOO_CLEAN_OPT` contains the make options used to clean the package. By default, the value is `clean`.
- `LIBFOO_UNINSTALL_STAGING_OPT`, contains the make options used to uninstall the package from the staging directory. By default, the value is `DESTDIR=$$(STAGING_DIR) uninstall`.
- `LIBFOO_UNINSTALL_TARGET_OPT`, contains the make options used to uninstall the package from the target directory. By default, the value is `DESTDIR=$$(TARGET_DIR) uninstall`.

With the autotools infrastructure, all the steps required to build and install the packages are already defined, and they generally work well for most autotools-based packages. However, when required, it is still possible to customize what is done in particular step:

- By adding a post-operation hook (after extract, patch, configure, build or install). See the reference documentation of the generic infrastructure for details.
- By overriding one of the steps. For example, even if the autotools infrastructure is used, if the package `.mk` defines its own `LIBFOO_CONFIGURE_CMDS` variable, it will be used instead of the default autotools one. However, using this method should be restricted to very specific cases. Do not use it in the general case.

#### Manual Makefile : tutorial

NOTE: new manual makefiles should not be created, and existing manual makefiles should be converted either to the generic infrastructure or the autotools infrastructure. This section is only kept to document the existing manual makefiles and help understanding how they work.

         1  #############################################################
         2  #
         3  # foo
         4  #
         5  #############################################################
         6  FOO_VERSION:=1.0
         7  FOO_SOURCE:=foo-$(FOO_VERSION).tar.gz
         8  FOO_SITE:=http://www.foosoftware.org/downloads
         9  FOO_DIR:=$(BUILD_DIR)/foo-$(FOO_VERSION)
        10  FOO_BINARY:=foo
        11  FOO_TARGET_BINARY:=usr/bin/foo
        12
        13  $(DL_DIR)/$(FOO_SOURCE):
        14          $(call DOWNLOAD,$(FOO_SITE),$(FOO_SOURCE))
        15
        16  $(FOO_DIR)/.source: $(DL_DIR)/$(FOO_SOURCE)
        17          $(ZCAT) $(DL_DIR)/$(FOO_SOURCE) | tar -C $(BUILD_DIR) $(TAR_OPTIONS) -
        18          touch $@
        19
        20  $(FOO_DIR)/.configured: $(FOO_DIR)/.source
        21          (cd $(FOO_DIR); rm -rf config.cache; \
        22                  $(TARGET_CONFIGURE_OPTS) \
        23                  $(TARGET_CONFIGURE_ARGS) \
        24                  ./configure \
        25                  --target=$(GNU_TARGET_NAME) \
        26                  --host=$(GNU_TARGET_NAME) \
        27                  --build=$(GNU_HOST_NAME) \
        28                  --prefix=/usr \
        29                  --sysconfdir=/etc \
        30          )
        31          touch $@
        32
        33  $(FOO_DIR)/$(FOO_BINARY): $(FOO_DIR)/.configured
        34          $(MAKE) CC=$(TARGET_CC) -C $(FOO_DIR)
        35
        36  $(TARGET_DIR)/$(FOO_TARGET_BINARY): $(FOO_DIR)/$(FOO_BINARY)
        37          $(MAKE) DESTDIR=$(TARGET_DIR) -C $(FOO_DIR) install-strip
        38          rm -Rf $(TARGET_DIR)/usr/man
        39
        40  foo: uclibc ncurses $(TARGET_DIR)/$(FOO_TARGET_BINARY)
        41
        42  foo-source: $(DL_DIR)/$(FOO_SOURCE)
        43
        44  foo-clean:
        45          $(MAKE) prefix=$(TARGET_DIR)/usr -C $(FOO_DIR) uninstall
        46          -$(MAKE) -C $(FOO_DIR) clean
        47
        48  foo-dirclean:
        49          rm -rf $(FOO_DIR)
        50
        51 #############################################################
        52 #
        53 # Toplevel Makefile options
        54 #
        55 #############################################################
        56 ifeq ($(BR2_PACKAGE_FOO),y)
        57 TARGETS+=foo
        58 endif

First of all, this Makefile example works for a package which comprises a single binary executable. For other software, such as libraries or more complex stuff with multiple binaries, it must be adapted. For examples look at the other `*.mk` files in the `package` directory.

At lines 6-11, a couple of useful variables are defined:

- `FOO_VERSION`: The version of foo that should be downloaded.
- `FOO_SOURCE`: The name of the tarball of foo on the download website or FTP site. As you can see `FOO_VERSION` is used.
- `FOO_SITE`: The HTTP or FTP site from which foo archive is downloaded. It must include the complete path to the directory where `FOO_SOURCE` can be found.
- `FOO_DIR`: The directory into which the software will be configured and compiled. Basically, it's a subdirectory of `BUILD_DIR` which is created upon decompression of the tarball.
- `FOO_BINARY`: Software binary name. As said previously, this is an example for a package with a single binary.
- `FOO_TARGET_BINARY`: The full path of the binary inside the target filesystem.

Lines 13-14 define a target that downloads the tarball from the remote site to the download directory (`DL_DIR`).

Lines 16-18 define a target and associated rules that uncompress the downloaded tarball. As you can see, this target depends on the tarball file so that the previous target (lines 13-14) is called before executing the rules of the current target. Uncompressing is followed by touching a hidden file to mark the software as having been uncompressed. This trick is used everywhere in a Buildroot Makefile to split steps (download, uncompress, configure, compile, install) while still having correct dependencies.

Lines 20-31 define a target and associated rules that configure the software. It depends on the previous target (the hidden `.source` file) so that we are sure the software has been uncompressed. In order to configure the package, it basically runs the well-known `./configure` script. As we may be doing cross-compilation, `target`, `host` and `build` arguments are given. The prefix is also set to `/usr`, not because the software will be installed in `/usr` on your host system, but because the software will bin installed in `/usr` on the target filesystem. Finally it creates a `.configured` file to mark the software as configured.

Lines 33-34 define a target and a rule that compile the software. This target will create the binary file in the compilation directory and depends on the software being already configured (hence the reference to the `.configured` file). It basically runs `make` inside the source directory.

Lines 36-38 define a target and associated rules that install the software inside the target filesystem. They depend on the binary file in the source directory to make sure the software has been compiled. They use the `install-strip` target of the software `Makefile` by passing a `DESTDIR` argument so that the `Makefile` doesn't try to install the software in the host `/usr` but rather in the target `/usr`. After the installation, the `/usr/man` directory inside the target filesystem is removed to save space.

Line 40 defines the main target of the software — the one that will be eventually be used by the top level `Makefile` to download, compile, and then install this package. This target should first of all depend on all needed dependencies of the software (in our example, uclibc and ncurses) and also depend on the final binary. This last dependency will call all previous dependencies in the correct order.

Line 42 defines a simple target that only downloads the code source. This is not used during normal operation of Buildroot, but is needed if you intend to download all required sources at once for later offline build. Note that if you add a new package providing a `foo-source` target is mandatory to support users that wish to do offline-builds. Furthermore it eases checking if all package-sources are downloadable.

Lines 44-46 define a simple target to clean the software build by calling the Makefiles with the appropriate option. The `-clean` target should run `make clean` on $(BUILD_DIR)/package-version and MUST uninstall all files of the package from $(STAGING_DIR) and from $(TARGET_DIR).

Lines 48-49 define a simple target to completely remove the directory in which the software was uncompressed, configured and compiled. The `-dirclean` target MUST completely rm $(BUILD_DIR)/ package-version.

Lines 51-58 add the target `foo` to the list of targets to be compiled by Buildroot by first checking if the configuration option for this package has been enabled using the configuration tool. If so, it then "subscribes" this package to be compiled by adding the package to the TARGETS global variable. The name added to the TARGETS global variable is the name of this package's target, as defined on line 40, which is used by Buildroot to download, compile, and then install this package.

### Conclusion

As you can see, adding a software package to Buildroot is simply a matter of writing a Makefile using an existing example and modifying it according to the compilation process required by the package.

If you package software that might be useful for other people, don't forget to send a patch to Buildroot developers!

## Resources

To learn more about Buildroot you can visit these websites:

- http://www.uclibc.org/
- http://www.busybox.net/