OpenWrt Buildroot

Usage and documentation by Felix Fietkau, based on uClibc Buildroot documentation by Thomas Petazzoni. Contributions from Karsten Kruse, Ned Ludd, Martin Herren.

Last modification : $Id$

About OpenWrt Buildroot

OpenWrt Buildroot is a set of Makefiles and patches that allows to easily generate both a cross-compilation toolchain and a root filesystem for your Wireless Router. The cross-compilation toolchain uses uClibc (http://www.uclibc.org/), a tiny C standard library.

A compilation toolchain is the set of tools that allows 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 application that runs on your system. If you're using a PC, your compilation toolchain runs on an x86 processor and generates code for a x86 processor. Under most Linux systems, the compilation toolchain uses the GNU libc as C standard library. This compilation toolchain is called the "host compilation toolchain", and more generally, 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 OpenWrt Buildroot has nothing to do with it.

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

You might wonder why such a tool is needed when you can compile gcc, binutils, uClibc and all the tools by hand. Of course, doing so is possible. But dealing with all configure options, with all problems of every gcc or binutils version it very time-consuming and uninteresting. OpenWrt 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 the MIPS architecture of most Broadcom based Wireless Routers.

Obtaining OpenWrt Buildroot

OpenWrt Buildroot is currently available as experimental snapshots

The latest snapshot is always available at http://openwrt.org/downloads/experimental/,

Using OpenWrt Buildroot

OpenWrt 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 run everything as a normal user. There is no need to be root to configure and use the Buildroot. The first step is to run the configuration assistant:

 $ make menuconfig

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

Once everything is configured, the configuration tool has generated 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 download, configure and compile all the selected tools, and finally generate target firmware images and additional packages (depending on your selections in make menuconfig. All the target files can be found in the bin/ subdirectory. You can compile firmware images containing two different filesystem types:

jffs2 contains a writable root filesystem, which will expand to the size of your flash image. Note that you if you use the generic firmware Image, you need to pick the right image for your Flash size, because of different eraseblock sizes.

squashfs contains a read-only root filesystem using a modified squashfs filesystem with LZMA compression. When booting it, you can create a writable second filesystem, which will contain your modifications to the root filesystem, including the packages you install.

Customizing the target filesystem

There are two ways to customize the resulting target filesystem:

Customizing the Busybox configuration

Busybox is very configurable, and you may want to customize it. Its configuration is completely integrated into the main menuconfig system. You can find it under "OpenWrt Package Selection" => "Busybox Configuration"

Customizing the uClibc configuration

Just like BusyBox, uClibc offers a lot of configuration options. They allow 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. Make a first compilation of buildroot without trying to customize uClibc.
  2. Go into the directory toolchain_build_ARCH/uClibc/ and run make menuconfig. The nice configuration assistant, similar to the one used in the Linux Kernel appears. Make your configuration 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 the Buildroot configuration, and the latter is used if you have selected locale support.
  4. Run the compilation again

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

How OpenWrt Buildroot works

As said above, OpenWrt is basically a set of Makefiles that download, configure and compiles software with the correct options. It also includes some patches for various software, mainly the ones involved in the cross-compilation tool chain (gcc, binutils and uClibc).

There is basically one Makefile per software, and they are named Makefile. Makefiles are split into three sections:

Each directory contains at least 2 files :

The main Makefile do the job through the following steps (once the configuration is done):

  1. Create the download directory (dl/ by default). This is where the tarballs will be downloaded. It is interesting to know that the tarballs are in this directory because it may be useful to save them somewhere to avoid further downloads.
  2. Create the build directory (build_ARCH/ by default, where ARCH is your architecture). This is where all user-space tools while be compiled.
  3. Create the toolchain build directory (toolchain_build_ARCH/ by default, where ARCH is your architecture). This is where the cross compilation toolchain will be compiled.
  4. Setup the staging directory (build_ARCH/staging_dir/ by default). This is where the cross-compilation toolchain will be installed. If you want to use the same cross-compilation toolchain for other purposes, such as compiling third-party applications, you can add build_ARCH/staging_dir/bin to your PATH, and then use arch-linux-gcc to compile your application. In order to setup this staging directory, it first removes it, and then it creates various subdirectories and symlinks inside it.
  5. Create the target directory (build_ARCH/root/ by default) and the target filesystem skeleton. This directory will contain the final root filesystem. To setup it up, it first deletes it, then it copies the skeleton available in target/default/target_skeleton and then removes useless CVS/ directories.
  6. Call the prepare, compile and install targets for the subdirectories toolchain, package and target

Using the uClibc toolchain

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

The toolchain generated by the Buildroot by default is located in build_ARCH/staging_dir/. The simplest way to use it is to add build_ARCH/staging_dir/bin/ to your PATH environment variable, and then to use arch-linux-gcc, arch-linux-objdump, arch-linux-ld, etc.

For example, you may add the following to your .bashrc (considering you're building for the MIPS architecture and that Buildroot is located in ~/buildroot/) :

export PATH=$PATH:~/buildroot/build_mipsel/staging_dir/bin/

Then you can simply do :

mipsel-linux-uclibc-gcc -o foo foo.c

Important : do not try to move the toolchain to an other directory, it won't work. There are some hard-coded paths in the gcc configuration. If the default toolchain directory doesn't suit your needs, please refer to the Using the uClibc toolchain outside of buildroot section.

Using the uClibc toolchain outside of buildroot

By default, the cross-compilation toolchain is generated inside build_ARCH/staging_dir/. But sometimes, it may be useful to install it somewhere else, so that it can be used to compile other programs or by other users. Moving the build_ARCH/staging_dir/ directory elsewhere is not possible, because they are some hardcoded paths in the toolchain configuration.

If you want to use the generated toolchain for other purposes, you can configure Buildroot to generate it elsewhere using the option of the configuration tool : Build options -> Toolchain and header file location, which defaults to $(BUILD_DIR)/staging_dir/.

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.

Extending OpenWrt with more software

This section will only consider the case in which you want to add user-space software.

Package directory

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

Config.in file

Then, create a file named Config.in. This file will contain the portion of options description related to our foo software that will be used and displayed in the configuration tool. It should basically contain :

config BR2_PACKAGE_FOO
        tristate "foo"
        default n
        help
	     This is a comment that explains what foo is.

Of course, you can add other options to configure particular things in your software.

Makefile in the package directory

To add your package to the build process, you need to edit the Makefile in the package/ directory. Locate the lines that look like the following:

package-$(BR2_PACKAGE_FOO) += foo

As you can see, this short line simply adds the target foo to the list of targets handled by OpenWrt Buildroot.

In addition to the default dependencies, you make your package depend on another package (e.g. a library) by adding a line: 

foo-compile: bar-compile

The .control file

Additionally, you need to create a control file which contains information about your package, readable by the ipkg package utility.

The file looks like this

     1  Package: foo
     2  Priority: optional
     3  Section: net
     4  Maintainer: Foo Software <foo@foosoftware.com>
     5  Source: http://foosoftware.com
     6  Description: Your Package Description

You can skip the usual Version: and Architecture fields, as they will be generated by the make-ipkg-dir.sh script called from your Makefile

The real Makefile

Finally, here's the hardest part. Create a file named Makefile. It will contain the Makefile rules that are in charge of downloading, configuring, compiling and installing the software. Below is an example that we will comment afterwards.

     1  #############################################################
     2  # foo
     3  #############################################################
     4  PKG_NAME:=foo
     5  PKG_VERSION:=1.0
     6  PKG_RELEASE:=1
     7  PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
     8  PKG_SITE:=http://www.foosoftware.org/downloads
     9  PKG_DIR:=$(BUILD_DIR)/$(PKG_NAME)-$(PKG_VERSION)
    10  PKG_IPK:=$(PACKAGE_DIR)/$(PKG_NAME)_$(PKG_VERSION)-$(PKG_RELEASE)_$(ARCH).ipk
    11  PKG_IPK_DIR:=$(PKG_DIR)/ipkg
    12
    13  $(DL_DIR)/$(PKG_SOURCE):
    14          $(WGET) -P $(DL_DIR) $(PKG_SITE)/$(PKG_SOURCE)
    15
    16  $(PKG_DIR)/.source: $(DL_DIR)/$(PKG_SOURCE)
    17          zcat $(DL_DIR)/$(PKG_SOURCE) | tar -C $(BUILD_DIR) $(TAR_OPTIONS) -
    18          touch $(PKG_DIR)/.source
    19
    20  $(PKG_DIR)/.configured: $(PKG_DIR)/.source
    21          (cd $(PKG_DIR); \
    22                  $(TARGET_CONFIGURE_OPTS) \
    23                  CFLAGS="$(TARGET_CFLAGS)" \
    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 $(PKG_DIR)/.configured;
    32
    33  $(PKG_DIR)/foo $(PKG_DIR)/.configured
    34          $(MAKE) CC=$(TARGET_CC) -C $(PKG_DIR)
    35
    36  $(PKG_IPK): $(PKG_DIR)/$(PKG_BINARY)
    37		$(SCRIPT_DIR)/make-ipkg-dir.sh $(PKG_IPK_DIR) $(PKG_NAME).control $(PKG_VERSION)-$(PKG_RELEASE) $(ARCH)
    38          $(MAKE) prefix=$(PKG_IPK_DIR)/usr -C $(PKG_DIR) install
    39          rm -Rf $(PKG_IPK_DIR)/usr/man
    40  	$(IPKG_BUILD) $(PKG_IPK_DIR) $(PACKAGE_DIR)
    41
    42  $(IPKG_STATE_DIR)/info/$(PKG_NAME).list: $(PKG_IPK)  
    43  	$(IPKG) install $(PKG_IPK) 
    44  
    45  prepare: $(PKG_DIR)/.source
    46  compile: $(PKG_IPK)
    47  install: $(IPKG_STATE_DIR)/info/$(PKG_NAME).list
    48  clean:
    49  	rm -rf $(PKG_DIR)
    50		rm -f $(PKG_IPK)

First of all, this Makefile example works for a single binary software. For other software such as libraries or more complex stuff with multiple binaries, it should be adapted. Look at the other Makefile files in the package directory.

At lines 4-11, a couple of useful variables are defined :

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

Lines 16-18 defines 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 (line 13-14) is called before executing the rules of the current target. Uncompressing is followed by touching a hidden file to mark the software has having been uncompressed. This trick is used everywhere in Buildroot Makefile to split steps (download, uncompress, configure, compile, install) while still having correct dependencies.

Lines 20-31 defines a target and associated rules that configures 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 it, it basically runs the well-known ./configurescript. 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 in the target filesystem. Finally it creates a .configured file to mark the software as configured.

Lines 33-34 defines a target and a rule that compiles 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 36-40 defines a target and associated rules that create the ipkg package which can optionally be embedded into the resulting firmware image. It depends on the binary file in the source directory, to make sure the software has been compiled. It uses the make-ipkg-dir.sh script, which will create the ipkg build directory for your package, copy your control file into that directory and add version and architecture information. Then it calls the install target of the software Makefile by passing a prefix argument, so that the Makefile doesn't try to install the software inside host /usr but inside target /usr. After the installation, the /usr/man directory inside the target filesystem is removed to save space. Finally IPKG_BUILD is called to create the package.

Line 42 and 43 define the installation target of your package, which will embed the software into the target filesystem.

Lines 45-50 define the main targets that the Makefile in the package dir calls.

Conclusion

As you can see, adding a software to buildroot is simply a matter of writing a Makefile using an already existing example and to modify it according to the compilation process of the software.

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

Resources

To learn more about OpenWrt Buildroot you can visit this website: http://openwrt.org/