Chapter 6. Developer Guidelines

6.1. Coding style

Overall, these coding style rules are here to help you to add new files in Buildroot or refactor existing ones.

If you slightly modify some existing file, the important thing is to keep the consistency of the whole file, so you can:

  • either follow the potentially deprecated coding style used in this file,
  • or entirely rework it in order to make it comply with these rules.

6.1.1. Config.in file

Config.in files contain entries for almost anything configurable in Buildroot.

An entry has the following pattern:

config BR2_PACKAGE_LIBFOO
        bool "libfoo"
        depends on BR2_PACKAGE_LIBBAZ
        select BR2_PACKAGE_LIBBAR
        help
          This is a comment that explains what libfoo is.

          http://foosoftware.org/libfoo/
  • The bool, depends on, select and help lines are indented with one tab.
  • The help text itself should be indented with one tab and two spaces.

The Config.in files are the input for the configuration tool used in Buildroot, which is the regular Kconfig. For further details about the Kconfig language, refer to http://kernel.org/doc/Documentation/kbuild/kconfig-language.txt.

6.1.2. The .mk file

  • Header: The file starts with a header. It contains the module name, preferably in lowercase, enclosed between separators made of 80 hashes. A blank line is mandatory after the header:

    ################################################################################
    #
    # libfoo
    #
    ################################################################################
  • Assignment: use = preceded and followed by one space:

    LIBFOO_VERSION = 1.0
    LIBFOO_CONF_OPT += --without-python-support

    It is also possible to align the = signs:

    LIBFOO_VERSION   = 1.0
    LIBFOO_SOURCE    = foo-$(LIBFOO_VERSION).tar.gz
    LIBFOO_CONF_OPT += --without-python-support
  • Indentation: use tab only:

    define LIBFOO_REMOVE_DOC
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/doc \
                    $(TARGET_DIR)/usr/share/man/man3/libfoo*
    endef

    Note that commands inside a define block should always start with a tab, so make recognizes them as commands.

  • Optional dependency:

    • Prefer multi-line syntax.

      YES:

      ifeq ($(BR2_PACKAGE_PYTHON),y)
      LIBFOO_CONF_OPT += --with-python-support
      LIBFOO_DEPENDENCIES += python
      else
      LIBFOO_CONF_OPT += --without-python-support
      endif

      NO:

      LIBFOO_CONF_OPT += --with$(if $(BR2_PACKAGE_PYTHON),,out)-python-support
      LIBFOO_DEPENDENCIES += $(if $(BR2_PACKAGE_PYTHON),python,)
    • Keep configure options and dependencies close together.
  • Optional hooks: keep hook definition and assignment together in one if block.

    YES:

    ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)
    define LIBFOO_REMOVE_DATA
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
    endef
    LIBFOO_POST_INSTALL_TARGET_HOOKS += LIBFOO_REMOVE_DATA
    endif

    NO:

    define LIBFOO_REMOVE_DATA
            $(RM) -fr $(TARGET_DIR)/usr/share/libfoo/data
    endef
    
    ifneq ($(BR2_LIBFOO_INSTALL_DATA),y)
    LIBFOO_POST_INSTALL_TARGET_HOOKS += LIBFOO_REMOVE_DATA
    endif

6.1.3. The documentation

The documentation uses the asciidoc format.

For further details about the asciidoc syntax, refer to http://www.methods.co.nz/asciidoc/userguide.html.

6.2. Adding new packages to Buildroot

This section covers how new packages (userspace libraries or applications) can be integrated into Buildroot. It also shows how existing packages are integrated, which is needed for fixing issues or tuning their configuration.

6.2.1. Package directory

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

Some packages have been grouped by topic in a sub-directory: multimedia, x11r7, efl and matchbox. If your package fits in one of these categories, then create your package directory in these. New subdirectories are discouraged, however.

6.2.2. 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/

The bool line, help line and other meta-informations about the configuration option must be indented with one tab. The help text itself should be indented with one tab and two spaces, and it must mention the upstream URL of the project.

You can add other sub-options into a if BR2_PACKAGE_LIBFOO…endif statement 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://kernel.org/doc/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"

Choosing depends on or select

The Config.in file of your package must also ensure that dependencies are enabled. Typically, Buildroot uses the following rules:

  • Use a select type of dependency for dependencies on libraries. These dependencies are generally not obvious and it therefore make sense to have the kconfig system ensure that the dependencies are selected. For example, the libgtk2 package uses select BR2_PACKAGE_LIBGLIB2 to make sure this library is also enabled. The select keyword express the dependency with a backward semantic.
  • Use a depends on type of dependency when the user really needs to be aware of the dependency. Typically, Buildroot uses this type of dependency for dependencies on toolchain options (target architecture, MMU support, C library, C++ support, large file support, thread support, RPC support, IPV6 support, WCHAR support), or for dependencies on "big" things, such as the X.org system. For dependencies on toolchain options, there should be a comment that is displayed when the option is not enabled, so that the user knows why the package is not available. The depends on keyword express the dependency with a forward semantic.

Note. The current problem with the kconfig language is that these two dependency semantics are not internally linked. Therefore, it may be possible to select a package, whom one of its dependencies/requirement is not met.

An example illustrates both the usage of select and depends on.

config BR2_PACKAGE_ACL
        bool "acl"
        select BR2_PACKAGE_ATTR
        depends on BR2_LARGEFILE
        help
          POSIX Access Control Lists, which are used to define more
          fine-grained discretionary access rights for files and
          directories.
          This package also provides libacl.

          http://savannah.nongnu.org/projects/acl

comment "acl requires a toolchain with LARGEFILE support"
        depends on !BR2_LARGEFILE

Note that these two dependency types are only transitive with the dependencies of the same kind.

This means, in the following example:

config BR2_PACKAGE_A
        bool "Package A"

config BR2_PACKAGE_B
        bool "Package B"
        depends on BR2_PACKAGE_A

config BR2_PACKAGE_C
        bool "Package C"
        depends on BR2_PACKAGE_B

config BR2_PACKAGE_D
        bool "Package D"
        select BR2_PACKAGE_B

config BR2_PACKAGE_E
        bool "Package E"
        select BR2_PACKAGE_D
  • Selecting Package C will be visible if Package B has been selected, which in turn is only visible if Package A has been selected.
  • Selecting Package E will select Package D, which will select Package B, it will not check for the dependencies of Package B, so it will not select Package A.
  • Since Package B is selected but Package A is not, this violates the dependency of Package B on Package A. Therefore, in such a situation, the transitive dependency has to be added explicitly:
config BR2_PACKAGE_D
        bool "Package D"
        select BR2_PACKAGE_B
        depends on BR2_PACKAGE_A

config BR2_PACKAGE_E
        bool "Package E"
        select BR2_PACKAGE_D
        depends on BR2_PACKAGE_A

Overall, for package library dependencies, select should be preferred.

Note that such dependencies will ensure that the dependency option is also enabled, but not necessarily built before your package. To do so, the dependency also needs to be expressed in the .mk file of the package.

Further formatting details: see the coding style Section 6.1.1, “Config.in file”.

6.2.3. The .mk file

Finally, here’s the hardest part. Create a file named libfoo.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:

Further formating details: see the writing rules Section 6.1.2, “The .mk file”.

6.2.4. Infrastructure for packages with specific build systems

By packages with specific build systems we mean all the packages whose build system is not one of the standard ones, such as autotools or CMake. This typically includes packages whose build system is based on hand-written Makefiles or shell scripts.

generic-package Tutorial

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_LICENSE = GPLv3+
11: LIBFOO_LICENSE_FILES = COPYING
12: LIBFOO_INSTALL_STAGING = YES
13: LIBFOO_CONFIG_SCRIPTS = libfoo-config
14: LIBFOO_DEPENDENCIES = host-libaaa libbbb
15:
16: define LIBFOO_BUILD_CMDS
17:     $(MAKE) CC="$(TARGET_CC)" LD="$(TARGET_LD)" -C $(@D) all
18: endef
19:
20: define LIBFOO_INSTALL_STAGING_CMDS
21:     $(INSTALL) -D -m 0755 $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
22:     $(INSTALL) -D -m 0644 $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
23:     $(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
24: endef
25:
26: define LIBFOO_INSTALL_TARGET_CMDS
27:     $(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
28:     $(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
29: endef
30:
31: define LIBFOO_DEVICES
32:     /dev/foo  c  666  0  0  42  0  -  -  -
33: endef
34:
35: define LIBFOO_PERMISSIONS
36:     /bin/foo  f  4755  0  0  -  -  -  -  -
37: endef
38:
39: define LIBFOO_USERS
40:     foo -1 libfoo -1 * - - - LibFoo daemon
41: endef
42:
43: $(eval $(generic-package))

The Makefile begins on line 7 to 11 with metadata information: the version of the package (LIBFOO_VERSION), the name of the tarball containing the package (LIBFOO_SOURCE) the Internet location at which the tarball can be downloaded from (LIBFOO_SITE), the license (LIBFOO_LICENSE) and file with the license text (LIBFOO_LICENSE_FILES). 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 12, 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 13, we specify that there is some fixing to be done to some of the libfoo-config files that were installed during LIBFOO_INSTALL_STAGING_CMDS phase. These *-config files are executable shell script files that are located in $(STAGING_DIR)/usr/bin directory and are executed by other 3rd party packages to find out the location and the linking flags of this particular package.

The problem is that all these *-config files by default give wrong, host system linking flags that are unsuitable for cross-compiling.

For example: -I/usr/include instead of -I$(STAGING_DIR)/usr/include or: -L/usr/lib instead of -L$(STAGING_DIR)/usr/lib

So some sed magic is done to these scripts to make them give correct flags. The argument to be given to LIBFOO_CONFIG_SCRIPTS is the file name(s) of the shell script(s) needing fixing. All these names are relative to $(STAGING_DIR)/usr/bin and if needed multiple names can be given.

In addition, the scripts listed in LIBFOO_CONFIG_SCRIPTS are removed from $(TARGET_DIR)/usr/bin, since they are not needed on the target.

Example 6.1. Config script: divine package

Package divine installs shell script $(STAGING_DIR)/usr/bin/divine-config.

So its fixup would be:

DIVINE_CONFIG_SCRIPTS = divine-config

Example 6.2. Config script: imagemagick package:

Package imagemagick installs the following scripts: $(STAGING_DIR)/usr/bin/{Magick,Magick++,MagickCore,MagickWand,Wand}-config

So it’s fixup would be:

IMAGEMAGICK_CONFIG_SCRIPTS = \
   Magick-config Magick++-config \
   MagickCore-config MagickWand-config Wand-config

On line 14, 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, lines 16..29, 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.

On line 31..33, we define a device-node file used by this package (LIBFOO_DEVICES).

On line 35..37, we define the permissions to set to specific files installed by this package (LIBFOO_PERMISSIONS).

On lines 39..41, we define a user that is used by this package (eg. to run a daemon as non-root) (LIBFOO_USERS).

Finally, on line 43, we call the generic-package function, which generates, according to the variables defined previously, all the Makefile code necessary to make your package working.

generic-package Reference

There are two variants of the generic target. The generic-package macro is used for packages to be cross-compiled for the target. The host-generic-package macro is used for host packages, natively compiled for the host. It is possible to call both of them in a single .mk file: once to create the rules to generate a target package and once to create the rules to generate a host package:

$(eval $(generic-package))
$(eval $(host-generic-package))

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 generic-package and/or host-generic-package macro must be at the end of the .mk file, after all variable definitions.

For the target package, the generic-package uses the variables defined by the .mk file and prefixed by the uppercased package name: LIBFOO_*. host-generic-package uses the HOST_LIBFOO_* variables. 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 information 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. It can also be a revision number, branch or tag for packages that are fetched directly from their revision control system. Examples: LIBFOO_VERSION = 0.1.2 LIBFOO_VERSION = cb9d6aa9429e838f0e54faa3d455bcbab5eef057 LIBFOO_VERSION = stable
  • LIBFOO_SOURCE may contain the name of the tarball of the package. If HOST_LIBFOO_SOURCE is not specified, it defaults to LIBFOO_SOURCE. 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 a space-separated list of patch file names, that will be downloaded from the same location as the tarball indicated in LIBFOO_SOURCE, and then applied to the package source code. If HOST_LIBFOO_PATCH is not specified, it defaults to LIBFOO_PATCH. Note that patches that are included in Buildroot itself use a different mechanism: all files of the form <packagename>-*.patch present in the package directory inside Buildroot will be applied to the package after extraction (see patching a package Section 6.3, “Patching a package”). Finally, patches listed in the LIBFOO_PATCH variable are applied before the patches stored in the Buildroot package directory.
  • LIBFOO_SITE provides the location of the package, which can be a URL or a local filesystem path. HTTP, FTP and SCP are supported URL types for retrieving package tarballs. Git, Subversion, Mercurial, and Bazaar are supported URL types for retrieving packages directly from source code management systems. A filesystem path may be used to specify either a tarball or a directory containing the package source code. See LIBFOO_SITE_METHOD below for more details on how retrieval works. Note that SCP URLs should be of the form scp://[user@]host:filepath, and that filepath is relative to the user’s home directory, so you may want to prepend the path with a slash for absolute paths: scp://[user@]host:/absolutepath. If HOST_LIBFOO_SITE is not specified, it defaults to LIBFOO_SITE. Examples: LIBFOO_SITE=http://www.libfoosoftware.org/libfoo LIBFOO_SITE=http://svn.xiph.org/trunk/Tremor/ LIBFOO_SITE=git://github.com/kergoth/tslib.git LIBFOO_SITE=/opt/software/libfoo.tar.gz LIBFOO_SITE=$(TOPDIR)/../src/libfoo/
  • LIBFOO_SITE_METHOD determines the method used to fetch or copy the package source code. In many cases, Buildroot guesses the method from the contents of LIBFOO_SITE and setting LIBFOO_SITE_METHOD is unnecessary. When HOST_LIBFOO_SITE_METHOD is not specified, it defaults to the value of LIBFOO_SITE_METHOD. The possible values of LIBFOO_SITE_METHOD are:

    • wget for normal FTP/HTTP downloads of tarballs. Used by default when LIBFOO_SITE begins with http://, https:// or ftp://.
    • scp for downloads of tarballs over SSH with scp. Used by default when LIBFOO_SITE begins with scp://.
    • svn for retrieving source code from a Subversion repository. Used by default when LIBFOO_SITE begins with svn://. When a http:// Subversion repository URL is specified in LIBFOO_SITE, one must specify LIBFOO_SITE_METHOD=svn. Buildroot performs a checkout which is preserved as a tarball in the download cache; subsequent builds use the tarball instead of performing another checkout.
    • git for retrieving source code from a Git repository. Used by default when LIBFOO_SITE begins with git://. The downloaded source code is cached as with the svn method.
    • hg for retrieving source code from a Mercurial repository. One must specify LIBFOO_SITE_METHOD=hg when LIBFOO_SITE contains a Mercurial repository URL. The downloaded source code is cached as with the svn method.
    • bzr for retrieving source code from a Bazaar repository. Used by default when LIBFOO_SITE begins with bzr://. The downloaded source code is cached as with the svn method.
    • file for a local tarball. One should use this when LIBFOO_SITE specifies a package tarball as a local filename. Useful for software that isn’t available publicly or in version control.
    • local for a local source code directory. One should use this when LIBFOO_SITE specifies a local directory path containing the package source code. Buildroot copies the contents of the source directory into the package’s build directory.
  • 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 dependencies 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.
  • LIBFOO_CONFIG_SCRIPTS lists the names of the files in $(STAGING_DIR)/usr/bin that need some special fixing to make them cross-compiling friendly. Multiple file names separated by space can be given and all are relative to $(STAGING_DIR)/usr/bin. The files listed in LIBFOO_CONFIG_SCRIPTS are also removed from $(TARGET_DIR)/usr/bin since they are not needed on the target.
  • LIBFOO_DEVICES lists the device files to be created by Buildroot when using the static device table. The syntax to use is the makedevs one. You can find some documentation for this syntax in the Section 11.1, “Makedev syntax documentation”. This variable is optional.
  • LIBFOO_PERMISSIONS lists the changes of permissions to be done at the end of the build process. The syntax is once again the makedevs one. You can find some documentation for this syntax in the Section 11.1, “Makedev syntax documentation”. This variable is optional.
  • LIBFOO_USERS lists the users to create for this package, if it installs a program you want to run as a specific user (eg. as a daemon, or as a cron-job). The syntax is similar in spirit to the makedevs one, and is described in the Section 11.2, “Makeuser syntax documentation”. This variable is optional.
  • LIBFOO_LICENSE defines the license (or licenses) under which the package is released. This name will appear in the manifest file produced by make legal-info. If the license appears in the following list Section 7.2, “License abbreviations”, use the same string to make the manifest file uniform. Otherwise, describe the license in a precise and concise way, avoiding ambiguous names such as BSD which actually name a family of licenses. This variable is optional. If it is not defined, unknown will appear in the license field of the manifest file for this package.
  • LIBFOO_LICENSE_FILES is a space-separated list of files in the package tarball that contain the license(s) under which the package is released. make legal-info copies all of these files in the legal-info directory. See Chapter 7, Legal notice and licensing for more information. This variable is optional. If it is not defined, a warning will be produced to let you know, and not saved will appear in the license files field of the manifest file for this package.
  • LIBFOO_REDISTRIBUTE can be set to YES (default) or NO to indicate if the package source code is allowed to be redistributed. Set it to NO for non-opensource packages: Buildroot will not save the source code for this package when collecting the legal-info.
  • LIBFOO_FLAT_STACKSIZE defines the stack size of an application built into the FLAT binary format. The application stack size on the NOMMU architecture processors can’t be enlarged at run time. The default stack size for the FLAT binary format is only 4k bytes. If the application consumes more stack, append the required number here.

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_EXTRACT_CMDS lists the actions to be performed to extract the package. This is generally not needed as tarballs are automatically handled by Buildroot. However, if the package uses a non-standard archive format, such as a ZIP or RAR file, or has a tarball with a non-standard organization, this variable allows to override the package infrastructure default behavior.
  • LIBFOO_CONFIGURE_CMDS lists the actions to be performed to configure the package before its compilation.
  • LIBFOO_BUILD_CMDS lists the actions to be performed to compile the package.
  • HOST_LIBFOO_INSTALL_CMDS lists 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 lists 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 have to be installed. Header files, static libraries and documentation will be removed again when the target filesystem is finalized.
  • LIBFOO_INSTALL_STAGING_CMDS lists 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, lists the actions to perform to clean up the build directory of the package.
  • LIBFOO_UNINSTALL_TARGET_CMDS lists the actions to uninstall the package from the target directory $(TARGET_DIR)
  • LIBFOO_UNINSTALL_STAGING_CMDS lists the actions to uninstall the package from the staging directory $(STAGING_DIR).
  • LIBFOO_INSTALL_INIT_SYSV and LIBFOO_INSTALL_INIT_SYSTEMD list the actions to install init scripts either for the systemV-like init systems (busybox, sysvinit, etc.) or for the systemd units. These commands will be run only when the relevant init system is installed (i.e. if systemd is selected as the init system in the configuration, only LIBFOO_INSTALL_INIT_SYSTEMD will be run).

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 hooks. These define further actions to perform after existing steps. Most 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. However, since they are provided by the generic infrastructure, they are documented here. The exception is LIBFOO_POST_PATCH_HOOKS. Patching the package and producing legal info are not user definable, so LIBFOO_POST_PATCH_HOOKS and LIBFOO_POST_LEGAL_INFO_HOOKS are useful for generic packages.

The following hook points are available:

  • LIBFOO_POST_DOWNLOAD_HOOKS
  • LIBFOO_POST_EXTRACT_HOOKS
  • LIBFOO_PRE_PATCH_HOOKS
  • LIBFOO_POST_PATCH_HOOKS
  • LIBFOO_PRE_CONFIGURE_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)
  • LIBFOO_POST_LEGAL_INFO_HOOKS

These 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

6.2.5. Infrastructure for autotools-based packages

autotools-package tutorial

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

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_INSTALL_STAGING = YES
11: LIBFOO_INSTALL_TARGET = NO
12: LIBFOO_CONF_OPT = --disable-shared
13: LIBFOO_DEPENDENCIES = libglib2 host-pkgconf
14:
15: $(eval $(autotools-package))

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 not install the package to the target directory. This directory contains what will become the root filesystem running on the target. For purely static libraries, it is not necessary to install them in the target directory because they will not be used at runtime. By default, target installation is enabled; setting this variable to NO is almost never needed. Also by default, packages are installed in this location using the make install 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 15, we invoke the autotools-package macro that generates all the Makefile rules that actually allows the package to be built.

autotools-package reference

The main macro of the autotools package infrastructure is autotools-package. It is similar to the generic-package macro. The ability to have target and host packages is also available, with the host-autotools-package macro.

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

First, all the package metadata 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 is 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 $(MAKE). If parallel building is not supported by the package, then it should be set to LIBFOO_MAKE=$(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_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. The default value is correct for most autotools packages, but it is still possible to override it 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 any 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 file 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.

6.2.6. Infrastructure for CMake-based packages

cmake-package tutorial

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

01: ################################################################################
02: #
03: # libfoo
04: #
05: ################################################################################
06:
07: LIBFOO_VERSION = 1.0
08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
09: LIBFOO_SITE = http://www.foosoftware.org/download
10: LIBFOO_INSTALL_STAGING = YES
11: LIBFOO_INSTALL_TARGET = NO
12: LIBFOO_CONF_OPT = -DBUILD_DEMOS=ON
13: LIBFOO_DEPENDENCIES = libglib2 host-pkgconf
14:
15: $(eval $(cmake-package))

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 not install the package to the target directory. This directory contains what will become the root filesystem running on the target. For purely static libraries, it is not necessary to install them in the target directory because they will not be used at runtime. By default, target installation is enabled; setting this variable to NO is almost never needed. Also by default, packages are installed in this location using the make install command.

On line 12, we tell Buildroot to pass custom options to CMake when it is configuring 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 15, we invoke the cmake-package macro that generates all the Makefile rules that actually allows the package to be built.

cmake-package reference

The main macro of the CMake package infrastructure is cmake-package. It is similar to the generic-package macro. The ability to have target and host packages is also available, with the host-cmake-package macro.

Just like the generic infrastructure, the CMake infrastructure works by defining a number of variables before calling the cmake-package macro.

First, all the package metadata information variables that exist in the generic infrastructure also exist in the CMake 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 CMake 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 main CMakeLists.txt file. This is useful, if for example, the main CMakeLists.txt file 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 CMake. By default, empty.
  • LIBFOO_CONF_OPT, to specify additional configure options to pass to CMake. By default, empty.
  • LIBFOO_MAKE, to specify an alternate make command. This is typically useful when parallel make is 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 $(MAKE). If parallel building is not supported by the package, then it should be set to LIBFOO_MAKE=$(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_INSTALL_STAGING_OPT contains the make options used to install the package to the staging directory. By default, the value is DESTDIR=$(STAGING_DIR) install, which is correct for most CMake packages. It is still possible to override it.
  • LIBFOO_INSTALL_TARGET_OPT contains the make options used to install the package to the target directory. By default, the value is DESTDIR=$(TARGET_DIR) install. The default value is correct for most CMake packages, but it is still possible to override it if needed.
  • LIBFOO_CLEAN_OPT contains the make options used to clean the package. By default, the value is clean.

With the CMake infrastructure, all the steps required to build and install the packages are already defined, and they generally work well for most CMake-based packages. However, when required, it is still possible to customize what is done in any 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 CMake infrastructure is used, if the package .mk file defines its own LIBFOO_CONFIGURE_CMDS variable, it will be used instead of the default CMake one. However, using this method should be restricted to very specific cases. Do not use it in the general case.

6.2.7. Gettext integration and interaction with packages

Many packages that support internationalization use the gettext library. Dependencies for this library are fairly complicated and therefore, deserve some explanation.

The uClibc C library doesn’t implement gettext functionality; therefore with this C library, a separate gettext must be compiled. On the other hand, the glibc C library does integrate its own gettext, and in this case the separate gettext library should not be compiled, because it creates various kinds of build failures.

Additionally, some packages (such as libglib2) do require gettext unconditionally, while other packages (those who support --disable-nls in general) only require gettext when locale support is enabled.

Therefore, Buildroot defines two configuration options:

  • BR2_NEEDS_GETTEXT, which is true as soon as the toolchain doesn’t provide its own gettext implementation
  • BR2_NEEDS_GETTEXT_IF_LOCALE, which is true if the toolchain doesn’t provide its own gettext implementation and if locale support is enabled

Packages that need gettext only when locale support is enabled should:

  • use select BR2_PACKAGE_GETTEXT if BR2_NEEDS_GETTEXT_IF_LOCALE in the Config.in file;
  • use $(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),gettext) in the package DEPENDENCIES variable in the .mk file.

Packages that unconditionally need gettext (which should be very rare) should:

  • use select BR2_PACKAGE_GETTEXT if BR2_NEEDS_GETTEXT in the Config.in file;
  • use $(if $(BR2_NEEDS_GETTEXT),gettext) in the package DEPENDENCIES variable in the .mk file.

6.2.8. Tips and tricks

Package name, config entry name and makefile variable relationship

In Buildroot, there is some relationship between:

  • the package name, which is the package directory name (and the name of the *.mk file);
  • the config entry name that is declared in the Config.in file;
  • the makefile variable prefix.

It is mandatory to maintain consistency between these elements, using the following rules:

  • the package directory and the *.mk name are the package name itself (e.g.: package/foo-bar_boo/foo-bar_boo.mk);
  • the make target name is the package name itself (e.g.: foo-bar_boo);
  • the config entry is the upper case package name with . and - characters substituted with _, prefixed with BR2_PACKAGE_ (e.g.: BR2_PACKAGE_FOO_BAR_BOO);
  • the *.mk file variable prefix is the upper case package name . and - characters substituted with _ (e.g.: FOO_BAR_BOO_VERSION).

How to add a package from github

Packages on github often don’t have a download area with release tarballs. However, it is possible to download tarballs directly from the repository on github.

FOO_VERSION = v1.0 # tag or (abbreviated) commit ID
FOO_SITE = http://github.com/<user>/<package>/tarball/$(FOO_VERSION)

Notes

  • The FOO_VERSION can either be a tag or a commit ID.
  • The tarball name generated by github matches the default one from Buildroot (e.g.: foo-1234567.tar.gz), so it is not necessary to specify it in the .mk file.
  • When using a commit ID as version, usually the first 7 characters of the SHA1 are enough.

6.2.9. 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 the Buildroot mailing list (see Section 10.1, “Submitting patches”)!

6.3. Patching a package

While integrating a new package or updating an existing one, it may be necessary to patch the source of the software to get it cross-built within Buildroot.

Buildroot offers an infrastructure to automatically handle this during the builds. It supports three ways of applying patch sets: downloaded patches, patches supplied within buildroot and patches located in a user-defined global patch directory.

6.3.1. Providing patches

Downloaded

If it is necessary to apply a patch that is available for download, then it to the <packagename>_PATCH variable. It is downloaded from the same site as the package itself. It can be a single patch, or a tarball containing a patch series.

This method is typically used for packages from Debian.

Within Buildroot

Most patches are provided within Buildroot, in the package directory; these typically aim to fix cross-compilation, libc support, or other such issues.

These patch files should be named <packagename>-<number>-<description>.patch.

A series file, as used by quilt, may also be added in the package directory. In that case, the series file defines the patch application order.

Notes

  • The patch files coming with Buildroot should not contain any package version reference in their filename.
  • The field <number> in the patch file name refers to the apply order.

Global patch directory

The BR2_GLOBAL_PATCH_DIR configuration file option can be used to specify a directory containing global package patches. See Section 3.4.4, “Customizing packages” for details.

6.3.2. How patches are applied

  1. Run the <packagename>_PRE_PATCH_HOOKS commands if defined;
  2. Cleanup the build directory, removing any existing *.rej files;
  3. If <packagename>_PATCH is defined, then patches from these tarballs are applied;
  4. If there are some *.patch files in the package directory or in the a package subdirectory named <packageversion>, then:

    • If a series file exists in the package directory, then patches are applied according to the series file;
    • Otherwise, patch files matching <packagename>-*.patch are applied in alphabetical order. So, to ensure they are applied in the right order, it is hightly recommended to named the patch files like this: <packagename>-<number>-<description>.patch, where <number> refers to the apply order.
  5. Run the <packagename>_POST_PATCH_HOOKS commands if defined.

If something goes wrong in the steps 3 or 4, then the build fails.

6.3.3. Format and licensing of the package patches

Patches are released under the same license as the software that is modified.

A message explaining what the patch does, and why it is needed, should be added in the header commentary of the patch.

You should add a Signed-off-by statement in the header of the each patch to help with keeping track of the changes and to certify that the patch is released under the same license as the software that is modified.

If the software is under version control, it is recommended to use the upstream SCM software to generate the patch set.

Otherwise, concatenate the header with the output of the diff -purN package-version.orig/ package-version/ command.

At the end, the patch should look like:

configure.ac: add C++ support test

signed-off-by John Doe <john.doe@noname.org>

--- configure.ac.orig
+++ configure.ac
@@ -40,2 +40,12 @@

AC_PROG_MAKE_SET
+
+AC_CACHE_CHECK([whether the C++ compiler works],
+               [rw_cv_prog_cxx_works],
+               [AC_LANG_PUSH([C++])
+                AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])],
+                               [rw_cv_prog_cxx_works=yes],
+                               [rw_cv_prog_cxx_works=no])
+                AC_LANG_POP([C++])])
+
+AM_CONDITIONAL([CXX_WORKS], [test "x$rw_cv_prog_cxx_works" = "xyes"])

6.3.4. Integrating patches found on the Web

When integrating a patch of which you are not the author, you have to add a few things in the header of the patch itself.

Depending on whether the patch has been obtained from the project repository itself, or from somewhere on the web, add one of the following tags:

Backported from: <some commit id>

or

Fetch from: <some url>

It is also sensible to add a few words about any changes to the patch that may have been necessary.

6.4. Download infrastructure

TODO