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OpenWrt Buildroot – Usage

→ go back to obtain.firmware or read →OpenWrt Buildroot – About


to generate an installable OpenWrt firmware image file with a size of e.g. 8MB:

  1. ca. 3-4 GB of available hard disk space
  2. environment variables:
    • SED should not be set. If it is, run `unset SED` before compiling. (See Ticket 10612.)
    • GREP_OPTIONS should not have –initial-tab or other options affecting its output
    • Add <buildroot dir>/staging_dir/host/bin and <buildroot dir>/staging_dir/toolchain-<platform>-<gcc_ver>-<libc_ver>/bin in front of your PATH variable in ~/.bashrc. The staging directory is created shortly after starting the build and the toolchain directory is created when the toolchain build begins. The build spawns multiple shells, some of which expect the toolchain binaries to be present in the PATH.


1. Do everything as non-root user
2. Issue all commands in the <buildroot dir> directory, e.g. ~/openwrt/trunk/
  1. Update OpenWrt sources.
  2. Update and install package feeds.
  3. Configure the firmware image you want to obtain.
  4. Start the build. This will automatically compile toolchain, cross-compile sources, package packages, and finally generate an image ready to be flashed.
  5. Proceed to Installing OpenWrt

Updating Sources with Git

:!: OpenWrt sources change frequently. It is recommended that you work with the latest sources.

git pull

Updating Feeds

see feeds

:!: Installing in context of ./scripts/feeds script means "making package available in make menuconfig" rather than really installing or compiling package.

  1. Update feeds:
    ./scripts/feeds update -a
  2. Make downloaded package/packages available in make menuconfig:
    • single package:
      ./scripts/feeds install <PACKAGENAME>
    • all packages:
      ./scripts/feeds install -a

Image Configuration

Typical actions:

  1. run make menuconfig and set target;
  2. run make defconfig;
  3. run make menuconfig and modify set of package;
  4. run scripts/ >mydiffconfig (save your changes in the text file mydiffconfig);
  5. run make V=s (build OpenWRT with console logging, you will look where build failed.).

Make menuconfig

The OpenWrt Buildroot configuration interface handles the selection of the target platform, packages to be compiled, packages to be included in the firmware file, some kernel options, etc.
Start the OpenWrt Buildroot configuration interface by issuing the following command:

make menuconfig

This will update the dependencies of your existing configuration automatically, and you can now proceed to build your updated images.

You have three options: y, m, n which are represented as follows:

  • pressing y sets the <*> built-in label
    This package will be compiled and included in the firmware image file.
  • pressing m sets the <M> package label
    This package will be compiled, but not included in the firmware image file. (E.g. to be installed with opkg after flashing the firmware image file to the device.)
  • pressing n sets the < > excluded label
    The source code will not be processed.

When you save your configuration, the file <buildroot dir>/.config will be created according to your configuration.


It has been the intention from the beginning, with the development of menuconfig, to create a simple, yet powerful, environment for the configuration of individual OpenWrt builds. menuconfig is more or less self-explanatory, and even the most specialized configuration requirements can be met by using it. Depending on the the particular target platform, package requirements and kernel module needs, the standard configuration process will include modifying:

  1. Target system
  2. Package selection
  3. Build system settings
  4. Kernel modules

Target system is selected from the extensive list of supported platforms, with the numerous target profiles – ranging from specific devices to generic profiles, all depending on the particular device at hand. Package selection has the option of either 'selecting all package', which might be un-practical in certain situation, or relying on the default set of packages will be adequate or make an individual selection. It is here needed to mention that some package combinations might break the build process, so it can take some experimentation before the expected result is reached. Added to this, the OpenWrt developers are themselves only maintaining a smaller set of packages – which includes all default packages – but, the feeds-script makes it very simple to handle a locally maintained set of packages and integrate them in the build-process.

The final step before the process of compiling the intended image(s) is to exit menuconfig – this also includes the option to save a specific configuration or load an already existing, and pre-configured, version.

Exit the TUI, and choose to save your settings.

Kernel configuration

Note that make kernel_menuconfig modifies the Kernel configuration templates of the build tree and clearing the build_dir will not revert them:

While you won't typically need to do this, you can do it:

make kernel_menuconfig CONFIG_TARGET=subtarget
CONFIG_TARGET allows you to select which config you want to edit. possible options: target, subtarget, env.

:FIXME: (git) SVN: The changes can be reviewed with

svn diff target/linux/
and reverted with
svn revert -R target/linux/

Source Mirrors

The 'Build system settings' include some efficient options for changing package locations which makes it easy to handle a local package set:

  1. Local mirror for source packages
  2. Download folder

In the case of the first option, you simply enter a full URL to the HTTP or FTP server on which the package sources are hosted. Download folder would in the same way be the path to a local folder on the build system (or network). If you have a web/ftp-server hosting the tarballs, the OpenWrt build system will try this one before trying to download from the location(s) mentioned in the Makefiles . Similar if a local 'download folder', residing on the build system, has been specified.

The 'Kernel modules' option is required if you need specific (non-standard) drivers and so forth – this would typically be things like modules for USB or particular network interface drivers etc.

Configure using config diff file

Beside make menuconfig another way to configure is using a configuration diff file. This file includes only the changes compared to the default configuration. A benefit is that this file can be version controlled with your OpenWRT based project. It's also less affected by OpenWRT updates, because it only contains the changes.

Creating diff file

This file is created using the <buildroot dir>/scripts/ script.

Using diff file

These changes can form the basis of a config file (<buildroot dir>/.config). By running make defconfig these changes will be expanded into a full config.

cat diffconfig > .config # append changes to bottom of .config
make defconfig # expand to full config

These changes can also be added to the bottom of the config file (<buildroot dir>/.config), by running make defconfig these changes will override the existing configuration.

cat diffconfig >> .config # append changes to bottom of .config
make defconfig # apply changes


OpenWrt Buildroot integrates quilt for easy patch management:

Custom files

Warning! Be carefull! OpenWrt failsafe relies on 100%-working base files on the SquashFS-partition (i.e. the /rom-directory), so in case you mis-configure or misspell something, you will end up with a non-working OpenWrt failsafe and be forced to used more rudimentary methods to debrick your device again!

In case you want to include some custom configuration files, the correct place to put them is:

  • <buildroot dir>/files/

For example, let's say that you want an image with a custom /etc/config/firewall or a custom etc/sysctl.conf, then create this files as:

  • <buildroot dir>/files/etc/config/firewall
  • <buildroot dir>/files/etc/sysctl.conf

E.g. if your <buildroot dir> is /openwrt/trunk and you want some files to be copied into firmware image's /etc/config directory, the correct place to put them is /openwrt/trunk/files/etc/config.


select your target before issuing defconfig

make defconfig
will produce a general purpose configuration of the build system including a check of dependencies and prerequisites for the build environment etc

will check for dependencies. Install missing and run again.

Building Images

Everything is now ready for building the image(s), which is done with one single command:

make world

This simple command will trigger a cascade of activity. As already stated, it will

  1. compile the toolchain
  2. then crosscompile the sources with this toolchain
  3. create opkg-packages
  4. generate a firmware image file ready to be flashed.

The make sequience

Top command make world calls the following sequence of the commands:
make target/compile
make package/cleanup
make package/compile
make package/install
make package/preconfig
make target/install
make package/index

You may run each command independency. For example, if the process of compilation of packages stops on error, you may fix problem and next continue without cleanup:
make package/compile
make package/install
make package/preconfig
make target/install
make package/index

Make options

Warnings, errors and tracing

The parameter V=xx specifies level of messages in the process of the build.

Value of V may be

  • 1 - print a messages containing the working directory before and after other processing.
  • w - same as 1;
  • 99 - trace of the build, ordinary messages yellow, error messages red, debug - black;
  • s - same as 99.


make V=s
Trace build.

make V=99


make V=sw

Same effect.

make V=w
Trace of directories.

Building on multi-core CPU

:!: Build may FAIL

The build process can be accelerated by running multiple concurrent job processes using the -j-option:

make -j 3

  • Use the standard formula <your number of CPU cores + 1>
  • If this produces random build errors try compiling again without the -j-option

Building in the background

If you intend to use your system while building, you can have the build process use only idle I/O and CPU capacity like this (dualcore CPU):

ionice -c 3 nice -n19 make -j 2

Building single Packages

When developing or packaging software for OpenWrt, it is convenient to be able to build only the package in question (e.g. with package cups):

make package/cups/compile V=s

For a rebuild:

make package/cups/{clean,compile,install} V=s

It doesn't matter what feed the package is located in, this same syntax works for any installed package.

Spotting build errors

If for some reason the build fails, the easiest way to spot the error is to do:

make V=s 2>&1 | tee build.log | grep -i error

The above saves a full verbose copy of the build output (with stdout piped to stderr) in /openwrt/trunk/build.log and only shows errors on the screen.

Another example:

ionice -c 3 nice -n 20 make -j 2 V=s CONFIG_DEBUG_SECTION_MISMATCH=y 2>&1 | tee build.log | egrep -i '(warn|error)'

The above saves a full verbose copy of the build output (with stdout piped to stderr) in build.log and outputs only warnings and errors while building using only background resources on a dual core CPU.

Yet another way to focus on the problem without having to wade through tons of output from Make as described above is to check the corresponding log in 'logs' folder. IE: if the build fails at "make[3] -C package/kernel/mac80211 compile", then you can go to <buildroot>/logs/package/kernel/mac80211 and view the compile.txt found there.

Getting beep notification

Depending on your CPU, the process will take a while, or while longer. If you want an acoustic notification, you could use echo -e '\a':

make V=s ; echo -e '\a'

Skipping failed packages

If you are building everything (not just packages enough to make a flashable image) and build stops on a package you don't care about you can skip failed packages by using IGNORE_ERRORS=1

IGNORE_ERRORS=1 make <make options>

Locating Images

After a successful build, the freshly built image(s) can be found in the newly created <buildroot_dir>/bin directory. The compiled files are additionally classified by the target platform, so e.g. a firmware built for an ar71xx device will be located in <buildroot_dir>/bin/ar71xx directory.

E.g. if your <buildroot_dir> is ~/openwrt/trunk, the binaries are in ~/openwrt/trunk/bin/ar71xx.

Cleaning Up

You might need to clean your build environment every now and then. The following make-targets are useful for that job:


make clean

deletes contents of the directories /bin and /build_dir. make clean does not remove the toolchain, it also avoids cleaning architectures/targets other than the one you have selected in your .config


make dirclean

deletes contents of the directories /bin and /build_dir and additionally /staging_dir and /toolchain (=the cross-compile tools) and /logs. 'Dirclean' is your basic "Full clean" operation.


make distclean

nukes everything you have compiled or configured and also deletes all downloaded feeds contents and package sources.

CAUTION: In addition to all else, this will erase your build configuration (<buildroot_dir>/.config), your toolchain and all other sources. Use with care!

There are numerous other functionalities in the OpenWrt build system, but the above should have covered some of the fundamentals.

Clean small part

In more time, you may not want to clean so many objects, then you can use some of the commands below to do it.

Clean linux objects.

make target/linux/clean

Clean package base-files objects.

make package/base-files/clean

Clean luci.

make package/luci/clean



  • Beware of unusual environment variables such as
    • GREP_OPTIONS which should not have –initial-tab or other options affecting its output
    • SED should not be set. If it is, run `unset SED` before compiling. (See Ticket 10612.)
  • First get more information on the problem using the make option "make V=s".

Errors building gcc

If the build fails with errors that include "[s-attrib] Killed" and "[all-gcc] Error 2" while compiling gcc, this indicates you are out of disk space or RAM. This could be likely if you are building within a virtual machine with < 1 Gb RAM allocated and no swap.

Missing source code file, due to download problems

First check if the URL path in the make file contains a trailing slash, then try with it removed (helped several times). Otherwise try to download the source code manually and put it into "dl" directory.

Compilation errors

Try to update the main source and all the feeds (Warning! May result in other problems). Check for a related bug in (TRAC), use the filters to find it. Otherwise report the problem there, by mentioning the package, the target data (CPU, image, etc.) and the code revisions (main & package). Compiling with make -j … sometimes gives random errors. Try compiling without -j first before reporting the problem.

I can't find my <package> in menuconfig

  1. Have you run this command? See this.
    ./scripts/feeds install <package>
  2. Maybe you are not looking for it in the right submenu. See the Makefile of the package to find out.
    cat feeds/<feedname>/<package>/Makefile
    You are looking for this:
    define Package/<package>/Default SUBMENU:=Firewall SECTION:=net CATEGORY:=Network
  3. Menuconfig appears to cache package lists and (especially) profiles. Try deleting the <buildroot_dir>/tmp directory and then running make menuconfig again.
    cd ~/openwrt/trunk
    rm -rf tmp
    make menuconfig
    Also try refreshing your package indexes:
    ./scripts/feeds update -i
    The -i is important if you're not using the latest revisions of any packages, as this flag prevents updates from the feed sources.

WARNING: skipping <package> -- package not selected

Run make menuconfig and enable compilation for your package. It should be labeled with <*> or <M> to work correctly. Read image.configuration further up in this article.


doc/howto/build.txt · Last modified: 2015/07/19 15:14 by grumbler_eburg