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Customization with the .userconfig file

You can customize certain aspects of the build process by putting some bash code into your B2G source's .userconfig file. This article looks at what you can achieve by making changes here, and how to do that.

The .userconfig file isn't checked into source code control, so your changes won't be overwritten when you update your source tree. It needs to be created in the root of the B2G tree; that is, in the same directory as,, and so forth. You should add this before you run your config and build steps.

The .userconfig file, if it exists, is sourced by the script, which is in turn sourced by these scripts:, (through,, and tools/ The run-*.sh scripts use it to determine where Gecko is for your build. The script is used by every B2G related mach command.

Important: Your .userconfig file should be in the root of your B2G source directory, not your home directory!

Changing the Gecko source tree

By default, the build uses the gecko tree, which is cloned from a tree in github. Some people like to use mozilla-inbound, or mozilla-central. To do this create your clone whereever you like and add a line to your .userconfig which sets GECKO_PATH, for example:

export B2G_DIR=${B2G_DIR:-$(cd $(dirname $0); pwd)}
echo "B2G_DIR = ${B2G_DIR}"

export GECKO_PATH=${B2G_DIR}/mozilla-inbound

Note: if building against a custom Gecko in Mac OS X, the mozilla-central directory must be in a case sensitive file system or else GECKO_PATH will be ignored. You can check whether the file system is case sensitive by running this in a Terminal window:

echo -n This file system is case->tmp; echo -n in>>TMP; echo sensitive>>tmp; cat tmp

Getting B2G_DIR the way it is above allows your .userconfig to work without having hard-coded paths.

Changing Gaia settings

Sometimes, you'd like to be able to change gaia settings. For example, enabling adb in a user build. The gaia Makefile passes in --override build/custom-settings.json when calling build/, so we can write some bash which will write {"devtools.debugger.remote-enabled": true} into the custom-settings.json file. We try to avoid changing custom-settings.json if we don't need to, so we actually write into and if the contents differ from custom-settings.json then we'll replace it.

export GAIA_PATH=${GAIA_PATH:-$(cd gaia; pwd)}
export CUSTOM_SETTINGS="${GAIA_PATH}/build/config/custom-settings.json"
cat > "${CUSTOM_SETTINGS}.new" <<EOF
{"devtools.debugger.remote-enabled": true}
if [[ -f ${CUSTOM_SETTINGS} ]] && cmp "${CUSTOM_SETTINGS}" "${CUSTOM_SETTINGS}.new" >& /dev/null; then
  rm "${CUSTOM_SETTINGS}.new"

Another easier way is to configure a build/config/custom-prefs.js file in the Gaia working directory, so that means in gaia/build/config/custom-prefs.js if you're in the B2G directory. See Gaia Build System Primer, Customizing the preferences.

Note:  Currently the build is not smart enough to look in a different directory based on GAIA_PATH.  This is different from how GECKO_PATH behaves.  If you wish to use a separate Gaia clone you can manually run make from that directory.

Creating different build types

You can create many different types of Gaia build automatically upon running the make command, by setting various options in your .userconfig — this section covers a number of different options. 

Note: You can also set many different build options dynamically at build time by including different options along with the make command when you build. For a complete reference, see our make options reference article.

Creating production and engineering builds

To create different production builds (built with the final apps you'd want users to see) and engineering builds (built with additional test apps and other engineering features), you can add the following lines into .userconfig:


On its own, this creates a production build. This can also be achieved on the fly by setting the production make option.

Alternatively, there are variants, which set various levels of engineering capabilities:




The differences between these variants are as follows:

  • The eng variant places the default apps in /data and includes many more apps than user/userdebug, for testing purposes. It also has Marionette enabled by default so you can run tests. This is the default value if no variant is specified.
  • The userdebug variant puts apps in /system, has Marionette enabled by default so you can run tests, and has the updater enabled, so you can get over-the-air (OTA/FOTA) updates.
  • The user variant places the default apps in /system and has the updater enabled, so you can get over-the-air (OTA/FOTA) updates.

Note: user and userdebug both imply PRODUCTION=1 when building locally for a real device/emulator.

Note: make production is really a way to build a user version of Gaia and flash it to the device. Specifying a VARIANT is the way to do this when building for Gaia in Nightly or B2G Desktop.

Creating a debug build

To build a debug build, put the following line in your .userconfig file:

export B2G_DEBUG=1

This can be achieved on the fly at build time by including the DEBUG=1 make option.

Creating a profiling build

To enable profiling (for best results with the built-in (SPS) platform profiler), add the following to your .userconfig file then rebuild:

export B2G_PROFILING=1

Important: Do NOT pair with B2G_NOOPT. The results will be meaningless!

Disabling the optimizer

To disable the optimizer (which may create builds that are easier to debug), add the following to your .userconfig file then rebuild:

export B2G_NOOPT=1

Disabling First Time User experience

If you build and reflash a lot, going through the First Time User experience constantly can be annoying. You can disable this by adding the following to your .userconfig:

export NOFTU=1

This can be achieved on the fly at build time by including the NOFTU=1 make option.

Building the updater and the update tools

By default the updater and the update tools are built only in userdebug and user builds.

You can force building the updater and the associated tools by adding the following to your .userconfig file:

export B2G_UPDATER=1

Enabling Gaia developer mode

If you plan to develop apps or hack gaia, you can automatically set various usefull settings and preferences to ease working with the device. For example, it will automatically enable the remote debugging feature and disable the prompt when an incoming debugging connection starts.

What you need is the following export added to your .userconfig:


Enabling Valgrind

Valgrind is a debugging tool useful for debugging memory or threading issues with your application. For more information on running valgrind, see Debugging B2G using valgrind.

To use valgrind under B2G, add the following to your .userconfig:

export B2G_VALGRIND=1

Changing the default host compiler

On some recent distributions that use GCC 4.7 or later as the default compiler you might need to specify an older version in order to be able to build, depending on your platform of choice. To do so add two lines to your .userconfig file, setting the CC and CXX variables to set the alternate C and C++ compilers, respectively. For example to set the GCC 4.6 compiler on Ubuntu 12.10 use:

export CC=gcc-4.6
export CXX=g++-4.6

Or if you're using a version built from sources provide the full path to the exectuables:

export CC=/opt/gcc-4.6.4/bin/gcc
export CXX=/opt/gcc-4.6.4/bin/g++

Specifying a custom Gecko object tree location

Once you start changing gecko source trees and other build options, you may want to also modify where your objects get stored (so, for example, all of your debug objects go into a different tree from your non-debug objects). So you might do something like:

export GECKO_OBJDIR=${GECKO_PATH}/objdir-gonk-debug

Using ${GECKO_PATH} makes it easy to switch between different gecko trees (eg: central, beta, aurora, etc).

Keeping both debug and non-debug objects

You can use your .userconfig file to switch back and forth between debug and release builds without having to rebuild everything each time!

export B2G_DIR=${B2G_DIR:-$(cd $(dirname $0); pwd)}
echo "B2G_DIR = ${B2G_DIR}"

export GECKO_PATH=${B2G_DIR}/mozilla-inbound

export B2G_DEBUG=1
echo "B2G_DEBUG = ${B2G_DEBUG}"

export GECKO_OBJDIR=${GECKO_PATH}/objdir-gonk
if [[ "${B2G_DEBUG}" != "0" ]]; then
if [[ "${GECKO_PATH/*mozilla-inbound*/mozilla-inbound}" == "mozilla-inbound" ]]; then

The echo commands help remind you what your current settings are. To switch between debug and release builds, simply change the value of B2G_DEBUG on line 7.