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Advanced Topics


Changing build flags

If you need to change any build flags (such as MOZ_SWITCHBOARD), instead of adding changes to the mozconfig and clobbering, you can test by adding the flags to mobile/android/, run

mach configure

then check that the changes are in $OJBDIR/config.status, and then build normally.

Building for the x86 architecture

If you want to build for x86, substitute:

ac_add_options --target=i386-linux-android

for the target specified above. Note: you cannot use ac_add_options --disable-optimize when building for x86. See bug 965870.

Building for the ARMv6 architecture

ARMv6 is no longer a supported target as of Firefox 33.

If you want to build for ARMv6 instead of ARMv7 (only required for very old, very low end phones), add:

ac_add_options --with-arch=armv6

Building different channels

If you want to build different channels (through the Try server, for example), you need to add this to mobile/android/config/mozconfigs/common.override:

ac_add_options --enable-update-channel=nightly

To change the branding, add:

ac_add_options --with-branding=browser/branding/nightly

These can be changed to any of the following channels (nightly, beta, release).

Unsupported build flags

Do not specify these flags because the resulting build will cause out-of-memory crashes:

ac_add_options --disable-install-strip # Do not use!

Updating the builders` SDK

i.e. Nightly builds & treeherder. See Mobile/Fennec/Android/Updating_SDK_on_builders.

Multilocale builds

Single-locale language repacks

There is a script in mozharness for this (scripts/ but it relies on buildbot information so it's not suitable for local repacks.

This assumes that $(AB_CD) is the locale you want to repack with; I tested with "ar" and "en-GB".

  • clone l10n-central/$(AB_CD) so that it is a sibling of your mozilla-central directory
  • I assume your object directory is "objdir-droid" and that you have built and packaged already
make -f && make -C objdir-droid package
  • copy your .mozconfig to .mozconfig.l10n and add the following lines
# L10n
ac_add_options --with-l10n-base=../../l10n-central

# Global options
ac_add_options --disable-tests

mk_add_options MOZ_OBJDIR=./objdir-l10n
  • cd to mozilla-central
  • configure and prepare objdir-l10n
MOZCONFIG=.mozconfig.l10n make -f configure
make -C objdir-l10n/config
  • copy your built package into objdir-l10n
cp ./objdir-droid/dist/fennec-*en-US*.apk ./objdir-l10n/dist
  • unpack. This files objdir-l10n/dist with the bits of the APK, ready for re-assembling.
make -C objdir-l10n/mobile/android/locales unpack
  • compare locales (you may need to install the compare-locales tool first). This writes locale differences into objdir-l10n/merged.
compare-locales -m objdir-l10n/merged mobile/android/locales/l10n.ini ../l10n-central $(AB_CD)
  • finally, re-assemble with the locale differences
LOCALE_MERGEDIR=objdir-l10n/merged make -C objdir-l10n/mobile/android/locales installers-$(AB_CD)

You should find an APK at "objdir-l10n/dist/fennec-*$(AB_CD)*.apk".

Enabling C++ debugging

If you want to create a build suitable for debugging the C++ code, add:

ac_add_options --enable-debug
ac_add_options --enable-debug-symbols


Don't set CC / CXX environmental variables

If you've set the environmental variables CC and CXX (e.g. via .bash_aliases or via your mozconfig), then you probably need to unset them before building for Android, or else your build may fail with something like:

checking whether the C compiler (gcc -mandroid -fno-short-enums (etc etc)) works... no

followed by errors about "C compiler cannot create executables", "Relocations in generic ELF (EM: 40)", and "crtbegin_dynamic.o: error adding symbols: File in wrong format". This is a sign that you're compiling with your platform's native compiler (due to having CC / CXX set), instead of the android-specific GCC version that ships with the NDK. See bug 977817 for more details; as noted there, the build system may trust your custom CC & CXX variables, when you probably don't want it to.

Coding Caveats

Closing resources

When handling resources (like Cursors), a try/finally block should be used to ensure these are closed properly. For example:

final Cursor c = getCursor();
try {
} catch (SomeSpecificException sse) {
} finally {

Once the try block is entered, the finally block will *always* get executed upon exit of the try block. The one exception is if there is a System.exit call inside the try block, which immediately exits the program and makes everything moot anyway. The finally block will get executed on caught and uncaught exceptions, as well as normal returns.

If you are casting the resource to something, make sure that you do the cast inside the try block, like so:

// GOOD!
InputStream is = getInputStream();
try {
    FileInputStream fis = (FileInputStream) is;
} finally {

rather than doing this:

// BAD!
FileInputStream fis = (FileInputStream) getInputStream();
try {
} finally {

This is so that in case of ClassCastExceptions you don't get a dangling open resource left behind.


TLDR: Google recommends using SystemClock.uptimeMillis() for general purpose interval timing of user interface events or performance measurements. If you're adding stuff for timing, use SystemClock.uptimeMillis(), rather than something like new Date().getTime().

Normally in Java the default time-getter is System.currentTimeMillis() since it avoids the overhead of creating a new Date object. This is also what new Date() does under the hood. However, currentTimeMillis() and the Date object are both subject to change in unexpected ways if the user changes the time on their device, or if daylight savings comes into effect, or there's a network time update, or whatever. So Android has generously provided android.os.SystemClock which has various functions that you can use to get a better timestamp. Refer to the class javadoc and pick whichever function is most suitable for what you're trying to measure.

Advanced Debugging

Beyond just using Android logcat, there are other options for debugging Firefox for Android, depending on your needs.

Logcat apps

ICS (4.1) and below

  • Install the aLogCat app. Use it to capture logs and attach the logs to bugs.

Once you have alogcat installed, just use Fennec as you would normally. Upon encountering a bug or issue, start the aLogcat app (as soon as possible after seeing the Fennec issue) and select "Share" or "Save" from the menu to send it via email or save it to the SD card. The log can then be attached to a bug or sent to a developer. As with adb logcat, it is better to have a log with timestamps than without timestamps. To enable timestamps in the log, select "Preferences" from the aLogcat menu, and change the "Format?" option to "Time".

If you need to, you can search for some kinds of Fennec-related output by using the "Filter" menu item and entering "Gecko". However, when submitting logs for bug reports, please make sure you clear the filter and include all of the available log data.

Jelly Bean (4.2) and above

JavaScript dump()

To use the dump() function in JavaScript to write to the log:

  1. Go to about:config and set browser.dom.window.dump.enabled to "true"
  2. Run the following ADB commands:
adb shell stop
adb shell setprop log.redirect-stdio true
adb shell start

Using JimDB

See Mobile/Fennec/Android/GDB

Using Debug Intent

Note: this is not useful with JimDB. If you want to use JimDB, just start it.

In order to attach before things get running, launch with:

  adb shell am start -a org.mozilla.gecko.DEBUG -n org.mozilla.fennec_foobar/.App

(Replace foobar by your username)

and just click launch once gdb is attached. If you need to debug a crash that happens before XRE_Main is called, the patch on bug 572247 may be useful.

this script [1] will attach gdbserver for you

Getting dalvik java stack dumps using gdb

(gdb) call dvmDumpAllThreads(true)

this will dump a stack trace to logcat

Note: this will only work if you have symbols for dalvik.

Debugging with jdb

JimDB can now launch JDB integration

You can also use eclipse for debugging in a similar way by setting up for debugging "Remote Java application".

Debugging with eclipse

You need to find the PID of your fennec process. Forward it to a local TCP socket as in "Debugging with jdb."

In Eclipse switch to the debug perspective. Go to Run > Debug configurations... Remote Java Application. Change the port to the TCP port you specified in your adb command. Under Source, navigate to your checkout, then into mobile/android.

Eclipse looks for source code in a specific location. You need to create the directory hierarchy:

       gecko -> ../../base

That is, in mozilla-central/mobile/android, create org/mozilla, and put the symlink gecko pointing to mozilla-central/mobile/android/base.

You may also want to add more debugging information and can do that like this:

diff --git a/config/ b/config/
index 4591239..a47726a 100644
--- a/config/
+++ b/config/
@@ -70,6 +70,6 @@ JAVAC_FLAGS = \
   -classpath $(JAVA_CLASSPATH) \
   -bootclasspath $(JAVA_BOOTCLASSPATH) \
   -encoding UTF8 \
-  -g:source,lines \
+  -g:source,lines,vars \
   -Werror \

Arguments and Environment Variables

If you need to set an environment variable at run time, append --es env# VAR=VAL to your activity manager command where # is the ordered number of variables for example:

 adb shell am start -a android.activity.MAIN -n org.mozilla.fennec_$USER/.App --es env0 VAR=val --es env1 FOO=bar

If you need to pass arguments at run time, append --es args "<your-args>" to your activity manager command. For example, to launch with a specific profile:

 adb shell am start -a android.activity.MAIN -n org.mozilla.fennec_$USER/.App --es args "--profile /mnt/sdcard/myprofile"

To launch with a specific URL, use the am -d option to set the intent's data URI:

 adb shell am start -a android.activity.MAIN -n org.mozilla.fennec_$USER/.App -d ''

C++ Logging

You can use the env vars as described above to enable MOZ_LOG logging from C++ code:

 adb shell am start -a android.activity.MAIN -n org.mozilla.fennec_$USER/.App --es env0 MOZ_LOG=all:5 --es env1 MOZ_LOG_FILE=/mnt/sdcard/log.txt

If no file is specified, logging is directed to the android logs:

 adb shell am start -a android.activity.MAIN -n org.mozilla.fennec_$USER/.App --es env0 MOZ_LOG=all:5

Look for lines marked "Gecko" with one of the requested log keys in the adb logcat output.

Using legacy GDB (non-JimDB)

See Fennec/Android/GDBNoRoot

Reading back the framebuffer

If you need to verify what is in the back buffer at a particular time, you can cleverly call functions from gdb to allocate memory, read back, and write that to disk.

You need to know the size of your framebuffer a priori; in this case, it's 480x699.

You also need to know what the GL enum values are, because unless you compile with -ggdb, you don't have #defines available to you in the debugger. Very helpful information: GL_RGBA = 0x1908, GL_UNSIGNED_BYTE = 0x1401. The rest you can find in gfx/gl/GLDefs.h.

You should be able to call glReadPixels directly, but in my experience that causes Fennec to crash. However, if you have a GLContext* lying around, as you often do, you can work around that problem.

(gdb) set $m = (int*)malloc(480*699*4)
(gdb) call aManager->mGLContext.mRawPtr->fReadPixels(0, 0, 480, 699, 0x1908, 0x1401, (void*)$m)
(gdb) set $f = fopen("/sdcard/outputfile", "wb+")
(gdb) call fwrite($m, 1, 480*699*4, $f)
$7 = 1342080
(gdb) call fclose($f)
$8 = 0

Now there is a file called /sdcard/outputfile that you can adb pull. But since it's just raw RGBA values, you need to be able to wrap that in PNG headers to display it. Jeff Muizelaar wrote a header called minpng.h that you can use to do so.

Get Jeff's minpng.h, and put it in a directory along with a driver c program:

#include "minpng.h"

int main(int argc, char* argv[])
	FILE* f = fopen(argv[1], "rb");
	int w = atoi(argv[2]);
	int h = atoi(argv[3]);
	char* d = (char*) malloc(w * h * 4);
	fread(d, w * h * 4, 1, f);
	write_png(argv[4], d, w, h);

Compile and run:

$ gcc -o minpng minpng.c
$ ./minpng outputfile 480 699 output.png

Using Rendertrace (Maple)

Rendertrace is a utility that will dump layer position and timing information (such as drawing, upload) to the console. This information can pasted into the rendertrace web front end to visualize the layer position and event timeline. This will let you understand where you're gecko is spending its time and why were checkerboarding.

To enable go in 'gfx/layers/RenderTrace.h' and uncomment '#define MOZ_RENDERTRACE'. Rebuild and run 'adb logcat | grep RENDERTRACE', paste the result in and hit 'reload'. For details talk to BenWa.

Using apitrace

Apitrace is a tool for tracing GL/EGL calls for debugging purposes. It basically uses an interim shared library called libapitrace that contains shadow gl* and egl* functions, which then get logged and then passed through to the real driver.

Use apitrace from to build for desktop and android.

apt-get install libegl1-mesa-dev libgles1-mesa-dev libgles2-mesa-dev libqt4-dev cmake
git clone
cd apitrace

# Build for Android
cmake -DANDROID_NDK=/path/to/your/ndk -DCMAKE_TOOLCHAIN_FILE=android/android.toolchain.cmake -DANDROID_API_LEVEL=9 -Bbuild-android -H.
make -C build-android -j8

# Build for desktop
cmake -H. -Bbuild
make -C build -j8

export EGL_SOFTWARE=true
./build/eglretrace -v /path/to/your/apitrace_log.trace

The Android build will create in build-android/wrappers, which you can then push to your device to /data/local:

adb push build-android/wrappers/ /data/local/tmp

Restarting Fennec will cause it to load the apitrace library and the apitrace log will be saved to /data/data/org.mozilla.fennec_username/firefox.trace

You can then adb pull /data/data/org.mozilla.fennec_username/firefox.trace and analyse it on your desktop.

You can also use qapitrace as a GUI to inspect your trace files. (be sure to switch qapitrace to the EGL api using the options dialog)

These instructions provide a trace that does not include the Java GL code. To get traces including java code is more complicated. You need to use the patch from this bug and this version of Further, you'll need to build your own image/modify the current one to replace /init.rc. You also need to disable hardware acceleration of the UI ( Ask jrmuizel for more information if you want to do this.


about:memory provides heaps (ha!) of useful memory information.

You can obtain a snapshot of memory info from a running Fennec instance using:

 adb shell am broadcast -a org.mozilla.gecko.MEMORY_DUMP

This dumps a json file to the SD card and prints out the exact filename to logcat. You can pull the json file to desktop using

 adb pull <absolute-path-to-file>

and view it in firefox's about:memory: use the "Read reports from a file" option at the bottom of the about:memory page.



Tweaking UI prefs

By default, all of these prefs are set to "-1" in Fennec, meaning they take the values listed below, which are maintained in

Fractional values are specified in 1/1000th of a value; to specify a value of 0.3, write 300.

Note: You need to restart Fennec after changing these values.

Pref Default value Description
ui.scrolling.friction_slow 850 This fraction in 1000ths of velocity remains after every animation frame when the velocity is low.
ui.scrolling.friction_fast 970 This fraction in 1000ths of velocity remains after every animation frame when the velocity is high.
ui.scrolling.velocity_threshold 10 Below this velocity (in pixels per frame), the friction changes from friction_fast to friction_slow.
ui.scrolling.max_event_acceleration 12 The maximum velocity change factor between events, per ms, in 1000ths.
ui.scrolling.overscroll_decel_rate 40 The rate of deceleration when the surface has overscrolled, in 1000ths.
ui.scrolling.overscroll_snap_limit 300 The fraction of the surface which can be overscrolled before it must snap back, in 1000ths.
ui.scrolling.min_scrollable_distance 500 The minimum amount of space that must be present for an axis to be considered scrollable, in 1/1000ths of pixels.
gfx.displayport.strategy 1 The strategy we use to determine how display ports are calculated. 0 = fixed margin, 1 = velocity bias, 2 = dynamic resolution, 3 = no margins
gfx.displayport.strategy_fm.multiplier 1500 When gfx.displayport.strategy = 0 (fixed margin), the 1000th of each dimension of the viewport the displayport is sized to.
gfx.displayport.strategy_fm.danger_x 100 When gfx.displayport.strategy = 0 (fixed margin), the 1000th of the width of the viewport the horizontal danger zone is set to.

Danger zone is defined as the space at the edge of the viewport at which the viewport (and hence displayport) starts being changed.

gfx.displayport.strategy_fm.danger_y 200 When gfx.displayport.strategy = 0 (fixed margin), the 1000th of the height of the viewport the vertical danger zone is set to.
gfx.displayport.strategy_vb.multiplier 1500 When gfx.displayport.strategy = 1 (velocity bias), the 1000th of each dimension of the viewport the displayport is sized to.
gfx.displayport.strategy_vb.threshold 32 When gfx.displayport.strategy = 1 (velocity bias), the threshold for velocity, in pixels/frame, when multiplied by the screen DPI.
gfx.displayport.strategy_vb.reverse_buffer 200 When gfx.displayport.strategy = 1 (velocity bias), the fraction of the buffer (in 1000ths) to be kept in the direction opposite the direction of the scroll.

Invalidate the JavaScript startup cache

To make life easier for developers, in local development builds only, bug 976216 invalidates the JavaScript startup cache every time Firefox for Android starts. On non-local development builds (including TBPL builds and try builds), the JavaScript startup cache is not invalidated at startup.

Background: JavaScript files and modules (like browser.js) are cached for fast startup. The cache is invalidated only when an internal build ID is updated, which only happens when certain C++ code is rebuilt. That doesn't happen for most patches that only touch Java and JavaScript within mobile/android. See bug 695145 for the details of this build ID handling. If you happen to be building a non-local development build in some way, you might need to invalidate the JavaScript startup cache by touching toolkit/xre/nsAndroidStartup.cpp and rebuilding libxul.

killer script

if [ $# -ne 1 ]
    echo "usage: $0  packagename.of.your.activity"
    echo "for example:"
    echo "  $0 org.mozilla.fennec"

p=`adb shell ps | grep $1 | awk '{print $2}'`
if [ "$p" = "" ];
    echo "ERROR: That doesn't seem to be a running process. Please make sure your"
    echo "application has been started and that you are using the correct"
    echo "namespace argument."

adb shell run-as $1 kill $p


This is an example .gdbinit that uses the symbols from a locally built rom and automatically attaches to gdbserver. Note that putting a .gdbinit file inside a directory will make gdb load it thus you will not pollute your regular gdb init with those configurations.

set solib-search-path /home/blassey/android/system/out/target/product/passion/symbols/system/bin:/home/blassey/android/system/out/target/product/passion/symbols/system/lib/:/home/blassey/src/ndk5-m-c/objdir-droid-dbg/dist/bin
set solib-absolute-prefix /home/blassey/android/system/out/target/product/passion/symbols/system/lib/
target remote localhost:12345

Rooting Android devices

See Rooting Android Devices.

Sign a Fennec build

Nightly builds are available unsigned, so that you can sign them with your local debug key and install them on top of your own debug builds (without uninstalling and losing your profile). To sign and install the unsigned nightly build:

 jarsigner -sigalg SHA1withRSA -digestalg SHA1 -keystore ~/.android/debug.keystore -storepass android -keypass android gecko-unsigned-unaligned.apk androiddebugkey
 zipalign -f -v 4 gecko-unsigned-unaligned.apk gecko-signed-aligned.apk
 adb install -r gecko-signed-aligned.apk

Or you can also re-sign a signed build. If "fennec.apk" is signed already, this will remove the signature and replace it with your own. The result will be saved as "fennec-resigned.apk":

 zip fennec.apk -d META-INF/*
 jarsigner -sigalg SHA1withRSA -digestalg SHA1 -keystore ~/.android/debug.keystore -storepass android -keypass android fennec.apk androiddebugkey
 zipalign -f -v 4 fennec.apk fennec-resigned.apk

If you get this error when you try to sign a package:

 jarsigner: unable to sign jar: invalid entry compressed size (expected 16716 but got 16964 bytes)

You should to follow some steps to complete your task:

* rename the .apk to .zip
* unzip the file in some folder
* remove the METAINF folder
* zip the remaining files
* change the name .zip to .apk
* sign again

To verify if everything is alright use the command

 jarsigner -verbose -verify

Useful Addons

There are some addons which may be useful for development purposes. See Mobile/Fennec/Android/Development/Addons.

Other Resources