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Chromium OS Developer Guide


  1. 1 Introduction
    1. 1.1 Target audience
    2. 1.2 Organization & content
    3. 1.3 Typography conventions
    4. 1.4 Modifying this document
    5. 1.5 Additional information
  2. 2 Prerequisites
    1. 2.1 Install git and curl
    2. 2.2 Install depot_tools
    3. 2.3 Tweak your sudoers configuration
    4. 2.4 Configure git
    5. 2.5 Preparations to submit changes
    6. 2.6 Double-check that you are running a 64-bit architecture
    7. 2.7 Verify that your default file permissions (umask) setting is correct
  3. 3 Get the Source
    1. 3.1 Decide where your source will live
    2. 3.2 Get the source code
  4. 4 Building Chromium OS
    1. 4.1 Create a chroot
    2. 4.2 Enter the chroot
    3. 4.3 Select a board
    4. 4.4 Initialize the build for a board
    5. 4.5 Set the chronos user password
    6. 4.6 Build the packages for your board
    7. 4.7 Build a disk image for your board
    8. 4.8 Look at your disk image (optional)
  5. 5 Installing Chromium OS on your Device
    1. 5.1 Put your image on a USB disk
    2. 5.2 Disable verified boot
    3. 5.3 Boot from your USB disk
    4. 5.4 Installing your Chromium OS image to your hard disk
    5. 5.5 Getting to a command prompt on Chromium OS
    6. 5.6 Building an image to run in a virtual machine
  6. 6 Making changes to packages whose source code is checked into Chromium OS git repositories
    1. 6.1 Keep the tree green
    2. 6.2 Run cros_workon start
    3. 6.3 Run repo sync
    4. 6.4 Find out which ebuilds map to which directories
    5. 6.5 Create a branch for your changes
    6. 6.6 Make your changes
    7. 6.7 Set your editor
    8. 6.8 Submit changes locally
    9. 6.9 Upload your changes and get a code review
    10. 6.10 Try seeing the mainline again
    11. 6.11 Work on something else while you're waiting
    12. 6.12 Send your changes to the Commit Queue
    13. 6.13 Make sure your changes didn't break things
    14. 6.14 Share your changes using the Gerrit sandbox
    15. 6.15 Clean up after you're done with your changes
  7. 7 Making changes to non-cros_workon-able packages
    1. 7.1 Adding small patches to existing packages
    2. 7.2 Making changes to the way that the chroot is constructed
    3. 7.3 Building an individual package
    4. 7.4 Making changes to the Chromium web browser on Chromium OS
  8. 8 Using Clang to get better compiler diagnostics
    1. 8.1 Description
    2. 8.2 Wrapper Options
    3. 8.3 Testing
  9. 9 Local Debugging
    1. 9.1 Debugging both x86 and non-x86 binaries on your workstation.
  10. 10 Remote Debugging
    1. 10.1 Setting up remote debugging by hand.
    2. 10.2 Automated remote debugging using gdb-${BOARD} script (gdb-lumpy, gdb-daisy, gdb-parrot, etc).
    3. 10.3 Examples of debugging using the gdb-${BOARD} script.
  11. 11 Troubleshooting
    1. 11.1 I lost my developer tools on the stateful partition, can I get them back?
  12. 12 Running Tests
    1. 12.1 Seeing which tests are implemented by an ebuild
    2. 12.2 Creating a recovery image that has been modified for test
  13. 13 Additional information
    1. 13.1 Toolchain Compilers
    2. 13.2 Developer mode
    3. 13.3 Attribution requirements
    4. 13.4 Documentation on this site
    5. 13.5 External documentation


This guide describes how to work on Chromium OS. If you want to help develop Chromium OS and you're looking for detailed information about how to get started, you're in the right place.  You can also use the quick-start guide instead, which gives just the basic steps that work for most people.

Target audience

The target audience of this guide is anyone who wants to obtain, build, or contribute to Chromium OS. That includes new developers who are interested in the project and who simply want to browse through the Chromium OS code, as well as developers who have been working on Chromium OS for a long time.

Organization & content

This guide describes the common tasks required to develop Chromium OS. The guide is organized linearly, so that developers who are new to Chromium OS can follow the tasks in sequence. The tasks are grouped into the following sections:

Typography conventions

  • Paths, files, and commands are shown in different colors to indicate whether they apply to (1) your build computer (the computer on which you're doing development), (2) the chroot on your build computer, or (3) your Chromium OS computer (the device on which you run the images you build):
    Color Paths, files, and commands:
    green text on your build computer
    purple text inside the chroot on your build computer
    crimson text on your Chromium OS computer
  • Notes are shown using the following conventions:
    • IMPORTANT NOTE describes required actions and critical information
    • SIDE NOTE describes explanations, related information, and alternative options
    • TODO describes questions or work that is needed on this document

Modifying this document

Only people with accounts can edit this document. Everyone else should post to the Chromium OS dev group.
If you're a Chromium OS developer, YOU SHOULD UPDATE THIS DOCUMENT and fix things as appropriate.  There's a "Sign In" text button at the very bottom of this page.  Sign in (with your account, of course) and you'll get an "Edit" button at the top of the page.  Bias towards action: 
  • If you see a red TODO and you know the right answer, fix it!
  • If you see something wrong, fix it. 
  • If you're not sure of the perfect answer, still fix it.   Stick in a red TODO noting your uncertainty if you aren't sure, but don't let anything you know to be wrong stick around.
Please try to abide by the following guidelines when you modify this document:
  • Put all general "getting started" information directly on this page. It's OK if this document grows big.
  • If some information is also relevant for other documents, put the information in this document and add links from the other documents to this document. If you do not have a choice and must put information relevant to getting started in other documents, add links in this document to discrete chunks of information in the other documents. Following a web of links can be very confusing, especially for people who are new to a project, so it's very important to maintain a linear flow as much as possible.
  • Where possible, describe a single way of doing things. If you specify multiple options, provide a clear explanation why someone following the instructions would pick one option over the others. If there is disagreement about how to do things, consider listing alternative options in a SIDE NOTE.
  • Keep Google-specific references to a minimum. A large number of the people working on Chromium OS work at Google, and this document may occasionally contain references that are only relevant to engineers at Google. Google engineers, please keep such references to a minimum – Chromium OS is an open source project that should stay as open as possible.

Additional information

This document provides an overview of the tasks required to develop Chromium OS. After you've learned the basics, check out the links in the additional information section at the end of this document for tips and tricks, FAQs, and important details (e.g., the Chromium OS directory structure, using the dev server, etc.).

Finally, if you build a Chromium OS image, please read this important note about attribution requirements.


You must have Linux to develop Chromium OS.  Any recent or up-to-date distribution should work.  However, we can't support everyone and their dog's Linux distro, so the only official supported environment is listed below.  If you encounter issues with other setups, patches are generally welcomed, but please do not expect us to figure out your distro.
  • Ubuntu Linux (version 14.04 - Trusty)
  • Most developers working on Chromium OS are using Trusty (the LTS version of Ubuntu). It is possible that things will work if you're running a different Linux distribution, but you will probably find life easier if you're on this one.
  • an x86_64 64-bit system for performing the build

  • an account with sudo access
  • You need root access to run the chroot command and to modify the mount table. NOTE: Do not run any of the commands listed in this document as root – the commands themselves will run sudo to get root access when needed.
  • many gigabytes of RAM
  • Per this thread, linking Chrome requires somewhere between 8 GB and 28 GB of RAM as of March 2017; you may be able to get by with less at the cost of slower builds with adequate swap space. Seeing an error like error: ld terminated with signal 9 [Killed] while building the chromeos-chrome package indicates that you need more RAM. If you are not building your own copy of Chrome, the RAM requirements will be substantially lower.
You will have a much nicer time if you also have:
  • a fast multi-processor machine with lots of memory
  • The build system is optimized to make good use of all processors, and an 8 core machine will build nearly 8x faster than a single core machine.
  • a good Internet connection
  • This will help for the initial download (minimum of about 2 GB) and any further updates.

Install git and curl

Install the git revision control system, the curl download helper, and lvm tools. On Ubuntu, the magic incantation to do this is:

sudo apt-get install git-core gitk git-gui curl lvm2 thin-provisioning-tools python-pkg-resources python-virtualenv

This command also installs git's graphical front end (git gui) and revision history browser (gitk).

Install depot_tools

To get started, follow the instructions at install depot_tools. This step is required so that you can use the repo command to get/sync the source code.

Tweak your sudoers configuration

You must tweak your sudoers configuration to turn off the tty_tickets option as described in This is required for using cros_sdk.

Configure git

Setup git now. If you don't do this, you may run into errors/issues later.  Replace and Your Name with your information:

git config --global ""
git config --global "Your Name"

Preparations to submit changes

IMPORTANT NOTEIf you are new to Chromium OS, you can skip this step (go to "decide where your source will live").  This is only for people who wish to make changes to the tree.  Remember: anyone can post changes to Gerrit for review.

Follow the instructions here to setup your code review account(s) and machine access credentials for the source repos. Remember a account is not required to submit code for review.

Double-check that you are running a 64-bit architecture

Run the following command:

uname -m

You should see the result: x86_64

If you see something else (for example, i686, which means you are on a 32-bit machine or a 64-bit machine running a 32-bit OS) then you won't be able to build Chromium OS.  The project would happily welcome patches to fix this. 

Verify that your default file permissions (umask) setting is correct

Sources need to be world-readable to properly function inside the chroot (described later). For that reason, the last digit of your umask should not be higher than 2, eg. '002' or '022'. Many distros have this by default, Ubuntu, for instance, does not. It is essential to put the following line into your ~/.bashrc file before you checkout or sync your sources.

umask 022

You can verify that this works by creating any file and checking if its permissions are correct.

$ touch ~/foo
$ ls -la ~/foo
-rw-r--r-- 1 user group 0 2012-08-30 23:09 /home/user/foo

Get the Source

Decide where your source will live

Chromium OS developers commonly put their source code in ${HOME}/chromiumos. If you feel strongly, put your own source elsewhere, but note that all commands in this document assume that your source code is in ${HOME}/chromiumos.

Create the directory for your source code with this command:

mkdir -p ${HOME}/chromiumos

IMPORTANT NOTE: If your home directory is on NFS, you must place your code somewhere else. Not only is it a bad idea to build directly from NFS for performance reasons, but builds won't actually work (builds use sudo, and root doesn't have access to your NFS mount, unless your NFS server has the no_root_squash option). Wherever you place your source, you can still add a symbolic link to it from your home directory (this is suggested), like so:

mkdir -p /usr/local/path/to/source/chromiumos
ln -s /usr/local/path/to/source/chromiumos ${HOME}/chromiumos

Get the source code

Chromium OS uses repo to sync down source code. repo is a wrapper for the git that helps deal with a large number of git repositories. You already installed repo when you installed depot_tools above.

Make sure you have followed the gerrit credentials setup instructions here.

(Note: If you are on a slow network connection or have low disk space, you can use the minilayout option below. This starts you out with a minimum amount of source code. This isn't a particularly well tested configuration and has been known to break from time-to-time, so we usually recommend against it.)

cd ${HOME}/chromiumos
# Note: Add the "-g minilayout" option to do a minilayout checkout.
repo init -u --repo-url [-g minilayout]
# Note: -j4 tells repo to concurrently sync up to 4 repositories at once. 
# You can adjust the number based on how fast your internet connection
# is. For the initial sync, it's generally requested that you use no
# more than 8 concurrent jobs. (For later syncs, when you already have
# the majority of the source local, using -j16 or so is generally okay).
repo sync -j4

Googlers: See goto/chromeos-building for internal notes.

Optionally add Google API keys

Secondly, decide whether you need to use features of Chromium that access Google APIs from the image you are building  (signing in, translating web pages, geolocation, etc).  If the answer is yes, you will need to have keys (see either in your include.gypi, or in a file in your home directory called ".googleapikeys".  If either of these file are present for step 1 of building (below) they will be included automatically.  If you don't have these keys, these features of chromium will be quietly disabled.

Branch Builds

If you want to build on a branch, pass the branch name to repo init (e.g: repo init -u <URL> [-g minilayout] -b 0.9.94.T).

When you use repo init you will be asked to confirm your name, email address, and whether you want color in your terminal. This command runs quickly. The repo sync command takes a lot longer.

More info can be found in the working on a branch page.

Building Chromium OS

Create a chroot

To make sure everyone uses the same exact environment and tools to build Chromium OS, all building is done inside a chroot.  This chroot is its own little world: it contains its own compiler, its own tools (its own copy of bash, its own copy of sudo), etc.  Now that you've synced down the source code, you need to create this chroot. Assuming you're already in ${HOME}/chromiumos (or wherever your source lives), the command to download and install the chroot is:


If this does not work, make sure you've added the depot_tools directory to your PATH already (as was needed above with using repo).

This will download and setup a prebuilt chroot from Chromium OS mirrors (under 400M). If you prefer to rather build it from source, or have trouble accessing the servers, use cros_sdk --bootstrap. Note that this will also enter the chroot. If you prefer to build only, use --download.

The command with --bootstrap takes about half an hour to run on a four core machine. It compiles quite a bit of software, which it installs into your chroot, and downloads some additional items (around 300MB). While it is building you will see a regular update of the number of packages left to build. Once the command finishes, the chroot will take up total disk space of a little over 3GB.

The chroot lives by default at ${HOME}/chromiumos/chroot. Inside that directory you will find system directories like /usr/bin and /etc. These are local to the chroot and are separate from the system directories on your machine. For example, the chroot has its own version of the ls utility. It will be very similar, but it is actually a different binary than the normal one you use on your machine.


  • You shouldn't have to create the chroot very often. Most developers create it once and never touch it again unless someone explicitly sends out an email telling them to recreate their chroot.
  • The cros_sdk command currently doesn't work behind a proxy server, but there is a workaround.
  • To help personalizing your chroot, the file ${HOME}/.cros_chroot_init is executed (if it exists) when the chroot is created.  The path to the chroot is passed as its argument ($1).

Enter the chroot

Most of the commands that Chromium OS developers use on a day-to-day basis (including the commands to build a Chromium OS image) expect to be run from within the chroot. You can enter the chroot by calling:


This is the same command used to create the chroot, but if the chroot already exists, it will just enter.

NOTE: if you want to run a single command in the chroot (rather than entering the chroot), prefix that command with cros_sdk -- .

This command will probably prompt you for your password for the sudo command (entering the chroot requires root privileges). Once the command finishes, that terminal is in the chroot and you'll be in the ~/trunk/src/scripts directory, where most build commands live. In the chroot you can only see a subset of the filesystem on your machine. However, through some trickery (bind mounts), you will have access to the whole src directory from within the chroot – this is so that you can build the software within the chroot.

Note in particular that the src/scripts directory is the same src/scripts directory found within the Chromium OS directory you were in before you entered the chroot, even though it looks like a different location. That's because when you enter the chroot, the ~/trunk directory in the chroot is mounted such that it points to the main Chromium OS directory ${HOME}/chromiumos. That means that changes that you make to the source code outside of the chroot immediately take effect inside the chroot.

Calling this will also install a chroot, if you don't have one yet, for example by not following the above.

While in the chroot you will see a special "(cros-chroot)" prompt to remind you that you are there:

(cros-chroot) johnnyrotten@flyingkite ~/trunk/src/scripts $ 

You generally cannot run programs on your filesystem from within the chroot. For example, if you are using eclipse as an IDE, or gedit to edit a text file, you will need to run those programs outside the chroot. As a consolation, you can use vim. If you are desperate for emacs, try typing sudo emerge emacs. Of course this command will build emacs from source so allow 5-10mins.


  • In this document, commands that are intended to be entered inside the chroot, and paths inside the chroot, are colored purple.
  • If you need to delete your chroot, use cros_sdk --delete to delete it properly. Using rm -rf could end up deleting your source tree due to the active bind mounts.


  • If you need to share lots of files inside and outside chroot (for example, settings for your favorite editor / tools, or files downloaded by browser outside chroot, etc.), read Tips and Tricks.
  • There is a file system loop because inside ~/trunk you will find the chroot again. Don't think about this for too long. If you try to use du -s ${HOME}/chromiumos/chroot/home you might get a message about a corrupted file system. This is nothing to worry about, and just means that your computer doesn't understand this loop either. (If you can understand this loop, try something harder.)

Select a board

Building Chromium OS produces a disk image (usually just called an "image") that can be copied directly onto the boot disk of a computer intended to run Chromium OS.  Depending on the specifics of that computer, you may want different files in the disk image. For example, if your computer has an ARM processor, you'll want to make sure that all executables in the image are compiled for the ARM instruction set. Similarly, if your computer has special hardware, you'll want to include a matching set of device drivers.
Different classes of computers are referred to by Chromium OS as different target "boards"  The following are some example boards:
  • amd64-generic - builds a generic image suitable for computers with a x86_64-compatible CPU (64 bit)
    • Use this board if you're unsure what you want ... most likely this is the one you want!
  • x86-generic - builds a generic image suitable for computers with a x86-compatible CPU (32 bit)
  • arm-generic - builds a generic image suitable for computers with an ARM CPU (32 bit)
You need to choose a board for your first build. Don't worry too much about this choice – you can always build for another board later. If you want a list of known boards, you can look in ~/trunk/src/overlays.

Each command in the build processes takes a --board parameter.  To facilitate this, it can be helpful to keep the name of the board in a shell variable. This is not strictly necessary, but if you do this, you can simply copy and paste the commands below into your terminal program. Enter the following inside your chroot (note: change amd64-generic to whatever board you want to build for):

export BOARD=amd64-generic

This setting only holds while you stay in the chroot. If you leave and come back, you need to do specify this setting again.

  • If you look at ~/trunk/src/overlaysyou may notice two different naming patterns: overlay-BOARD and overlay-variant-BOARD-VARIANT (note the dash here). If you intend to build for particular board variant, you'll want to use BOARD_VARIANT pair (note the underscore here) as your board name. E.g. for overlay-variant-peach-pi the correct value for BOARD would be peach_pi.

Initialize the build for a board

NOTE: this step is no longer required, as it is part of the build_packages step

To start building for a given board, issue the following command inside your chroot (you should be in the ~/trunk/src/scripts directory):

./setup_board --board=${BOARD}

This command sets up the board target with a default sysroot of /build/${BOARD}. The command downloads a small amount of stuff and takes a few minutes to complete.

  • If you pass the --default flag to setup_board, the command writes the board name in the file ~/trunk/src/scripts/.default_board (it does the same thing as echo ${BOARD} > ~/trunk/src/scripts/.default_board). This makes it so that you don't need to specify a --board argument to subsequent commands. These instructions do not use the --default flag so that you can explicitly see what commands are board-specific.
  • If you have previously set up this board, the setup_board command will fail. If you really want to clobber your old board files and start fresh, try passing the --force flag, which deletes the old /build/${BOARD} directory for you. Like cros_sdk, most people only re-run setup_board when told to (they don't re-run it even after a repo sync).
  • You can delete the board sysroot at any time with: sudo rm -rf /build/${BOARD}

Set the chronos user password

On a Chromium OS computer, you can get command line access (and root access through the sudo command) by logging in with the shared user account "chronos".  You should set a password for the chronos user by entering the command below from inside the ~/trunk/src/scripts directory:


You will be prompted for a password, which will be stored in encrypted form in /etc/shared_user_passwd.txt

  • The encrypted password is stored inside chroot. That means that if you recreate your chroot, you have to run this command again.
  • If you don't set a shared user password, the password for the chronos account may end up being any number of things depending on some complicated (and constantly evolving) formula that includes whether you build a developer image, whether you boot into developer mode, and the current state of the scripts.  The password might be empty, or a well-known string (such as "test0000" on test images), or it might be impossible to login with chronos.  It is strongly recommended that you simply set a shared user password.  TODO: put a link to some place with more info about this.

Build the packages for your board

To build all the packages for your board, run the following command from inside the ~/trunk/src/scripts directory:

./build_packages --board=${BOARD}

This step is the rough equivalent of make all in a standard Makefile system.  This command handles incremental builds; you should run it whenever you change something and need to rebuild it (or after you run repo sync). 

Normally, the build_packages command builds the stable version of a package (i.e. from committed git sources), unless you are working on a package (with cros_workon). If you are working on a package, build_packages will build using your local sources. See below for information about cros_workon.

  • Even though there are some flags that can optimize your build (like --nowithdev, --nowithautotest, etc), you should not use these flags (even if you don't plan on building a developer / test image). There are some issues with virtual packages that can cause some hard-to-debug differences if you use one of these flags.
  • The --nowithdebug flag is an exception to the previous point.  By default, packages other than Chrome will be compiled in debug mode; that is, with  NDEBUG undefined and with debugging constructs like  DCHECK,  DLOG, and the red "Debug image" message present.  If you supply --nowithdebug, then  NDEBUG will be defined and the debugging constructs will be removed.
  • The first time you run the build_packages command, it will take a long time (around 90 minutes on a four core machine), as it must build every package, and also download about 1.7GB of source packages and 1.3GB of binary packages. See here for more information about what the build_packages command actually does. In short, it tries to download existing binary packages to avoid building anything (and puts them in/build/${BOARD}/packages for subsequent builds). Failing that, it downloads source packages, puts them in /var/lib/portage/distfiles-target, and builds them.

Build a disk image for your board

Once the build_packages step is finished, you can build a Chromium OS-base developer image by running the command below from inside the ~/trunk/src/scripts directory:

./build_image --board=${BOARD} --noenable_rootfs_verification test

The args for build_image specify what type of build you want (test in the example above). It is more convenient to use test images, but developers could also build developer (dev) images.  A developer image provides a Chromium OS-based image with additional developer packages.  To build a test image with additional test-specific packages that also accepts an incoming ssh connection, use 'test' instead of 'dev'. If building a test image, the password set using will be ignored and "test0000" will be the password instead. The --noenable_rootfs_verification turns off verified boot allowing you to freely modify the root file system.  The system is less secure using this flag, however, for rapid development you may want to set this flag.  If you would like a more secure, locked-down version of Chromium OS, then simply remove the --noenable_rootfs_verification flag.  Finally if you want just the pristine Chromium OS-based image (closest to Chrome OS but not quite the same), pass in base rather than dev.  Use build_image --help for more information.

The image produced by build_image will be located in ~/trunk/src/build/images/${BOARD}/versionNum/ (where versionNum will actually be a version number).  The most recent image produced for a given board will be symlinked to ~/trunk/src/build/images/${BOARD}/latest.

IMPORTANT NOTE: It's up to you to delete old builds that you don't need. Every time you run build_image, the command creates files that take up over 4GB of space(!).

Look at your disk image (optional)

The preferred way to mount the image you just built to look at its contents is:

./ --board=${BOARD} --safe -f $(./ --board=${BOARD})

If you built a test image, also make sure to add -i chromiumos_test_image.bin to this command.

The --safe option ensures you do not make accidental changes to the Root FS. 

Again, don't forget to unmount the root filesystem when you're done:

./ --board=${BOARD} -u 

Optionally, you can unpack the partition as separate files and mount them directly:

cd ~/trunk/src/build/images/${BOARD}/latest
./ chromiumos_image.bin
mkdir -p rootfs
sudo mount -o loop,ro part_3 rootfs

This will do a loopback mount of the rootfs from your image to the location ~/trunk/src/build/images/${BOARD}/latest/rootfs in your chroot.

If you built with "--noenable_rootfs_verification" you can omit the "ro" option to mount it read write.

If you built an x86 Chromium OS image, you can probably even try chrooting into the image:

sudo chroot ~/trunk/src/build/images/${BOARD}/latest/rootfs

This is a little hacky (the Chromium OS rootfs isn't really designed to be a chroot for your host machine), but it seems to work pretty well. Don't forget to exit this chroot when you're done.

When you're done, unmount the root filesystem:

sudo umount ~/trunk/src/build/images/${BOARD}/latest/rootfs 

Installing Chromium OS on your Device

Put your image on a USB disk

The easiest way to get your image running on your target computer is to put the image on a USB flash disk (sometimes called a USB key), and boot the target computer from the flash disk. The first step is to insert a USB flash disk (4GB or bigger) into your build computer. This disk will be completely erased, so make sure it doesn't have anything important on it.  Wait ~10 seconds for the USB disk to register, then type the following command:

cros flash usb:// ${BOARD}/latest

For more details on using this tool, see the Cros Flash pageNote that auto-mounting of USB devices should be turned off as it may corrupt the disk image while it's being written.

When the cros flash command finishes, you can simply unplug your USB key and it's ready to boot from.

IMPORTANT NOTE: To emphasize again, cros flash completely replaces the contents of your USB disk. Make sure there is nothing important on your USB disk before you run this command.

  • If you want to create a test image (used for allowing Chromium OS to talk to autotest), see the Running Tests section.

Disable verified boot

You need to disable verified boot to be able to use your own image. Sometimes it can be achieved by holding the reset shortcuts (Esc-F3-Power) for a long time. Try with your machine for a while until you figure it out. Disabling verified boot takes some time, and is often accompanied by entering developer mode.

How this is accomplished for non Google Chrome OS devices depends heavily on the BIOS and can vary drastically, so we will not attempt to cover that.

For Google Chrome OS devices, see the Developer Hardware page. (Pages of each devices will show how to enter dev mode which will disable verified boot.)

Boot from your USB disk

For Google Chrome OS devices, you must first put the device into developer mode. There are two potential ways to do this, depending on your device. If you have a physical developer switch, move it to ON position. Otherwise if your device is using “keyboard based developer / recovery switch”, do that by first Esc-F3-Power to recovery mode, then Ctrl-D and ENTER to activate the virtual developer switch (Note if you’re already in developer mode, Ctrl-D won’t do anything - just move to next step).

Entering developer mode takes a while, the device removes all personal data and resets itself.

Then you should set your system to boot from USB. Let it boot, login and open a shell (or switch to terminal 2 via Ctrl+Alt+F2). Run the following command:

sudo crossystem

You should see "dev_boot_usb" equal to 0. Set it to 1 to enable USB boot:

sudo crossystem dev_boot_usb=1

Now reboot. In the white screen (developer mode enabled) screen, plug-in the USB and press Ctrl+U (or whatever key boots from USB on your device).

For specific information about what works on various different machines, see the Developer Hardware page.

Installing your Chromium OS image to your hard disk

Once you've booted from your USB key and gotten to the command prompt, you can install your Chromium OS image to the hard disk on your computer with this command:


IMPORTANT NOTE: Installing Chromium OS onto your hard disk will WIPE YOUR HARD DISK CLEAN.

Getting to a command prompt on Chromium OS

Since you set the shared user password (with when you built your image, you have the ability to login as the chronos user:
  1. After your computer has booted to the Chromium OS login screen, press [ Ctrl ] [ Alt ] [ F2 ] to get a text-based login prompt. ( [ F2 ] may appear as  ] on your Notebook keyboard.)
  2. Login with the chronos user and enter the password you set earlier.
Because you built an image with developer tools, you also have an alternate way to get a terminal prompt. The alternate shell is a little nicer (in the very least, it keeps your screen from dimming on you), even if it is a little harder to get to. To use this alternate shell:
  1. Go through the standard Chromium OS login screen (you'll need to setup a network, etc.) and get to the web browser.  It's OK to login as guest.
  2. Press [ Ctrl ] [ Alt ] [ T ] to get the crosh shell.
  3. Use the shell command to get the shell prompt. NOTE: you don't need to enter the chronos password here, though you will still need the password if you want to use the sudo command.

Building an image to run in a virtual machine

Many times it is easier to simply run Chromium OS in a virtual machine like kvm.  You can adapt the previously built Chromium OS image so that it is usable by kvm (which uses qemu images) by entering this command from the ~/trunk/src/scripts directory:

./ --board=${BOARD}

This command creates the file ~/trunk/src/build/images/${BOARD}/latest/chromiumos_qemu_image.bin.

  • WARNING: After crbug/710629, 'betty' is the only board regularly run through pre-CQ and CQ VMTest and so is the most likely to work at ToT. 'betty' is based on 'amd64-generic', though, so 'amd64-generic' is likely to also work.
  • Only KVM/QEMU VM's are actively supported at the moment.
  • You can specify source/destination paths with the --from and --to parameters.
  • If you're interested in creating a test image (used for allowing Chromium OS to talk to autotest), see the Running Tests section.

Making changes to packages whose source code is checked into Chromium OS git repositories

Now that you can build and run Chromium OS, you're ready to start making changes to the code.

NOTE: If you skipped to this section without building your own system image, you may run into hard-to-fix dependency problems if build your own versions of system packages and try to deploy them to a system image that was built by a builder. If you run into trouble, try going through the full Building Chromium OS process first and installing your own system image.

Keep the tree green

Before you start, take a moment to understand Chromium's source management strategy of "keeping the tree green". For the Chromium OS project, keeping the tree green means:
  1. Any new commits should not destabilize the build:
    • Images built from the tree should always have basic functionality working.
    • There may be minor functionality not working, and it may be the case, for example, that you will need to use Terminal to fix or work around some of the problems. 

  2. If you must introduce unstable changes to the tree (which should happen infrequently), you should use parameterization to hide new, unstable functionality behind a flag that's turned off by default. The Chromium OS team leaders may need to develop mechanisms for parameterizing different parts of the system (such as the init script).

  3. Internal "dev channel" releases will be produced directly from the tree, with a quick branch to check-point the release version. Any fixes required for a release will be pulled from the tree, avoiding merges back to tree.
This strategy has many benefits, including avoiding separate build trains for parallel development (and the cost of supporting such development), as well as avoiding large, costly merges from forked branches.

SIDE NOTE: "Keep the tree green" means something a bit different for Chromium OS than for Chromium, which is much further along in its life cycle.

The steps in this section describe how to make changes to a Chromium OS package whose source is checked into the Chromium OS source control system. Specifically, this is a package where:
  • The ebuild for the package lives in the src/third_party/chromiumos-overlay or src/overlays/overlay-${BOARD} directories.
  • There is an ebuild for the package that ends with 9999.ebuild.
  • The ebuild inherits from the cros-workon class.
  • The ebuild has a KEYWORDS in the ebuild containing this architecture name (like "x86").
You can see a list of all such packages by running the following command from inside the ~/trunk/src/scripts directory:

cros_workon --board=${BOARD} --all list 

Run cros_workon start

The first thing you need to do is to mark the package as active. Use the command below, replacing ${PACKAGE_NAME} with your package name (e.g., chromeos-wm):

cros_workon --board=${BOARD} start ${PACKAGE_NAME}

This command:
  • Indicates that you'd like to build the 9999 version of the ebuild instead of the stable, committed version.
  • Indicates that you'd like to build from source every time.
  • If you specified that you wanted the minilayout when you did your repo init, this command adds a clause to your .repo/local_manifest.xml to tell repo to sync down the source code for this package next time you do a repo sync.

Run repo sync

After running cros_workon, sync down the sources. This is critical if you're using the minilayout, but is probably a good idea in any case to make sure that you're working with the latest code (it'll help avoid merge conflicts later).  Run the command below from outside the chroot, anywhere under your ~/chromiumos directory:

repo sync

Find out which ebuilds map to which directories

The cros_workon tool can help you find out what ebuilds map to each directory. You can view a full list of ebuilds and directories using the following command:

cros_workon --board=${BOARD} --all info

If you want to find out which ebuilds use source code from a specific directory, you can use grep to find them. For example:

cros_workon --board=${BOARD} --all info | grep platform/ec

This returns the following output:
    chromeos-base/ec-utils chromiumos/platform/ec src/platform/ec

This tells you the following information:
  1. The name of the ebuild is chromeos-base/ec-utils
  2. The path to the git repository on the server is chromiumos/platform/ec
  3. The path to the source code on your system is src/platform/ec
You can similarly find what source code is associated with a given ebuild by grepping for the ebuild name in the list.

To find out where the ebuild lives:

equery-${BOARD} which ${PACKAGE_NAME}

As an example, for PACKAGE_NAME=ec-utils, the above command might display:

SIDE NOTE: If you run the same command without running cros_workon first, you can see the difference:

Create a branch for your changes

Since Chromium OS uses repo/git, you should always create a local branch whenever you make changes.

First, find the source directory for the project you just used cros_workon on. This isn't directly related to the project name you used with cros_workon. (This isn't very helpful - someone with more experience, actually tell us how to find it reliably? --Meredydd)

cd into that directory, in particular the "files/" directory in which the actual source resides. In the command below, replace ${BRANCH_NAME} with a name that is meaningful to you and that describes your changes (nobody else will see this name):

repo start ${BRANCH_NAME} .

The branch that this creates will be based on the remote branch (which one? --Meredydd). If you've made any other local changes, they will not be present in this branch.

Make your changes

You should be able to make your changes to the source code now.  To incrementally compile your changes, use 

cros_workon_make --board=${BOARD} ${PACKAGE_NAME}

This will build your package inside your source directory.  Change a single file, and it will rebuild only that file and re-link.  If your package contains test binaries, using

cros_workon_make --board=${BOARD} ${PACKAGE_NAME} --test

will build and run those binaries as well.  Call cros_workon_make --help to see other options that are supported.

You probably want to get your changes onto your device now.  You need to install the changes you made by using 

cros_workon_make --board=${BOARD} ${PACKAGE_NAME} --install

You can then rebuild an image with build_image and reimage your device, but you might also want to take a peek at cros deploy if you want something a little quicker.

Set your editor

Many of the commands below (in particular git) open up an editor. You probably want to run one of the three commands below depending on your favorite editor. 

If you're not a *nix expert, nano is a reasonable editor:

export EDITOR='nano'

If you love vi:

export EDITOR='vi'

If you love emacs (and don't want an XWindow to open up every time you do something):

export EDITOR='emacs -nw'

You should probably add one of those lines to your .bashrc (or similar file) too.

Submit changes locally

When your changes look good, commit them to your local branch using git. Full documentation of how to use git is beyond the scope of this guide, but you might be able to commit your changes by running something like the command below from the project directory:

git commit -a

The git commit command brings up a text editor. You should describe your changes, save, and exit the editor. Note that the description you provide is only for your own use. When you upload your changes for code review, the repo upload command grabs all of your previous descriptions, and gives you a chance to edit them.

Upload your changes and get a code review

Once your changes are committed locally, upload your changes using repo upload. The repo upload command takes all of the changes that are unmerged and asks them if you want to upload them.  You can specifically say to only look for unmerged changes in your current repo by passing in '.'.  Please note that you must have a Gerrit account before you can upload changes.

repo upload [.|${PROJECT-NAME}] [--current-branch]

It is important to note that repo uses the Change-Id in your git commits to track code reviews.  So in order to work on a CL the standard work flow is to use git commit --amend rather than make a new commit.  This differs from our old git-cl style.

In your local commit logs make sure to add a BUG= field and TEST= field.  For BUG=, you should put something that looks like: BUG=bug-tracker:number. You can get a tracker name by looking in the upper-left corner of the tracker page (e.g. in the image on the right, the tracker name is "chromium-os"). If your changes are related to more than one tracker issue, you can list all the issues separated with commas.

For TEST=, you should describe what you did to test the changes.

Here's what a sample description should look like:

# Enter a description of the change.
# This will displayed on the codereview site.
# The first line will also be used as the subject of the review.
Here's a SHORT, one-line summary of my change.

And here are more details
...this can be as long as I want.

BUG=chromium-os:99999, chromium:88888
TEST=Ran all the white box tests

Change-Id: I8d7f86d716f1da76f4c85259f401c3ccc9a031ff
Once you run repo upload, this uploads the changes and prints out a URL for the code review (if it's a new code review). Go to that URL (log in with your account, which you might want to do in an "incognito" window in Chrome), and use the "Review->Publish Comments" link to mail your changes to your reviewers.

You should pick reviewers that know the code you're working on well and that will do the best reviews. Picking reviewers who will just rubber-stamp your changes is a bad idea. The point of submitting changes is to submit good code, not to submit as much code as you can. If you don't know who should review your changes, start by looking at the git log for the project that you're working on. Simply type the command below in a directory related to your project:

git log

Your reviewers will likely provide comments about changes that you should make before submitting your code. You should make such changes, submit them locally, and then re-upload your changes for code review by amending your changes to your git commit and re-running repo upload.
# make some changes
git add -u .
git commit --amend

If you have a chain of commits (which repo upload . converts to a chain of CLs), and you need to modify any commits that are not at the top of the chain, use interactive rebase:
git rebase -i
# This shows a list of cherry-picks into a temporary branch.
# Change some of the "pick" keywords to "edit".  Then exit the editor.
git log # shows you are at the first "edit"ed commit.  All earlier commits are cherry-picked.
# Make some modifications.
git add -u .
git commit --amend
git rebase --continue # goes to the next "edit"ed commit.
repo upload . --current-branch # when all modifications are ready to be uploaded again.

Try seeing the mainline again

While you're working on your changes, you might want to go back to the mainline for a little while (maybe you want to see if some bug you are seeing is related to your changes, or if the bug was always there). If you want to go back to the mainline without abandoning your changes, you can run the following command from within a directory associated with your project:

git checkout cros/master

When you're done, you can get back to your changes by running:

git checkout ${BRANCH_NAME} 

Work on something else while you're waiting

If you want to start on another (unrelated) change while waiting for your code review, you can repo start another branch. When you want to get back to your first branch, run the following command from within a directory associated with your project:

git checkout ${BRANCH_NAME} 

Send your changes to the Commit Queue

Eventually, all your reviewers will be happy and will give you a Looks Good and Approved (the latter is only available to owners of the code) message in Gerrit.  When a reviewer is satisfied with the CL but wants other reviewers to approve they may give a +1. If they choose to test it they can also mark Verified but typically you'll need to test the CL and mark Verified yourself.  Both Looks Good and Approved (+2) and Verified must be set in order to commit your CL.  Once they are set, set the Commit Ready bit on your CL and the commit queue will test the change.

Before the Commit Queue picks up your CL it must pass a pre-CQ trybot.  This trybot run is triggered automatically when your CL is marked Looks Good and Approved.  You can trigger this trybot earlier by checking 'Trybot-Ready' and, if the run passes, it will not be required again before the Commit Queue picks up the CL.

Note it is possible that your change will be rejected because of a merge conflict.  If it is, rebase against any new changes and re-upload your patch.  This patch will have to be re-approved before it is allowed to be committed. (Note: If it's just a trivial rebase, you can approve the rebase yourself.)

More details on the Commit Queue can be found in the Commit Queue Overview.

Make sure your changes didn't break things

After you commit, make sure you didn't break the build by checking the buildbot.

Share your changes using the Gerrit sandbox

It is possible to upload changes to a personal sandbox on Gerrit. This way, a change can be shared between developers before it is ready for code review.

project_url=$(git config remote.cros.projectname)
git push ${project_url} HEAD:refs/sandbox/${USER}/${BRANCH_NAME}

Other developers can then fetch your changes using the following commands:
project_url=$(git config remote.cros.projectname)
git fetch ${project_url} refs/sandbox/${USER}/${BRANCH_NAME}
git checkout FETCH_HEAD

In a given repository, you can explore sandboxes using the ls-remote command:

git ls-remote cros "refs/sandbox/${USER}/*"
git ls-remote cros "refs/sandbox/*"

Once you're finished with a sandbox, you can delete it using the following commands:

project_url=$(git config remote.cros.projectname)
git push $project_url :refs/sandbox/${USER}/${BRANCH_NAME}

Clean up after you're done with your changes

After you're done with your changes, you're ready to clean up. The most important thing to do is to tell cros_workon that you're done by running the following command:

cros_workon --board=${BOARD} stop ${PACKAGE_NAME}

This command tells cros_workon to stop forcing the -9999.ebuild and to stop forcing a build from source every time.

You can also delete the branch that repo created. There are a number of ways to do so; here is one way:


  • If you don't specify a project name, the repo abandon command will throw out any local changes across all projects. You might also want to look at git branch -D or repo prune.
  • If you're using the minilayout, doing a cros_workon stop will not remove your source code. The code will continue to stay on your hard disk and get synced down.

Making changes to non-cros_workon-able packages

If you want to make to changes to something other than packages whose source is checked into the Chromium OS source control system, you can follow the instructions in the previous section, but skip the cros_workon step. Note specifically that you still need to run repo start to create a branch for your changes.

The types of changes that fall into this category include:
  • changes to build scripts (pretty much anything in src/scripts)
  • changes to ebuild files themselves (like the ones in src/third_party/chromiumos-overlay)
    • To change these files, you need to "manually uprev" the package. For example, if we're making a modification to for the Pixel overlay (link), then 
    • cd src/overlays/overlay-link/chromeos-base/chromeos-bsp-link
    • mv chromeos-bsp-link-0.0.2-r29.ebuild chromeos-bsp-link-0.0.2-r30.ebuild
    • chromeos-bsp-link-0.0.2-r29.ebuild is a symlink that points to chromeos-bsp-link-0.0.2.ebuild. To uprev the package, simply increment the revision (r29) number.
  • adding small patches to existing packages whose source code is NOT checked into Chromium OS git repositories
  • changes to eclass files (like the ones in src/third_party/chromiumos-overlay/eclass)
  • changes to the buildbot infrastructure (in crostools)
  • TODO: anything else?

Adding small patches to existing packages

When you need to add small patches to existing packages whose source code is not checked into a Chromium OS git repository (e.g. it comes from portage, and is not a cros_workon-able package), you need to do the following:

First, find the package ebuild file under third_party/chromiumos-overlay.

Then, create a patch file from the exact version of the package that is used by the current ebuild.  If other patches are already in the ebuild, you'll want to add your patch LAST, and build the patch off of the source that has already had the existing patches applied (either do it by hand, or set FEATURES=noclean and build your patch off of the temp source).  Note that patch order is significant, since the ebuild expects each patch line number to be accurate after the previous patch is applied.

Place your patch in the "files" subdir of the directory that contains the ebuild file (e.g. third_party/chromiumos-overlay/dev-libs/mypackage/files/mypackage-1.0.0-my-little-patch.patch).

Then, in the prepare() section of the ebuild (create one if it doesn't exist), add an epatch line:

epatch "${FILESDIR}"/${P}-my-little-patch.patch

Lastly, you'll need to bump the revision number in the name of the ebuild file (or symlink) so the build system picks up the change.  The current wisdom is that the ebuild file should be symlinked instead of being renamed.  For example, if the original ebuild file is "mypackage-1.0.0.ebuild", you should create a "mypackage-1.0.0-r1.ebuild" symbolic link that points at the original ebuild file.  If that symlink already exists, create the next higher "rN" symlink.

Making changes to the way that the chroot is constructed

TODO: This section is currently a placeholder, waiting for someone to fill it in.  However, a few notes:
  • Many of the commands that take a --board=${BOARD} parameter also take a --host parameter, which makes the commands affect the host (i.e. the chroot) rather than the board.
    • Most notably, cros_workon --host says that you want to build a package used in the chroot from source.

Building an individual package

TODO: Document this better, and add the new cros_workon_make.

SIDE NOTE:To build an individual portage package, for a particular board, use emerge-${BOARD}.
For example, if you want to build dash to test on your device:

emerge-${BOARD} dash

To install the package to the device, see cros deploy.

Making changes to the Chromium web browser on Chromium OS

If you just want to make modifications to the Chromium web browser and quickly deploy your changes to an already-built Chromium OS image, see Making changes to the Chromium web browser on Chromium OS.

To use your local checkout of the Chromium source code when building a Chromium OS image, set the --chrome_root flag appropriately when entering the chroot, e.g.

cros_sdk --chrome_root=${HOME}/chrome

Within the chroot, you'll also need to either start working on the chromeos-chrome package:

cros_workon --board=${BOARD} start chromeos-chrome

or set the CHROME_ORIGIN environment variable appropriately:


See src/third_party/chromiumos-overlay/chromeos-base/chromeos-chrome/chromeos-chrome-9999.ebuild for additional possible values for the CHROME_ORIGIN variable.

If you have an internal checkout of the Google Chrome source and want to build the browser with official branding, export USE=chrome_internal.

Using Clang to get better compiler diagnostics


The ChromeOS toolchain provides a feature to get Clang syntax-only compiler diagnostics without having to do a separate build with the Clang compiler. To enable this feature, add '-clang' to the C[XX]FLAGS used by the package for its build. 

Addition of the '-clang' option to the build is interpreted by the compiler driver wrapper script, which then invokes Clang with -fsyntax-only option, and after a successful Clang run, invokes the gcc compiler. Any errors generated by Clang will stop compilation just like a regular build does. In addition to Clang warnings, you will also see warning from gcc, in some cases for the same source construct. 

The presence of a few specific gcc options, for example, '-print-*' or '-E' will disable a clang run, even if '-clang' is specified. This is to allow package configure scripts to run correctly even in the presence of the '-clang' option. 

Wrapper Options

The wrapper script also interprets a few other options. All options specific to the wrapper only are tabulated below:

 Option Description
 -clang Invoke Clang front-end with -fsyntax-only and all other options specified on the command line. On successful completion of Clang compile, continue the build with gcc or g++. 
The presence of '-print-*', '-dump*', '@*', '-E', '-' or '-M' will disable clang invocation. 
 -Xclang-only=<option> This is a special option that can be used to pass <option> to Clang and not to gcc or g++. This can be used, for example, to turn off a specific Clang warning.
Example: -Xclang-only=-Wno-c++11-extensions will add -Wno-c++11-extensions to the Clang invocation.
-print-cmdline In addition to doing the builds, print the exact command-line used for both Clang and gcc. 


You can test your package with Clang before adding '-clang' to your ebuild or Makefiles using the CFLAGS or CXXFLAGS variable. While using this, you need to be careful not to overwrite existing CFLAGS or CXXFLAGS. Here's an example: 

(cr) $ CFLAGS="$(portageq-$board envvar CFLAGS) -clang" CXXFLAGS="$(portageq-$board envvar CXXFLAG) -clang" emerge-$board chromeos-chrome

After your package builds cleanly with Clang, you can add '-clang' to your e-build. 

Local Debugging

Debugging both x86 and non-x86 binaries on your workstation.

If you build your projects incrementally, write unit tests and use them to drive your development, you may want to debug your code without shipping it over to a running device or VM.

gdb-${BOARD} sets up gdb in your board sysroot and ensures that gdb is using the proper libraries, debug files, etc. for debugging, allowing you to run your target-compiled binaries. 
It should already be installed in your chroot.  If you do not have the script, update your repository to get the latest changes, then re-build your packages:

repo sync

./build_packages --board=...

This should install gdb-${BOARD} in the /usr/local/bin directory inside the chroot.   These board-specific gdb wrapper scripts correctly handle both local and remote debugging (see next section for more information on remote debugging).  When used for local debugging, these scripts will run inside a special chroot-inside-your-chroot, rooted in the board's sysroot. For example if you are using gdb-lumpy, it will run inside a chroot based entirely in your /build/lumpy sysroot.  The libraries that it will load and use are the libraries in the sysroot, i.e. the target libraries for the board; the gdb binary it will use is the gdb  binary in that tree.  While debugging with gdb-lumpy (for local debugging), you will not be able to see/access any files outside of the /build/lumpy tree.  While for the most part this is very good, as it ensures the correct debugging environment, it does mean that if you want to use this script to debug a lumpy binary, such as a unit test, that you built outside of the /build/lumpy tree, you will need to copy the binary to the /build/lumpy tree first.  Also, if you want the debugger to be able to see the source files when debugging, you will need to make sure they exist inside the /build/lumpy tree as well (see example below).

IMPORTANT NOTE 1:  Local and remote debugging functionality are combined in this single script.  Some of the options shown below only work for remote debugging.

IMPORTANT NOTE 2:  When doing local debugging of x86 binaries, they will try to execute on your desktop machine (using the appropriate libraries and gdb binaries).  It is possible that for some x86 boards, the binaries may use instructions not understood by your hardware (particularly some vector instructions), in which case you will need to do remote debugging with the actual hardware instead.

IMPORTANT NOTE 3:  You can use this script with *some* debugging functionality for local debugging of non-x86 binaries.  The script loads qemu and runs the non-x86 binaries in qemu.  However qemu
has some unfortunate limitations.  For example you can "set" breakpoints in the binary (to see what addresses correspond to locations in the source), examine the source or assembly code, and execute the program.  But qemu does not actually hit the breakpoints, so you cannot suspend execution in the middle when running under qemu.   For full debugging functionality with non-x86 binaries, you must debug them remotely running on the correct hardware (see next section on remote debugging).  You can see this in the example below, where gdb-daisy does not actually stop at the breakpoint it appears to set, although it does correctly execute the program.

(cr) $ gdb-daisy -h

usage: cros_gdb [-h]
                [--log-level {fatal,critical,error,warning,notice,info,debug}]
                [--log_format LOG_FORMAT] [--debug] [--nocolor]
                [--board BOARD] [-g GDB_ARGS] [--remote REMOTE] [--pid PID]
                [--remote_pid PID] [--no-ping] [--attach ATTACH_NAME] [--cgdb]
                [binary-to-be-debugged] [args-for-binary-being-debugged]

Wrapper for running gdb.

This handles the fun details like running against the right sysroot, via
qemu, bind mounts, etc...

positional arguments:
  inf_args              Arguments for gdb to pass to the program being
                        debugged. These are positional and must come at the
                        end of the command line. This will not work if
                        attaching to an already running program.

(cr) $ gdb-daisy /bin/grep shebang /bin/ls
15:51:06: INFO: RunCommand: file /build/daisy/bin/grep
Reading symbols from /bin/grep...Reading symbols from /usr/lib/debug/bin/grep.debug...done.
(daisy-gdb) b main
Breakpoint 1 at 0x2814: file grep.c, line 2111.
(daisy-gdb) disass main
Dump of assembler code for function main:
   0x00002814 <+0>: ldr.w r2, [pc, #3408] ; 0x3568 <main+3412>
   0x00002818 <+4>: str.w r4, [sp, #-36]!
   0x0000281c <+8>: movs r4, #0
   0x0000281e <+10>: strd r5, r6, [sp, #4]
   0x00002822 <+14>: ldr.w r3, [pc, #3400] ; 0x356c <main+3416>
   0x00002826 <+18>: movs r5, #2
   0x00002828 <+20>: strd r7, r8, [sp, #12]
(daisy-gdb) run
Starting program: /bin/grep shebang /bin/ls
qemu: Unsupported syscall: 26
#!/usr/bin/coreutils --coreutils-prog-shebang=ls
qemu: Unsupported syscall: 26
During startup program exited normally.
(daisy-gdb) quit

Note in the example above that, like "regular" gdb when given --args, you can pass the arguments for the program being debugged to the gdb wrapper script just by adding them to the command line after the name of the program being debugged (except that --args isn't needed). 

The commands below show how to copy your incrementally-compiled unit test binary and source file(s) to the appropriate sysroot and then start gdb with that binary (using the correct libraries, etc).

(cr) $ cd /build/lumpy/tmp/portage
(cr) $ mkdir shill-test
(cr) $ cd shill-test
(cr) $ cp <path-to-binary>/shill_unittest .
(cr) $ cp <path-to-src>/ .
(cr) $ gdb-lumpy
(gdb-lumpy) directory /tmp/portage/shill-test          # Tell gdb to add /tmp/portage/shill-test to the paths it searches for source files
(gdb-lumpy) file ./shill_unittest

If gdb is still looking for the source file in the wrong directory path, you can use 'set substitute-path <from> <to>' inside gdb to help it find the right path (inside your sysroot) for searching for source files.

Remote Debugging

Setting up remote debugging by hand.

If you want to manually run through all the steps necessary to set up your system for remote debugging and start the debugger, see Remote Debugging in Chromium OS.

Automated remote debugging using gdb-${BOARD} script (gdb-lumpy, gdb-daisy, gdb-parrot, etc).

gdb-${BOARD} is a script that automates many of the steps necessary for setting up remote debugging with gdb.  It should already be installed in your chroot.  If you do not have the script, update your repository to get the latest changes, then re-build your packages:

repo sync

./build_packages --board=...

This should install gdb_remote in the /usr/bin directory inside the chroot.  The gdb-${BOARD} script takes several options.  The most important ones are mentioned below.

"--gdb_args" ("-g") are arguments to be passed to gdb itself (rather than to the program gdb is debugging).  If multiple arguments are passed, each argument requires a separate -g flag. 
E.g gdb-lumpy --remote=  -g "-core=/tmp/core" -g "-directory=/tmp/source"

"--remote" is the ip_address or name for your chromebook, if you are doing remote debugging.  If you omit this argument, the assumption is you are doing local debugging in the sysroot on your desktop (see section above).  if you are debugging in the VM, then you need to specify either ':vm:' or 'localhost:9222'.

"--pid" is the pid of a running process on the remote device to which you want gdb/gdbserver to attach.

"--attach" is the name of the running process on the remote device to which you want gdb/gdbserver to attach.  If you want to attach to the Chrome browser itself, there are three special names you can use:  'browser' will attach to the main browser process; 'gpu-process' will attach to the GPU process; and 'renderer' will attach to the renderer process if there is only one.  If there is more than one renderer process --attach=renderer will return a list of the renderer pids and stop.

To have gdb/gdbserver start and attach to a new (not already running) binary, give the name of the binary, followed by any arguments for the binary, at the end of the command line:

$ gdb-daisy --remote= /bin/grep "test" /tmp/myfile

When doing remote debugging you *must* use the --pid or the --attach option, or specify the name of a new binary to start.  You cannot start a remote debugging session without having specified the program to debug in one of these three ways.

When you invoke gdb-${BOARD} --remote=..., it will connect to the notebook or VM (automatically setting up port-forwarding on the VM), make sure the port is entered into the iptables, and start up gdbserver, using the correct port and binary, either attaching to the binary (if a remote pid or name was specified) or starting up the binary.  It will also start the appropriate version of gdb (for whichever type of board you are debugging) on your desktop and connect the gdb on your desktop to the gdbserver on the remote device.

Examples of debugging using the gdb-${BOARD} script.

Below are three examples of using the board-specific gdb wrapper scripts to start up debugging sessions.  The first two examples show connecting to a remote chromebook.  The first one automatically finds the browser's running GPU process, attaches gdbserver to the running process, starts gdb on the desktop, and connects the local gdb to gdbserver.  It also shows the user running  the 'bt' (backtrace) command after gdb comes up.  The second example shows the user specifying the pid of a process on the chromebook.  Again the script attaches gdbserver to the process, starts gdb on the desktop, and connects the two.  The third example shows the user connecting to the main browser process in ChromeOS running in a VM on the user's desktop.    For debugging the VM, you can use either
--remote=:vm: or --remote=localhost:9222  (":vm:" gets translated into "localhost:9222").

Example 1:

$ gdb-lumpy --remote= --attach=gpu-process

14:50:07: INFO: RunCommand: ping -c 1 -w 20
14:50:09: INFO: RunCommand: file /build/lumpy/opt/google/chrome/chrome
14:50:10: INFO: RunCommand: x86_64-cros-linux-gnu-gdb --quiet '--eval-command=set sysroot /build/lumpy' '--eval-command=set solib-absolute-prefix /build/lumpy' '--eval-command=set solib-search-path /build/lumpy' '--eval-command=set debug-file-directory /build/lumpy/usr/lib/debug' '--eval-command=set prompt (lumpy-gdb) ' '--eval-command=file /build/lumpy/opt/google/chrome/chrome' '--eval-command=target remote localhost:38080'
Reading symbols from /build/lumpy/opt/google/chrome/chrome...Reading symbols from/build/lumpy/usr/lib/debug/opt/google/chrome/chrome.debug...done.
(lumpy-gdb) bt 
#0  0x00007f301fad56ad in poll () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f3020d5787c in g_main_context_poll (priority=2147483647, n_fds=3,   fds=0xdce10719840, timeout=-1, context=0xdce1070ddc0) at gmain.c:3584 
#2  g_main_context_iterate (context=context@entry=0xdce1070ddc0,block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>)  at gmain.c:3285
#3  0x00007f3020d5798c in g_main_context_iteration (context=0xdce1070ddc0may_block=1) at gmain.c:3351
#4  0x00007f30226a4c1a in base::MessagePumpGlib::Run (this=0xdce10718580, delegate=<optimized out>) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/message_loop/
#5  0x00007f30226666ef in base::RunLoop::Run (this=this@entry=0x7fff72271af0) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/
#6  0x00007f302264e165 in base::MessageLoop::Run (this=this@entry=0x7fff72271db0) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/base/message_loop/
#7  0x00007f30266bc847 in content::GpuMain (parameters=...) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/gpu/
#8  0x00007f30225cedee in content::RunNamedProcessTypeMain (process_type=..., main_function_params=..., delegate=0x7fff72272380 at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/
#9  0x00007f30225cef3a in content::ContentMainRunnerImpl::Run (this=0xdce106fef50) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/
#10 0x00007f30225cd551 in content::ContentMain (params=...) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/content/app/
#11 0x00007f3021fef02a in ChromeMain (argc=21, argv=0x7fff722724b8) at ../../../../../chromeos-cache/distfiles/target/chrome-src-internal/src/chrome/app/
#12 0x00007f301fa0bf40 in __libc_start_main (main=0x7f3021fee760 <main(int, char const**)>, argc=21, argv=0x7fff722724b8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>,stack_end=0x7fff722724a8) at libc-start.c:292
#13 0x00007f3021feee95 in _start ()

Example 2:

$ gdb-daisy --pid=626 --remote=
14:50:07: INFO: RunCommand: ping -c 1 -w 20
14:50:09: INFO: RunCommand: file /build/daisy/usr/sbin/cryptohomed
14:50:10: INFO: RunCommand: armv7a-cros-linux-gnueabi-gdb --quiet '--eval-command=set sysroot /build/daisy' '--eval-command=set solib-absolute-prefix /build/daisy' '--eval-command=set solib-search-path /build/daisy' '--eval-command=set debug-file-directory /build/daisy/usr/lib/debug' '--eval-command=set prompt (daisy-gdb) ' '--eval-command=file /build/daisy/usr/sbin/cryptohomed' '--eval-command=target remote localhost:38080'
Reading symbols from /build/daisy/usr/sbin/cryptohomed...Reading symbols from/build/daisy/usr/lib/debug/usr/bin/cryptohomed.debug...done.

Example 3:

$ gdb-lumpy --remote=:vm: --attach=browser
15:18:28: INFO: RunCommand: ping -c 1 -w 20 localhost
15:18:31: INFO: RunCommand: file /build/lumpy/opt/google/chrome/chrome
15:18:33: INFO: RunCommand: x86_64-cros-linux-gnu-gdb --quiet '--eval-command=setsysroot /build/lumpy' '--eval-command=set solib-absolute-prefix /build/lumpy' '--eval-command=set solib-search-path /build/lumpy' '--eval-command=set debug-file-directory /build/lumpy/usr/lib/debug' '--eval-command=set prompt (lumpy-gdb) ' '--eval-command=file /build/lumpy/opt/google/chrome/chrome' '--eval-command=target remote localhost:48062'
Reading symbols from /build/lumpy/opt/google/chrome/chrome...Reading symbols from /build/lumpy/usr/lib/debug/opt/google/chrome/chrome.debug...done.
Remote debugging using localhost:48062

If you find problems with the board-specific gdb scripts, please file a bug ( and add 'build-toolchain' as one of the labels in the bug.


I lost my developer tools on the stateful partition, can I get them back?

This happens sometimes because the security system likes to wipe out the stateful partition and a lot of developer tools are in /usr/local/bin. But all is not lost because there is a tool for updating the stateful partition from an image created by the auto-update part of the dev_server. Sadly, it is normally found in /usr/local so will have been lost too and you need to copy it over manually. This works for me:

$ cd /tmp
$ scp me@myworkstation:/path/to/chromiumos/chroot/build/x86-whatever/usr/bin/stateful_update .
$ sudo sh stateful_update
$ sudo reboot

Note you can clobber the stateful partition (remove user accounts etc and force OOBE) as part of this process by using a flag:

$ cd /tmp
$ scp me@myworkstation:/path/to/chromiumos/chroot/build/x86-whatever/usr/bin/stateful_update .
$ sudo sh stateful_update --stateful_change=clean
$ sudo reboot

Running Tests

Many of the automated tests that are part of the Chromium OS project run using the autotest framework. See Autotest User Documentation for details.

Various quick links:

Getting an image that has been modified for test - This doesn't work if you want to create a recovery image and boot a Cr48 with it, see below

If you wish to produce a VM image instead, you should omit the --test flag to build_image and let ./ produce the test image:
./ --board=${BOARD} --test_image 

Note: this difference between `cros flash` and ./ arises because ./ does not yet support the --image_name flag and by default looks for chromiumos_image.bin.  We expect this to change in the future.

Note that creating a test image will change the root password of the image to test0000.  The --test_image flag causes the image_to_xxx commands to make a copy of your chromiumos_image.bin file called chromiumos_test_image.bin (if that file doesn't already exist), modify that image for test, and use the test image as the source of the command.

  • If the directory already has a file called chromiumos_test_image.bin, that file is reused. That means that calling both cros flash and doesn't waste time by creating the test image twice.

Creating a recovery image that has been modified for test

After building a test image using ./build_image test as described above, you may wish to encapsulate it within a recovery image:

./ \
    --board=${BOARD} \
    --nominimize_image \
    --image ~/trunk/src/build/images/${BOARD}/latest/chromiumos_test_image.bin \
    --to ~/trunk/src/build/images/${BOARD}/latest/recovery_test_image.bin

If desired, you may specify a custom kernel with --kernel_image ${RECOVERY_KERNEL}.

You can write this recovery image out to the USB device like so:

cros flash usb:// ~/trunk/src/build/images/${BOARD}/latest/recovery_test_image.bin

Note that there are some downsides to this approach which you should keep in mind.

  • Your USB image will be a recovery mode/test image, but the ordinary image in your directory will be a non-test image.
  • If you use devserver, this will serve the non-test image not the test-image.
  • This means a machine installed with a test-enabled USB image will update to a non-test-enabled one.

Additional information

Toolchain Compilers

At any given time in the chroot, to see what cross-compiler version is the current default one, do:

  • For ARM: armv7a-cros-linux-gnueabi-gcc -v
  • For x86: i686-pc-linux-gnu-gcc -v
  • For amd64: x86_64-cros-linux-gnu-gcc -v

Developer mode

It can be confusing to figure out what "developer mode" means: Some parts of this document talk about switching your hardware to developer mode, while other parts talk about entering developer mode by running an image built with the --withdev flag. For the most part, the two actions enable the same set of things (like the shell command in crosh). Thus, you can think of either action as enabling developer mode.

To provide some details:

  • Putting your hardware into developer mode makes /bin/cros_boot_mode print developer. It also makes the file /mnt/stateful_partition/.developer_mode appear.
  • Building with the --withdev flag creates a file called /root/.dev_mode (in addition to adding a whole bunch of useful developer tools to your stateful partition). The presence of the /root/.dev_mode file enables developer mode and also prevents wiping of the stateful partition when you switch your hardware between developer mode and release mode.

Attribution requirements

When you produce a Chromium OS image, you need to fulfill various attribution requirements of third party licenses. Currently, the images generated by a build don't do this for you automatically. You must modify ~/chromium/src/chrome/browser/resources/chromeos/about_os_credits.html.

Documentation on this site

You now understand the basics of building, running, modifying, and testing Chromium OS, but you've still got a lot to learn. Here are links to a few other pages on the site that you are likely to find helpful (somewhat ordered by relevance):

External documentation

Below are a few links to external sites that you might also find helpful (somewhat ordered by relevance):