Eclipse CDT


Please contact Jonathan Watt before editing this page. In fact the instructions on this page are not working yet, so probably you shouldn't even bother reading it until this notice is removed. The old text on this page was years out of date and no longer correct/working. Jonathan is in the middle of a complete rewrite, but it's taking a fair bit of investigative work and coordination with the Eclipse CDT team to get various bugs worked and and to get indexing working well with the Mozilla source.

Eclipse CDT (C/C++ Development Tools) is an open-source IDE for C and C++ development. It supports complex projects that have their own build system, like Mozilla. If you want to use an IDE for Mozilla C++ development with advanced code assistance (inheritance/call graph explorer, jump to definition, refactoring, autocomplete, syntax highlighting, etc.), then Eclipse CDT might be for you.

System requirements

Eclipse provides many features that you'll find yourself unable to live without once you start using them, but it does come with some fairly hefty system requirements. In order to index the Mozilla source it requires 2-3 GB of free RAM. If you don't already have 16 GB of RAM installed, Eclipse provides you with the perfect excuse for an upgrade. :-)

Code assistance

Out of the box, Eclipse can provide some code assistance for the Mozilla source, but it will be fairly incomplete and often just plain broken. If you set up Eclipse as detailed below it will do a much, much better job.

Important background

To help you make sense of the instructions that follow (and so that you can modify them to meet your own needs if necessary) this section provides some background on what IDEs need in order to provide good code assistance, and what Eclipse CDT needs in particular.

To be able to provide advanced code assistance for a project's source code, IDEs like Eclipse need to carry out a thorough static analysis of the project's source files to build up a complete picture of the code. (What Eclipse is trying to do when it "indexes" the source.) Static analysis involves parsing the source files, so naturally enough it can only produce good results if it has a sensible set of preprocessor defines, include paths and pre-include files for each source file. Since Eclipse doesn't initially have any of this information, the code assistance that it's able to provide out of the box is pretty broken.

For projects the size and complexity of Mozilla, it's impractical to manually configuring Eclipse with a valid set of defines and include paths for each part of the source code. Happily, Eclipse makes manual configuration unnecessary. Like some other IDEs, for projects that have their own build system, Eclipse provides a tool that can collect the options that are passed to the compiler for each file that's compiled during an actual build. This process is sometimes called "compiler option discovery".

The way that Eclipse does compiler option discovery is to scan the console output from a real build looking for lines where a compiler was invoked. For each line that invokes a compiler, it tries to figure out which source file was being built and what its include paths were. If relative paths are used to specify the source file or any of its include paths (common in Mozilla), and if you fail to take steps to make sure the build output specify which directory the compiler is invoked from, this will not be possible. If Eclipse can successfully identify which source file was being compiled, then it can associate with that file the (resolvable) include paths, preprocessor defines and pre-include files that were passed to the compiler for that file.

Note the requirements that this method of compiler option discovery imposes on us. First, a build must be invoked from inside Eclipse so it can process the build output and find the compiler options. The build process must be explicitly instructed to output information that allows the directory that the compiler is being invoked from to be identified (i.e. make must be instructed to output "Entering directory..."/"Leaving directory..." lines). Finally, the build must not be parallelized (since that would result in interleaving of build output from different directories, breaking resolution of relative paths by interleaving the "Entering directory..."/"Leaving directory..." lines) and the build must not be silenced/quietened.

Conversely, note this very carefully: if you configure Eclipse to invoke a build process that is parallelized, silenced, or that fails to identify the directory that the compiler is being run from, then it will mess up the compiler options that Eclipse associates with your files, and that in turn will significantly degrade the quality of the code assistance that Eclipse will provide after next re-indexing the code.

Unfortunately, these requirements don't fit very well with the way that most Mozilla developers use parallelized and silenced builds in order to minimize their build times.

The consequences of the above observations are that:

  • It is strongly recommended that you invoke your normal (re)builds from the command line, externally of Eclipse.
  • We will configure Eclipse's build step below so that you only occasionally use it to manually trigger a special "build" that is purely for the purposes of setting/updating the compiler options Eclipse associates with each source file.

(Not doing your normal builds from inside Eclipse does loose you some minor benefits that Eclipse has to offer, but these are worth sacrificing for much better code assistance. See the "Build" section below if you're interested in what you loose.)

Creating a build log

This section assumes that you already have a copy of the Mozilla source and know how to build it.

As explained above, we're going to set Eclipse's build step to trigger a special "build" that you will only use occasionally whenever you need to set up or update the preprocessor defines, include paths and preincludes that Eclipse associates with each source file. (Doing this once every few months generally seems to be sufficient.) Rather than have Eclipse trigger a real build, we're going to trick it into reading a build log. This has the advantages that it's quicker to have Eclipse reprocess a build log if something goes wrong than to have it do a full rebuild. Additionally, with a quick search and replace edit, the build log can be reused to quickly set up Eclipse with any other source trees you may have.

We'll do this step first so that the build can be running while you're following the steps below to install and configure Eclipse.

First, note that some of the include paths in the build log will point into a specific object directory. These include paths need to resolve whenever Eclipse reindexes the source, so you want the object directory you choose below to be your normal object directory for your source tree.

Second, the build needs to be a full rebuild, so delete that object directory before you proceed so that it will be recreated from scratch.

That done, create a special mozconfig just for the purpose of creating the build log for Eclipse, and give it the following content.

# IMPORTANT!!! Do NOT use -j or -s in here!
# explains why!

# IMPORTANT!!! We absolutely require make to output "Entering"/
# "Leaving" lines:
mk_add_options MOZ_MAKE_FLAGS="-w"

# IMPORTANT!!! Make sure this is your normal objdir, and that you empty it
# before building so that you get a full rebuild:
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-debug

# A debug build is best so that Eclipse will index '#ifdef DEBUG'
# sections of the code:
ac_add_options --enable-debug --disable-optimize
ac_add_options --enable-debug-symbols

Make sure you edit the MOZ_OBJDIR line!

Now create the build log by running the following command, replacing '.mozconfig-eclipse' with the name that you gave the mozconfig file above:

MOZCONFIG=.mozconfig-eclipse make -f 1>build-for-eclipse.log 2>&1

Continue with the steps below while you wait for the build to finish.

Installing Eclipse

These instructions currently direct you to install Eclipse Juno, a developer build of Eclipse. ("Juno" is the code name of the next version of Eclipse, coincidentally scheduled for released on June 27, 2012.) Using Juno nightly is important because it contains improvements and fixes that allow it to provide much better code assistance with the Mozilla source.

Download "Eclipse IDE for C/C++ Developers" from the Eclipse Developer builds page, extract the file, and put the resulting directory somewhere sensible. (Consider renaming the Eclipse executable to "Eclipse-Juno" to remind you what version it is later.)

When you open Eclipse, it will ask you to "Select a workspace" (a directory where Eclipse will store files that it generates during indexing, etc.). It's recommended that you have a separate workspace for each Mozilla source tree, and that you choose a directory outside the Mozilla source. Something like $HOME/eclipse-workspaces/mozilla-tree-1, for example. After selecting an appropriate directory, click OK.

When the main Eclipse window opens, close the "Welcome" tab.

Now to use Eclipse's software update mechanism to update Juno to the latest Juno nightly...

From Eclipse's "Help" menu select "Install New Software...", then in the "Install" window that opens, click "Add...". In the "Add Repository" dialog that opens, set "Name" to something like "Juno Nightly", "Location" to "", then click "OK" to close the dialog. A "CDT Main Features" option should now have been added in the "Install" window. Tick this (all of its sub-options should then be ticked), click "Next" twice, accept the license agreement, and then click "Finish". Eclipse should now update itself and ask you to restart.

Troubleshooting: If you get an error when trying to add "Juno Nightly", try clicking "Available Software Sites" in the "Install" window, untick "Juno", tick "Juno Nightly", highlight "Juno Nightly", click "Reload", "OK", and then try again.

Increasing memory limits

When Eclipse's indexer tries to process the Mozilla source, Eclipse will need considerably more memory than it allows itself out of the box. You should increase its memory limits using an eclipse.ini. Set something like -Xms1G -Xmx3G -XX:MaxPermSize=1G (initial heap space of 1 GB, max heap space of 3 GB, max perm-gen space of 1 GB).

Restart Eclipse and check that your changes have taken affect and are present in Eclipse > About Eclipse > Installation Details > Configuration.

Initial global configuration

Whenever you create a new workspace for a Mozilla source tree, you should be sure to turn off the following two settings in Eclipse's global preferences (Window > Preferences, or Eclipse > Preferences) before you create a project in that workspace:

  • in "General > Workspace", disable "Build automatically"
  • in "C/C++ > Indexer", disable "Automatically update the index"

Turning off automatic indexing prevents the CPU and RAM intensive indexer from running at various stages in the steps below before we're ready.

Eclipse CDT will format new C/C++ code as you type. However, it's default formats are not a good match to Mozilla's style rules. Download this first pass at an Eclipse formatter configuration for Mozilla C/C++ coding style, and install it by opening Eclipse's global preferences, selecting "C/C++ > Code Style > Formatter", and then using the "Import" button to import that file. The "Active profile" field should then automatically change to "Mozilla". Depending on the area of the code that you work on, you may need to tweak this configuration using the "Edit" button. (Note that the format settings in "General > Editors > Text Editors" have no effect in C/C++ views, since the C/C++ settings are more specific and override these settings. However, you may still want to set some of these settings if you'll be editing other file types in Eclipse.)

Select "General > Workspace" and select "Refresh using native hooks or polling" and "Refresh on access" to prevent Eclipse giving you annoying "Resource is out of sync" messages when the source code change from under it due to Mercurial activity.

Select "C/C++ > Build > Console" and set "Limit console output (number of lines)" to something large like "1000000".

Select "C/C++ > Editor" and set "Workspace default" to "Doxygen".

Select "C/C++ > Editor > Content Assist" and set the Auto-Activation delay to 0 so that autocomplete suggestions don't seem to be laggy. (Sadly there is no auto-activation option to activate autocomplete suggestion as soon as you type any alphabetical character to begin a symbol name like there is in other IDEs.)

Select "C/C++ > Editor > Save Actions" and deselect "Ensure newline at the end of file".

Select "C/C++ > Editor > Scalability" and set "Enable scalability mode when the number of lines in the file is more than:" to 100000.

Select "C/C++ > Indexer" and set "Index unused headers".

Select "Run/Debug > Console" and deselect "Limit console output".

If you want to increase the text size in the editor, select "General > Appearance > Colors and Fonts", select "Basic > Text Font", and edit the font size.

Creating an Eclipse project

To create an Eclipse project for your Mozilla source tree select "File > New > Makefile Project with Existing Code". In the "Import Existing Code" window that opens, enter a meaningful Project Name that identifies your Mozilla source tree, set the code location to the root of your source tree, select an appropriate Toolchain (e.g. "MacOSX GCC"), and click Finish.

The status bar at the bottom right of the window should now show that Eclipse is "Refreshing the workspace" (gathering a list of all the files in the source tree). Click on the little green button beside this message to open the "Progress" tab, and keep an eye on the "Refreshing workspace" item as you continue with the steps below. If an "indexing" item starts, click the little red box beside that item to cancel it, since we want to configure the project before the indexer runs.

Initial project configuration

So that the indexer will run faster and give better results, you should add some resource filters to have Eclipse ignore certain files and directories. Note, you must not make Eclipse ignore the object directory that you chose to create the build log in the section above. That object directory is needed to resolve include paths to the various headers that the build process generates/copies there.

To create resource filters, open the project (not global) properties window (select Properties from the context menu for the project in the Project Explorer tab on the left, or from the menu bar select Project > Properties) and select "Resource > Resource Filters" on the left. Use the Add button to add the following filters:

  • Add an "Exclude all" filter for folders with a Project Relative Path matching ".hg". (This directory doesn't show in the Project Explorer tab, but Eclipse still indexes it without this filter!)
  • Add an "Exclude all", regular expression filter for folders with a Project Relative Path matching "obj(?!-debug(?:$|/)).+". This particular regular expression will exclude all object directories except "obj-debug", so if "obj-debug" is not the name of your main object directory, then adjust the regular expression as necessary.
  • Add recursive "Exclude all" filters for files with names matching the regular expression "^.+\.(?:rej|orig|o|pp)$" to exclude .rej, .orig, .pp and .o files.
  • Add "Exclude all" filters for any other non-source directories (e.g. Xcode or MSVC project directories).

Close the filters window, click "OK", wait for Eclipse to finish processing your filters, then make sure the filtered directories and files have disappeared from the Project Explorer tab on the left. Also check that your main object directory has not disappeared.

Reopen the project properties window, and select "C/C++ General > Preprocessor include Paths, Macros etc.". Select the Providers tab. Make sure that "CDT GCC Build Output Parser" is selected, and that "CDT Managed Build Settings Entries" is not selected. Highlight (select) "CDT GCC Build Output Parser", then in the "Language Settings Provider Options" that appear below, make sure that "Share setting entries between projects (global provider)" is not ticked.

Select "C/C++ Build" from the left of the project properties window. Select the "Builder Settings" tab, untick "Use default build command", set the build command to "cat build-for-eclipse.log", and set the build directory to "${ProjDirPath}/." Select the "Behaviour" tab, delete the word "all" from the "Build (incremental build)" field, and disable the Clean checkbox.

Using the build log

Hopefully it won't be too long now before the build that you started during the steps above has finish creating the build log you need to get Eclipse's static analysis working properly.

Reading in the build log

Once the build has finished, check the end of the log file to make sure that the build completed successfully, and to double check that it contains the "Entering"/"Leaving" lines that Eclipse needs. A handy way to do this on Mac/Linux is to print the last 50 lines using:

tail -n 50 build-for-eclipse.log

If everything went well, and if you configured the build commands and build directory as directed above, then click the Build button (the button with the hammer symbol) or select "Project > Build Project" from the main menu. Select the "Console" tab at the bottom of the main Eclipse window, and you should now see the contents of the build log flying by as Eclipse processes it. This may take about 10 minutes or so.

Building the index

Once you see the end of the build log in the Console tab, it's time to index the source! Right click the project in the Project Explorer tab and select "Index > Rebuild". You will now see "Indexing..." in the status bar at the bottom right. It will take 10-20 minutes on a decent developer machine for a full rebuild of the index. Once the indexing has finished you should find that Eclipse's code assistance works much better. :-)

As the source changes, you'll want to update the index to keep things working. You can either set the index to update automatically by opening the global preferences, selecting "C/C++ > Indexer" and reenebling "Automatically update the index", or else, if you find that too disruptive, you can leave that option disabled and update the index manually as necessary. To update the index manually, use the context menu for the project (or an individual directory) in the Project Explorer tab on the left side of the window, and select either "Index > Rebuild" or "Index > Freshen All Files".

Reusing the build log

A build log is source tree specific, since it contains lots of absolute paths pointing into a specific source tree. However, it's trivial to reuse the build log to set up Eclipse with other copies of the Mozilla source (rather than doing a special build for each tree). You just need to do a quick search and replace in the log file first. Specifically:

  • replace the absolute path to the root of the original source tree with the absolute path to the root of the tree you're setting up Eclipse with
  • replace the name of the object directory, if necessary

Your editor should tell you it's replaced about 10,000+ lines, then you're good to go.

Refreshing the build log

In my experience (Jonathan Watt's, during mid-2012), it seems to take a few months before changes to the Mozilla build system cause the compiler options that Eclipse has associated with the source to become noticeably stale. Your mileage may vary. When that happens, just create a new build log using the steps above, read it in, and rebuild the index.


To create a debug configuration, open the project properties window, and select "Run/Debug Settings" on the left. Click "New" then select "C/C++ Application". In the window that opens enter the path to your firefox binary (something like obj-debug/dist/ and select "Disable auto build". Select the Arguments tab and enter any args you want to pass to firefox (such as "-no-remote -p my-testing-profile"). If you're on Linux, you may also need to set the "Working directory" to "${ProjDirPath}/obj-debug/dist/bin", and then select the Environment tab and set LD_LIBRARY_PATH to ".:./plugins:.", and LIBRARY_PATH to ".:./components:.". (Is this really necessary?)

In the global preferences, you may want to go to "C/C++ > Debug > GDB" and deselect "Stop on startup at" so that Eclipse won't automatically break in main() when it starts Firefox for debugging.

To debug, click the Debug button on the toolbar, or select "Run > Debug" from the menu bar.

After you've finished debugging, you can get back to the C/C++ perspective via the menubar using "Window > Open Perspective".

Upgrading gdb on Mac

The ancient, partly maintained fork of gdb that comes with Xcode on Mac is really horrible. If you use Mac, you may want to build the latest FSF version of GDB and set Eclipse to use that gdb for debugging. One suggested configuration for building gdb is '--prefix="$HOME" --disable-debug --with-python=/usr' (create an optimized build with support for Python to better control when you break etc., and install it in $HOME). That done, open Eclipse's global preferences, select "C/C++ > Debug > GDB", and set "GDB Debugger" to the full path of your new gdb executable.

Building from Eclipse

In short, don't do this. Eclipse doesn't have good facilities for building incrementally in individual directories in the way that Mozilla developers generally require. More importantly, unless you set Eclipse to run a build that is NOT parallelized, NOT silenced, and that IS built with -w to output "Entering"/"Leaving" lines, Eclipse's code assistance will be seriously degraded when you build from inside Eclipse (see the Code Assistance section above for why!). Since avoiding these problems would require Eclipse to do verbose, slow rebuilds of the entire tree each time you hit the Build button, it is strongly recommended that you do NOT configure Eclipse to trigger your normal (re)builds, and that instead you (re)build outside Eclipse from your console as normal. In other words, rather than following the instructions below to configure Eclipse's "build" action to build the source, its almost certainly better for you to follow the instructions above to make the "build" action read in an appropriate build log file.

Nevertheless, if you understand the above and you still want to configure Eclipse's "Build" button to run 'make -f', then this is how you would do it...

Select "Project > Properties > C/C++ Build" and then select the "Builder Settings" tab. Untick "Use default build command" and set the "Build command" to "make -w -f". Set "Build directory" to "${ProjDirPath}/.". Now select the Behaviour tab and remove the "all" from the "Build (Incremental build)" field.

Next, select "Project > Properties > C/C++ Build > Build Variables", and add a variable "MOZCONFIG" and set it to the name of the mozconfig file you want to buid with. Set any other environment variables you want to set for the build, then close the project properties window.

Now when you hit the Build button (the little hammer icon) you should see the source build in the Console tab at the bottom of the window.

The benefit of building from inside Eclipse is that build errors will appear in the Problems tab at the bottom of the window, and from there you can double click on the build error and it will take you straight to the source line that caused the problem. For this to work reliably though, you need to build with similar make flags to those used to create the build log in the instructions above. Specifically, in order to relibably identify the problem source file, you should not do a parallel build, and you should pass the -w flag to make so that it outputs "Entering"/"Leaving" lines. It also used to be necessary to add the following two lines to your mozconfig to make the compiler output errors all on a single line, but i don't think Eclipse requires this any more:

export CFLAGS="-fmessage-length=0"
export CPPFLAGS="-fmessage-length=0"

Usage tips

Keyboard shortcuts

The keyboard shortcuts given below are the defaults. You can change the key bindings by opening the global preferences (Eclipse > Preferences, or Window > Preferences) and selecting "General > Keys". You can set the scheme to "Emacs" if that's your thing, or change individual key bingings. When changing individual key bindings, note that bindings are context sensitive, and that any changes you make may be ignored if they conflict with existing bindings or if they are overriden by a binding for a more specific context. For example, changing the Find Next command to cmd-G/ctrl-G is not sufficient. For that to work you also either need to find the existing bindings for that key combination (using the Bindings column to sort by key combination helps with this), or else you need to make your binding very specific by setting the "When" field to "C/C++ Editor" instead of the more general "Editing Text".

Use Ctrl-M to toggle maximization of the current editor view.

To jump to the definition of a symbol (or the declaration of a symbol if already at the definition), hover over the symbol, depress Ctrl, move the mouse slightly to linkify the symbol, then click on it.

You can quickly open a file by name using Cmd-Shift-R/Ctrl-Shift-R.

To quickly switch between a source file and its header file, use Ctrl-Tab.

To quickly switch to a recent document use Cmd-F6/Ctrl-F6. If you want to change this key binding, the command you need to rebind is "Next Editor".

To show a filterable list of open documents, use Cmd-E/Ctrl-E. (Similar to Emacs buffers.)

To do a C++ symbol search etc., use Ctrl-H.

To tab to another view, use Cmd-F7/Ctrl-F7. This is useful if you have maximized the editor and you want to quickly see your search results without un-maximizing the editor.

To get a list of autocomplete options, start typing the name of an identifier and then type Ctrl-Space. Unfortunately this list cannot currently be configured to appear automatically until after you've typed -> or some other seperator.

Context menu shortcuts

Select a symbol, then from the context menu select "Open Declaration". This will actually jump to the definition.

Known Issues

There are various knows limitations and bugs when it comes to using Eclipse with Mozilla.

It would be nice if Eclipse could pass information about what files have changed to the build process, which could then decide on a faster way to do the build (e.g., "just make in layout/"). I (roc) have actually written a small change to the CDT Make builder that lets you specify that as an option, in which case Eclipse sends the names of all changed files to your build tool. The build tool is a Perl script that figures out if a faster build is possible and if so, does it.

Duplicate files

Sometimes when searching for files or symbols you will be given the option between a file in the source tree, and a file with the same name under the object directory. This is just one of those things that it's best to put up with. Some source and header files are copied to the object directory so copies exist in both places. In the case of include files, some source files include the header from the source directory, while others include it from the object directory. Setting up a resource filter to exclude the copies in the object directory would break indexing for the source files that include the headers from there.


Here are some questions that have been/may be asked.

Is there a Mercurial plugin for Eclipse?

There is MercurialEclipse, but probably most Mozilla developers will probably just prefer to just use the command line. If you think we need our own documentation on MercurialEclipse, please consider adding a separate page for that tool since this page is long enough.

Isn't there a better method of compiler option discovery?

Yes, but Eclipse doesn't currently support it. Instead of reading build console output, Eclipse could use something like LD_PRELOAD to load its own little library into all the processes that are invoked as part of the build process. This library could then check whether the processor is a compiler instance and, if so, use the processes' current working directory and the arguments that were passed to it to reliably obtain the information it needs for each source file. This would significantly simplify the instructions above since there would be no need to mess about creating special non-parallelized, non-silenced builds or to mess about with build logs. You could also build from Eclipse and get the benefits that brings.


Here is a list of problems people have encountered, and suggestions for solutions.

Problem Occurred (Java heap space)

See the "Increasing memory limits" section above.

Resource is out of sync with the file system

If you get get the message "Resource is out of sync with the file system", refresh the project (or an individual directory/file) using the Refresh item from the context menu for the project (or directory/file) in the Project Explorer tab. You may also want no enable "General > Workspace > Refresh automatically".


Everything that follows is old content that will be deleted (or integrated in).

Initial setup

The following instructions are for Linux. If you have experience with Win32, please note it here.

  • Install Eclipse IDE for C/C++ Developers.
  • Obtain a clean copy of the Mozilla source.
  • Make a buildable Mozilla tree with the object files in an objdir. For example, sources in "src/", object files in "src/obj-debug".
  • Things will be a little quicker if you delete any unnecessary files from the source directory, e.g., "*~" emacs backups, "*.rej", "*.orig" and ".#*" patch detritus.
  • Start Eclipse and when prompted to "Select a workspace" just accept the default - we'll give our Mozilla project(s) their own workspace(s) rather than using this one anyway.
  • Fire up Eclipse and do File > New > C++ Project to start the C++ Project wizard.
    • Enter a meaningful project name identifying the Mozilla source tree this project will manage. (The Eclipse help docs say "do not use spaces or special characters in the project name".)
    • Uncheck the "Use default location" box, and enter the path of the directory containing the Mozilla source.
    • Pick "Makefile Project"
  • Now you can finish the wizard. You should get a Project view in the left with a directory tree you can browse to find files to edit. (Tip: Ctrl-F brings up a fast-find textbox for quick selection of files from opened directories.)
  • Doubleclick on a file to edit it. It should be syntax highlighted. There should be an outline in the right-hand pane that shows the gross constructs in the file. (Sometimes elements are missing because the parser was recovering from an error --- this view doesn't parse include files).
  • You can tweak preferences to improve your life: menu Window | Preferences
    • In the General | Keys page, in the Modify tab, you can select the Emacs scheme if your fingers demand it
  • Turn off "Build Automatically" in the Project menu.

Editing Tips

Alt-/ emacs-style abbrev completion just works.

In the default key bindings, ctrl-J does incremental search. Some users find that they like to use ctrl-G for "next match" (because Firefox does) but in Eclipse that tries to locate the definition of an identifier, which can be very slow in Mozilla with the CDT. You may want to rebind ctrl-G to "next match" if it isn't already so bound.

You probably need to adjust the CDT editor preferences to always use spaces instead of tabs, and to indent by 2 spaces.

Eclipse doesn't exactly provide the emacs buffers setup, but it does go some way towards it. Press ctrl-E to get a list of active editors; typing into it gives you autocompletion of the editor/buffer name for you to switch to.

Extra CDT features


You can configure CDT to build Mozilla by calling "make". Select the project in the project view, right click, choose Properties, C/C++ Make project, then you can configure how "make" is called. (For example, you may want to input the command for building your project as "make -f build".) Do a build by choosing Project | Build Project in the menus. It works OK but the obvious approach does a full build which is quite slow in Mozilla (we go through all directories twice, once for exports and once for the real build).

If you're brave you can turn on "auto build on save" in the project properties under "C++ Make Project" in the "Make Builder" tab, and/or by turning on Build Automatically in the project menu. I don't recommend it.

Build output appears in the Console view near the bottom of the window. Eclipse will do an OK job of matching compiler errors/warnings to source lines if you configure your Mozilla build with the gcc option "-fmessage-length=0". Just add the lines

export CFLAGS="-fmessage-length=0"
export CPPFLAGS="-fmessage-length=0"

to your .mozconfig. Errors and warnings will be marked in the source when you open an affected file. They will also appear in the "Problems" view near the bottom of the window.

Be aware that Eclipse's parsing of the build output can be screwed up if you use parallel make.

Intelligent Code Navigation

The CDT offers intelligent code completion, and source navigation and indexing. These features are currently too slow and have a few bugs that stop me from using them regularly with Mozilla, but they're fun to play with.

First you have to teach the Eclipse parser which files get built and with what options. If you do a clean build from Eclipse and it doesn't get screwed up, Eclipse will discover all the include files/directories and predefined preprocessor symbols for each file in the project. These are visible via Properties (in the context menu) for each file. Eclipse will also learn which files get built and which don't. Maybe a more reliable way to teach Eclipse this information is to do a clean build, redirect the output to a file, then in project properties "C++ Make Project" Discovery Options you can specify the build log file and choose "Load".

Now you should be able to use intelligent code completion. Start typing an identifier and press ctrl-space. You should get a popup list of the completions that are valid in the current scope --- including global functions and variables, applicable "this" members, members if you're completing a member access, and even macros. Unfortunately in the current CDT this always does a parse of the current translation unit up to the completion point, which takes several seconds for a large Mozilla file with a decent number of includes, so this isn't good.

You should also be able to navigate within the current compilation unit by holding down ctrl and then clicking on an identifier to jump to its definition. Again this requires parsing of the compilation unit.

For really slick navigation you need to index the Mozilla project by turning on the indexer (Project properties, C++ Make Project, C++ indexer). Starting with Eclipse 3.3.2, the indexation process was greatly improved and is worth executing once.

If you like living dangerously, in Preferences | C/C++ you can select "follow #includes when parsing working copies". Eclipse will then follow #includes when it does parsing to update the outline view. This slows things down massively and is not recommended, but without it, many of our macros will not be defined and either cause syntax errors in the quick parser which cause some constructs to be omitted from the outline due to error recovery, or some constructs will not be parsed at all because they're #ifdefed and Eclipse doesn't know that the macro is defined.


You can use Eclipse as a front end to gdb. It simplifies debugging and variable watching.

First, you need to create a debug configuration.

 Select Run->Open Debug Dialog… from the Eclipse menu
 Right-click on C/C++ Local Application and select New
 Project: firefox (or select your project name)
 C/C++ Application: (select browse and choose mozilla/../obj-i686-pc-linux-gnu/dist/bin/firefox-bin)

Specific configuration

  1. In the Arguments tab, change the working directory to mozilla/../obj-i686-pc-linux-gnu/dist/bin/
  2. In the Debugger tab, remove the checkbox on "Stop on startup at:"
  3. In the Environment tab, create two variables (make sure you trim the names and values)
 Value: .:./plugins:.
 Value: .:./components:.
GDB Timeouts

Out of the box, you may/will get GDB connection timeouts. This is because Eclipse is trying to push every subfolder in GDB's environment. The easiest way to resolve this issue is to remove any source entry from the debug configuration (Run->Open Debug Dialog...) in the Source tab. Doing so will unfortunately remove the binding between the binaries and the source code. To keep this feature working, you need to add a "Path Mapping" by clicking "Add..." in the Source tab. Once a "Path Mapping" is created, select "Edit..." and add an entry with these values

 Compilation path: / 
 Local file system path: /

This is the only known workaround to bind binaries to source files. It has been tested and works perfectly under Eclipse Europa (3.3.2) with Eclipse-CDT (4.0.3).

Build Parallelism

You can use distcc and the MOZ_MAKE_FLAGS build variable to distribute the compilation across a network. This greatly improves the speed of the build. The complete informations are available in the Mozilla Team Documentation - distcc.

CDT Issues

The CDT has some problems with Mozilla that need to be worked on.

  • Code completion and other operations that require a full parse tree are really slow because they parse the compilation unit from scratch every time. This can be fixed -- there is a plan. But it's quite a lot of work.
  • The parser has bugs. I get a few internal parser errors when parsing the common Mozilla include files that use slightly tricky constructs (e.g., nsCOMPtr). Some of these bugs should be easy to fix, others maybe not. Apparently these bugs are only in the "old" parser which is deprecated and will be completely replaced by the "new" parser, "soon", so it's not really worth working on these bugs.
  • It would be nice if Eclipse could pass information about what files have changed to the build process, which could then decide on a faster way to do the build (e.g., "just make in layout/"). I've actually written a small change to the CDT Make builder that lets you specify that as an option, in which case Eclipse sends the names of all changed files to your build tool. The build tool is a Perl script that figures out if a faster build is possible and if so, does it.
  • Debugging.
  • Some editing operations get sluggish when you have lots of files open (say, more than 20).
  • The GDB thread terminates unexpectedly and detaches from the Firefox bin. This is caused by the breakpoints currently set. When you restart your Eclipse environment and face such issue. Delete and readd the breakpoints in your list.

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17.19 kB18:05, 21 May 2012Jonathan WattActions

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