Debugging Firefox with GDB

This page is an import from MDN and the contents might be outdated

This page details how you can more easily debug Firefox and work around some GDB problems.

Use GDB 5, or higher. A more recent version of GDB can be obtained from sourceware or your Linux distro repo. If you are running less than 256 MB of RAM, be sure to see Using gdb on wimpy computers.

Where can I find general gdb documentation?

Using GDB is beyond the scope of this document. Documentation is likely available on your system if you have GDB installed, in the form of info, man pages, or the gnome help browser. Additionally, you can use a graphical front-end to GDB like ddd or insight. For more information see

How do I run Firefox under gdb?

The preferred method, is using the Mach command-line tool to run the debugger, which can bypass several optional defaults. Use “mach help run” to get more details. If inside the source directory, you would use “./mach”. Please note that mach is aware of mozconfigs.

$ ./mach run --debug [arguments to pass to firefox]

If you need to direct arguments to gdb, you can use ‘–debugger-args’ options via the command line parser, taking care to adhere to shell splitting rules. For example, if you wanted to run the command ‘show args’ when gdb starts, you would use:

$ ./mach run --debug --debugger-args "-ex 'show args'"

Alternatively, you can run gdb directly against Firefox. However, you won’t get some of the more useful capabilities this way. For example, mach sets an environment variable (see below) to stop the JS engine from generating synthetic segfaults to support the slower script dialoging mechanism.

(gdb) OBJDIR/dist/bin/firefox

How do I pass arguments in prun?

Set the arguments in GDB before calling prun. Here’s an example on how to do that:

(gdb) set args
(gdb) prun

How do I set a breakpoint in a library that hasn’t been loaded?

GDB 6.1 and above has support for “pending breakpoints”. This is controlled by the “set breakpoint pending” setting, and is enabled by default. If a breakpoint cannot be immediately resolved, it will be re-checked each time a shared library is loaded, by the process being debugged. If your GDB is older than this, you should upgrade.

In older versions, there isn’t a way to set breakpoints in a library that has not yet been loaded. See more on setting a breakpoint when a component is loaded. If you have to set a breakpoint you can set a breakpoint in _dl_open. This function is called when a new library is loaded, when you can finally set your breakpoint.

How do I set a breakpoint when a component is loaded?

In Firefox Version 57 (and possibly earlier) XPCOM_BREAK_ON_LOAD does not seem to exist.

There’s a facility in XPCOM which allows you to set an environment variable to drop into the debugger when loading a certain component. You have to set XPCOM_BREAK_ON_LOAD variable before you run Firefox, setting it to a string containing the names of libraries you want to load. For example, if you wish to stop when a library named raptor or necko is loaded, you set the variable to raptor:necko. Here’s an example:

(gdb) set env XPCOM_BREAK_ON_LOAD raptor:necko
(gdb) prun

Why can’t I set a breakpoint?

You probably can’t set a breakpoint because its library hasn’t been loaded. Most Firefox functionality is in libraries loaded mid-way through the main()function. If you break on main(),and step through until the libraries are loaded, with a call to InitCOMGlue(), you should be able to set breakpoints on many more symbols, source files, and continue running.

(gdb) break main
(gdb) run
Breakpoint 1, main(argc=4, argv=0x7fffffffde98, envp=0x7ffffffffdec0) .....
256    {
(gdb) next
293      nsresult rv = InitXPCOMGlue()
(gdb) next

If you still can’t set the breakpoints, you need to confirm the library has loaded. You can’t proceed until the library loads. See more on loading shared libraries. If you wish to break as soon as the library is loaded, see the section on breaking when a component is loaded and breaking on a library load.

How do I display PRUnichar’s?

One suggestion is this:

(gdb) print ((PRUnichar*)uri.mBuffer)[0]@16
$47 = {114, 100, 102, 58, 110, 117, 108, 108, 0, 0, 8, 0, 0, 0, 37432,
(gdb) print aURI
$1 = (const PRUnichar *) 0x855e6e0
(gdb) x/32ch aURI
0x855e6e0:      104 'h' 116 't' 116 't' 112 'p' 58 ':'  47 '/'  47 '/'  119 'w'
0x855e6f0:      119 'w' 119 'w' 46 '.'  109 'm' 111 'o' 122 'z' 105 'i' 108 'l'
0x855e700:      108 'l' 97 'a'  46 '.'  111 'o' 114 'r' 103 'g' 47 '/'  115 's'
0x855e710:      116 't' 97 'a'  114 'r' 116 't' 47 '/'  0 '\0'  25 '\031'       0 '\0'
  • Define helper functions in your .gdbinit

# Define a "pu" command to display PRUnichar * strings (100 chars max)
# Also allows an optional argument for how many chars to print as long as
# it's less than 100.
def pu
  set $uni = $arg0
  if $argc == 2
    set $limit = $arg1
    if $limit > 100
      set $limit = 100
    set $limit = 100
  # scratch array with space for 100 chars plus null terminator.  Make
  # sure to not use ' ' as the char so this copy/pastes well.
  set $scratch = "____________________________________________________________________________________________________"
  set $i = 0
  set $scratch_idx = 0
  while (*$uni && $i++ < $limit)
    if (*$uni < 0x80)
      set $scratch[$scratch_idx++] = *(char*)$uni++
      if ($scratch_idx > 0)
    set $scratch[$scratch_idx] = '\0'
    print $scratch
    set $scratch_idx = 0
      print /x *(short*)$uni++
  if ($scratch_idx > 0)
    set $scratch[$scratch_idx] = '\0'
    print $scratch

# Define a "ps" command to display subclasses of nsAC?String.  Note that
# this assumes strings as of Gecko 1.9 (well, and probably a few
# releases before that as well); going back far enough will get you
# to string classes that this function doesn't work for.
def ps
  set $str = $arg0
  if (sizeof(*$str.mData) == 1 && ($str.mFlags & 1) != 0)
    print $str.mData
    pu $str.mData $str.mLength

This is hard. Give me a .gdbinit that already has the functions.

  • Define a small helper function “punichar” in #ifdef NS_DEBUG code somewhere.

How do I display an nsString?

You can call the ToNewCString() method on the nsString. It leaks a little memory but it shouldn’t hurt anything if you only do it a few times in one gdb session. (via

(gdb) p string.ToNewCString()

Another method (via bent) is the following (replace n with: the returned length of your string):

(gdb) p string.Length()
$1 = n
(gdb) x/ns string.BeginReading()

You can of course use any of the above unichar-printing routines instead of x/s.

This is hard. Give me a .gdbinit that works.

See Boris Zbarsky’s .gdbinit. It contained several function definitions including:

  • “prun” to start the browser and disable library loading.

  • “pu” which will display a (PRUnichar *) string.

  • “ps” which will display a nsString.

How do I determine the concrete type of an object pointed to by an interface pointer?

You can determine the concrete type of any object pointed to, by an XPCOM interface pointer, by looking at the mangled name of the symbol for the object’s vtable:

(gdb) p aKidFrame
$1 = (nsIFrame *) 0x85058d4
(gdb) x/wa *(void**)aKidFrame
0x4210d380 <__vt_14nsRootBoxFrame>: 0x0
(gdb) p *(nsRootBoxFrame*)aKidFrame
 [ all the member variables of aKidFrame ]

If you’re using gcc 3.x, the output is slightly different from the gcc 2.9x output above. Pay particular attention to the vtable symbol, in this case __vt_14nsRootBoxFrame. You won’t get anything useful if the shared library containing the object is not loaded. See How do I load shared libraries? and How do I see what libraries I already have loaded?

Or use the gdb command set print object on.

How can I debug JavaScript from gdb?

If you have JavaScript Engine code on the stack, you’ll probably want a JS stack in addition to the C++ stack.

(gdb) call DumpJSStack()

See for more JS debugging tricks.

How can I debug race conditions and/or how can I make something different happen at NS_ASSERTION time?

[submitted by Dan Mosedale]
As Linux is unable to generate useful core files for multi-threaded applications, tracking down race-conditions which don’t show up under the debugger can be a bit tricky. Unless you’ve given the --enable-crash-on-assert switch to configure, you can now change the behavior of NS_ASSERTION (nsDebug::Break) using the XPCOM_DEBUG_BREAK environment variable.

How do I run the debugger in emacs/xemacs?

Emacs and XEmacs contain modes for doing visual debugging. However, you might want to set up environment variables, specifying the loading of symbols and components. The easiest way to set up these is to use the script, located in the dist/bin directory of your build. This script sets up the environment to run the editor, shell, debugger, or defining a preferred setup and running any commands you wish. For example:

$ ./ /bin/bash

GDB 5 used to work for me, but now Firefox won’t start. What can I do?

A recent threading change (see bug 57051 for details) caused a problem on some systems. Firefox would get part-way through its initialization, then stop before showing a window. A recent change to gdb has fixed this. Download and build the latest version of Insight, or if you don’t want a GUI, the latest version of gdb.

“run” or “prun” in GDB fails with “error in loading shared libraries.”

Running mozilla-bin inside GDB fails with an error message like:

Starting program:
/u/dmose/s/mozilla/mozilla-all/mozilla/dist/bin/./mozilla-bin: error
in loading shared libraries: cannot open shared
object file: No such file or directory

Your LD_LIBRARY_PATH is probably being reset by your .cshrc or .profile. From the GDB manual:

*Warning:* GDB runs your program using the shell indicated by your ‘SHELL’ environment variable if it exists (or ‘/bin/sh’ if not). If your ‘SHELL’ variable names a shell that runs an initialization file – such as ‘.cshrc’ for C-shell, or ‘.bashrc’ for BASH–any variables you set in that file affect your program. You may wish to move the setting of environment variables to files that are only run when you sign on, such as ‘.login’ or ‘.profile’.

Debian’s GDB doesn’t work. What do I do?

Debian’s unstable distribution currently uses glibc 2.1 and GDB 4.18. However, there is no package of GDB for Debian with the appropriate threads patches that will work with glibc 2.1. I was able to get this to work by getting the GDB 4.18 RPM from Red Hat’s rawhide server and installing that. It has all of the patches necessary for debugging threaded software. These fixes are expected to be merged into GDB, which will fix the problem for Debian Linux. (via Bruce Mitchener)

Firefox is aborting. Where do I set a breakpoint to find out where it is exiting?

On Linux there are two possible symbols that are causing this: PR_ASSERT() and NS_ASSERTION(). To see where it’s asserting you can stop at two places:

(gdb) b abort
(gdb) b exit

I keep getting a SIGSEGV in JS/JIT code under gdb even though there is no crash when gdb is not attached. How do I fix it?

Set the JS_DISABLE_SLOW_SCRIPT_SIGNALS environment variable (in FF33, the shorter and easier-to-remember JS_NO_SIGNALS). For an explanation, read Jan’s blog post.

I keep getting a SIG32 in the debugger. How do I fix it?

If you are getting a SIG32 while trying to debug Firefox you might have turned off shared library loading before the pthreads library was loaded. For example, set auto-solib-add 0 in your .gdbinit file. In this case, you can either:

  • Remove it and use the method explained in the section about GDB’s memory usage

  • Use handle SIG32 noprint either in gdb or in your .gdbinit file

Alternatively, the problem might lie in your pthread library. If this library has its symbols stripped, then GDB can’t hook into thread events, and you end up with SIG32 signals. You can check if your libpthread is stripped in file /lib/libpthread* and looking for 'stripped'.To fix this problem on Gentoo Linux, you can re-emerge glibc after adding "nostrip" to your FEATURES in /etc/make.conf.

How do I get useful stack traces inside system libraries?

Many Linux distributions provide separate packages with debugging information for system libraries, such as gdb, Valgrind, profiling tools, etc., to give useful stack traces via system libraries.


On Fedora, you need to enable the debuginfo repositories, as the packages are in separate repositories. Enable them permanently, so when you get updates you also get security updates for these packages. A way to do this is edit /etc/yum.repos.d/fedora.repo and fedora-updates.repo to change the enabled=0 line in the debuginfo section to enabled=1. This may then flag a conflict when upgrading to a new distribution version. You would the need to perform this edit again.

You can finally install debuginfo packages with yum or other package management tools. The best way is install the yum-utils package, and then use the debuginfo-install command to install all the debuginfo:

$ yum install yum-utils
$ debuginfo-install firefox

This can be done manually using:

$ yum install GConf2-debuginfo ORBit2-debuginfo atk-debuginfo \
cairo-debuginfo dbus-debuginfo dbus-glib-debuginfo expat-debuginfo \
fontconfig-debuginfo freetype-debuginfo gcc-debuginfo glib2-debuginfo \
glibc-debuginfo gnome-vfs2-debuginfo gtk2-debuginfo gtk2-engines-debuginfo \
hal-debuginfo libX11-debuginfo libXcursor-debuginfo libXext-debuginfo \
libXfixes-debuginfo libXft-debuginfo libXi-debuginfo libXinerama-debuginfo \
libXrender-debuginfo libbonobo-debuginfo libgnome-debuginfo \
libselinux-debuginfo pango-debuginfo popt-debuginfo scim-bridge-debuginfo

Debugging electrolysis (e10s)

mach run and mach test both accept a --disable-e10s argument. Some debuggers can’t catch child-process crashes without it.

You can find some (outdated) information on You may also like to read for a more up-to-date blog post.

To get the child process id use:


See also

Original Document Information