# Debugging Firefox with LLDB

See <http://lldb.llvm.org/index.html>.

## Mozilla-specific lldb settings

There's an
`.lldbinit` {searchfox}`file <.lldbinit>`
in the Mozilla source tree, which applies recommended settings and
includes a few type summaries and Mozilla-specific debugging commands
via the lldbutils module (see
{searchfox}`python/lldbutils/README.txt`).
For information about available features see the links above and the [Using
LLDB to debug Gecko](http://mcc.id.au/blog/2014/01/lldb-gecko) blog
post.

The in-tree `.lldbinit` should be loaded automatically in most cases
when running lldb from the command line (e.g. using
{ref}`mach`), but **not**
when using Xcode. See {ref}`Debugging on macOS` for information on setting up
Xcode.

:::{warning}
LLDB warning: Xcode 5 only comes with lldb (gdb is gone). The
introduction and use of UNIFIED_SOURCES in the source starting around
November 2013 has broken the default LLDB configuration so that it
will not manage to resolve breakpoints in files that are build using
UNIFIED_SOURCES (the breakpoints will be listed as "pending", and
lldb will not stop at them). To fix this add the following to your
\$HOME/.lldbinit file:

```
# Mozilla's use of UNIFIED_SOURCES to include multiple source files into a
# single compiled file breaks lldb breakpoint setting. This works around that.
# See http://lldb.llvm.org/troubleshooting.html for more.
settings set target.inline-breakpoint-strategy always
```

Restart Xcode/lldb and restart your debugging session. If that still
doesn't fix things then try closing Xcode/lldb, doing a clobber
build, reopening Xcode/lldb, and restarting your debugging session.
:::

## Starting a debugging session

### Attaching to an existing process

You can attach to Firefox with following command:

```
(lldb) process attach --name firefox
```

Some versions of lldb causes crashes after attaching to Firefox.

### Running a new process

To start Firefox under the debugger, run `lldb` followed by "--",
followed by the command line you'd like to run, like this:

```bash
lldb -- obj-ff-dbg/dist/Nightly.app/Contents/MacOS/firefox -profile /path/to/profile
```

Then set breakpoints you need and start the process:

```
(lldb) breakpoint set --name nsInProcessTabChildGlobal::InitTabChildGlobal
Breakpoint created: 1: name = 'nsInProcessTabChildGlobal::InitTabChildGlobal', locations = 0 (pending)
WARNING: Unable to resolve breakpoint to any actual locations.

(lldb) r
Process 7602 launched: '/.../obj-ff-opt/dist/Nightly.app/Contents/MacOS/firefox' (x86_64)
1 location added to breakpoint 1
```