Saturday, June 23, 2012

lldb: VI mode and tab-completion

lldb uses libedit, which is a BSD licensed alternative to GNU readline. A feature of libedit is the ability to configure per-application settings for command-line bindings via ~/.editrc. Detailed documentation for this configuration file can be found using man editrc or via online documentation, such as developer.apple.com.

As I prefer VI bindings, I initially configured my .editrc as follows, to replace the default emacs bindings:

lldb:bind -v

Unfortunately, when I re-ran lldb, tab-completion was not functioning.  I added the following line, which is exported in libedit, and what I assumed would be the default completer function:

lldb:bind ^I rl_complete

Although tab-completion started working again, I was seeing frequent segfaults in lldb, along with an error that it could not bind the rl_complete command.  How was I going to find this elusive tab-completion command?  I could dig through lldb source, but figured there must be a quicker way.  I reviewed all the commands in the editrc docs and found 

ed-command
           Editline extended command.
I cleared my .editrc and added:
lldb:bind ^P ed-command

tab-completion was working again, and pressing ^P presented me a :, puting me in Editline's extended command mode. Typing bind listed all the current bindings; I was looking for ^I, which showed up as

"^I"           ->  lldb_complete

lldb_complete was the elusive command I needed to fix tab-completion in VI mode, so I settled on the following for my .editrc

lldb:bind -v
lldb:bind ^I lldb_complete
lldb:bind ^P ed-command

I suspect this technique will come in handy for other programs that use libedit and a non-standard completion function, hence this long post.

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