[lldb-dev] Debugging a running process with lldb

Greg Clayton gclayton at apple.com
Thu May 15 10:21:04 PDT 2014

So to clear things up:

-t or --tty will attempt to launch the process in a _separate_terminal_window_ (open a new terminal window and launch your process within it so the process gets its own window and we don't have to shared IO) and should never be used with -n (--no-stdio). If they currently can be run together then this is a bug. --tty isn't supported on Linux as far as I know and the linux launch code should return an error saying this isn't supported. The --tty option is designed to be used when you want to debug anything that uses curses (emacs, vi, etc) and where having a different terminal window with its own stdin/out/err is required.

--no-stdio should, as you surmised, launch a process with /dev/null as the stdin/out/err. It also means we should always have a  prompt.

To to summarize:

(lldb) process launch ...

No LLDB prompt while running as all input goes to process (though if CTRL+C is pressed and we will stop the target for you)

(lldb) process launch --tty -- ...

LLDB prompt always visible, process launches in a different terminal window, not supported on linux, should never be used with --no-stdio and if our options allow it, this is a bug

(lldb) process launch --no-stdio -- ...

LLDB prompt always visible (if not, then a bug, probably only on the linux side).

Let me know if I 
> On May 15, 2014, at 1:53 AM, Matthew Gardiner <mg11 at csr.com> wrote:
> Matthew Gardiner wrote:
>> Greg Clayton wrote:
>>> If we launch a process and hookup a pty to its stdin/out/err, then we shouldn't get a prompt.
>> Sorry, I don't understand this. Surely if we attach the inferior's (i.e. what you termed the process) standard IOs to a pty, then we *will* get a prompt, since then lldb can use it's terminal with no corruption from the inferiors in/out?
>> (I'm not actually clear about the relationship between the "process launch" -t and -n commands. I do accept that they are mutually exclusive. I assume, -n means that the stdin/out/err of the inferior is closed, so that lldb has complete use of the foreground, so it could use it's prompt. I assume -t means attach the inferior to a different pty, so again, lldb should still be able to use it's prompt.)
>>>  It is probably a bug where if you launch with --no-stdio that the LLDB prompt isn't always active.
>>> For attach, we will always have a prompt.
>>> So seems this is a bug that --no-stdio is still pushing a Process IOHandler and taking over the console while the process is running.
>> So you are saying if I do
>> (lldb) process launch -s --no-stdio
>> then
>> (lldb) process continue
>> then I should always see:
>> (lldb)
>> back on my terminal?
>> Todd, could you try this (a couple of times, in case I'm seeing something intermittent) please, to see if your observations agree?
>> thanks
>> Matt
> Hi Greg,
> Actually, my colleague just brought his MacBook (OSX 10.9.2) into work, this morning. He installed pre-built xcode/lldb (lldb-310.2.37) on his Mac, and I can confirm that on mac, when we invoked:
> (lldb) process launch -s -n
> Process 1002 lanched....
> (lldb) process continue
> Process 1002 resuming
> (lldb)
> then we see the interpreter prompt return after the "continue". So what I reported originally is a linux build bug. (Perhaps I'll start digging into this, if I get any time...).
> However, using  "process launch -s -t" on the mac, was buggy, a terminal-emulator was spawned, but (presumably) lldb and the child terminal fail to synchronise using the UNIX socket.
> (lldb) process launch -s -t
> error: failed to launch or debug process
> (lldb)
> Anyway, sorry for my earlier rant ;-), but at least now I'm a little clearer about the intended semantics...
> Matt
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