Error when testing on iOS simulator: Couldn't register with the bootstrap server

status: this has been seen as recently as Mac OS 10.8 and Xcode 4.4.

tl;dr: This can occur in two contexts: when running on the device and when running on the simulator. When running on the device, disconnecting and reconnecting the device seems to fix things.

Mike Ash suggested

launchctl list|grep UIKitApplication|awk '{print $3}'|xargs launchctl remove

This doesn't work all the time. In fact, it's never worked for me but it clearly works in some cases. Just don't know which cases. So it's worth trying.

Otherwise, the only known way to fix this is to restart the user launchd. Rebooting will do that but there is a less drastic/faster way. You'll need to create another admin user, but you only have to do that once. When things wedge, log out as yourself, log in as that user, and kill the launchd that belongs to your main user, e.g.,

sudo kill -9 `ps aux | egrep 'user_id .*[0-9] /sbin/launchd' | awk '{print $2}'`

substituting your main user name for user_id. Logging in again as your normal user gets you back to a sane state. Kinda painful, but less so than a full reboot.

details:

This has started happening more often with Lion/Xcode 4.2. (Personally, I never saw it before that combination.)

The bug seems to be in launchd, which inherits the app process as a child when the debugger stops debugging it without killing it. This is usually signaled by the app becoming a zombie, having a process status of Z in ps.

The core issue appears to be in the bootstrap name server which is implemented in launchd. This (to the extent I understand it) maps app ids to mach ports. When the bug is triggered, the app dies but doesn't get cleaned out of the bootstrap server's name server map and as result, the bootstrap server refuses to allow another instance of the app to be registered under the same name.

It was hoped (see the comments) that forcing launchd to wait() for the zombie would fix things but it doesn't. It's not the zombie status that's the core problem (which is why some zombies are benign) but the bootstrap name server and there's no known way to clear this short of killing launchd.

It looks like the bug is triggered by something bad between Xcode, gdb, and the user launchd. I just repeated the wedge by running an app in the iphone simulator, having it stopped within gdb, and then doing a build and run to the ipad simulator. It seems to be sensitive to switching simulators (iOS 4.3/iOS 5, iPad/iPhone). It doesn't happen all the time but fairly frequently when I'm switching simulators a lot.

Killing launchd while you're logged in will screw up your session. Logging out and logging back in doesn't kill the user launchd; OS X keeps the existing process around. A reboot will fix things, but that's painful. The instructions above are faster.

I've submitted a bug to Apple, FWIW. rdar://10330930

Try quitting and restarting the simulator? If "worse comes to worst" you can always try restarting: in my experience this should fix it.

I find I have started having this issue with Lion + Xcode 4.2. I have also experienced the issue in Xcode 4.3.

I have tried all the suggestions but none of them have worked other than a full reboot.

Here is how you determine if you require a reboot quickly.

List out all your Zombie processes:

ps -el | grep 'Z'

If you see your app listed as a Zombie process you will need to reboot your machine. The error message states "This generally means that another instance of this process was already running or is hung in the debugger". Well, Xcode is detecting this Zombie process which you can't kill. The only way you can then fix it is with a system reboot. :(

EDIT, 20120823: I have some better knowledge of Zombie processes so I wanted to update this answer. A Zombie process is created when a parent process does not call wait() (wait for process to change state) on a terminating child process. You can't run 'kill' directly on a Zombie process but if you kill the parent process, the zombie child process will be 'reaped' and removed from the process table.

I haven't seen this issue in a long while so haven't inspected to see what the parent process is in this scenario. The alternative to killing the parent process is to reboot your system. :)