In xcode, the iPhone app build result will be run on lldb.

It's normal if you run the app and the iOS simulator starts or the app screen appears on the actual machine. The display (lldb) indicates that the debugger is accepting commands and is probably attached to the debugger because it is booting from Xcode. If you tap the app directly from the actual machine or simulator and start it, you should be able to start it without attaching the debugger. Recent Xcode uses a debugger called lldb by default, so if you want to use the old gdb, it should be resolved by changing the debugger from EditScheme to gdb.

Up to Xcode 4.x, both gdb and lldb were included, and lldb was able to change from EditScheme to gdb by default, but Xcode 6.x does not include gdb as a standard, so it cannot be changed to gdb. (Sorry it didn't solve the problem...)