WWDC2014_IntroToLLDBAndTheSwiftREPL

WWDC2014_IntroToLLDBAndTheSwiftREPL.md

Intro To LLDB and the Swift REPL

Sean Callanan

Basic debugging

• lldb commands
  • ti/ thread info
    • EXC_BAD_INSTRUCTION = CPU doesn't understand an instruction
      • typically used in assertions -> Assertion failure
  • bt / thread backtrace
    • prints all frames on the stack of the current thread
  • f 1 / frame select 1
  • p foo / expression foo
• In Xcode, you can also see where blocks get dispatched

Symbolic breakpoints

• br s -r timestwo.*String
• methods of a class: -r Account\\.
• Functions in a module: -r main\\.

Breakpoint commands

• br co a \ breakpoint command add
  • >p valueInCents
  • >continue
  • >DONE
• Caveat: You must set separate conditions and actions for Obj-C and Swift locations
• (lldb) br m --c "a[3]==2"3.1
• (lldb) br m --c "a[3]==2"3.1

REPL-enabled debugging workflows

• (lldb) repl when your app is running

  • add code to an existing session(!)
  • : to get back to LLDB
  • :help to issue LLDB commands directly from the repl

• $ xcrun swift in any shell

• The REPL adds global code

• use expression command for investigating stack variables

• use LLDB REPL for global variables, classes, functions

• See Advanced Debugging with LLDB 2013 video