ax-lldb

LLDB

Quick Patterns

Crash Triage (When Debugger Stops)

(lldb) thread info              # What happened?
(lldb) bt                       # Where did it happen?
(lldb) frame select 3           # Jump to your code frame
(lldb) v                        # Inspect all variables
(lldb) v self.someProperty      # Inspect specific property

The Four Print Commands

Command	Best For	Reliability
v	Swift structs, enums, locals, stored properties	Most reliable (reads memory directly)
p	Computed properties, function calls	Compiles expression
po	Classes with CustomDebugStringConvertible	Calls debugDescription
expr	Modifying state, calling methods	Full expression evaluation

Default to v -- it never fails for stored properties. Use p when v can't reach it (computed properties, function calls). Use po only for class descriptions.

Exception Breakpoints (Set These Always)

(lldb) breakpoint set -E swift   # Break on all Swift errors
(lldb) breakpoint set -E objc    # Break on all ObjC exceptions

These catch errors at the throw site instead of the crash site.

Hang Diagnosis

(lldb) process interrupt         # Pause hung app
(lldb) bt all                    # All thread backtraces
(lldb) thread list               # Thread states (look for deadlocks)

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Decision Tree

What do you need?
├─ App crashed, debugger stopped
│  ├─ Read stop reason -> thread info
│  ├─ Get backtrace -> bt
│  ├─ Find your frame -> frame select N
│  ├─ Inspect state -> v self, v localVar
│  └─ Classify: EXC_BAD_ACCESS? EXC_BREAKPOINT? SIGABRT?
├─ App is frozen/hung
│  ├─ Pause -> process interrupt
│  ├─ All backtraces -> bt all
│  ├─ Check main thread (Thread 0)
│  │  ├─ __psynch_mutexwait -> Blocked on mutex
│  │  ├─ _dispatch_sync_f_slow -> dispatch_sync deadlock
│  │  └─ Your code at top -> CPU-bound, move to background
│  └─ Check for deadlocks -> thread list (multiple threads waiting)
├─ Need to inspect a variable at runtime
│  └─ Set breakpoint -> v self.property
├─ po doesn't work / shows garbage
│  └─ Use v instead (see Swift Value Inspection)
├─ Need to test a fix without rebuilding
│  └─ expr self.property = newValue
├─ Want to break on specific condition
│  └─ breakpoint set -f File.swift -l 42 -c "value == nil"
└─ Need to log without stopping
   └─ Logpoint: breakpoint + command add + continue