Quick Tour of LLDB

You use the LLDB debugger to run programs step-by-step, set breakpoints, and inspect and modify program state.

You can get a basic understanding of the debugger’s capabilities by interacting with a small example.

The following Swift code defines a Greeter type that is responsible for greeting individuals. The Greeter type keeps track of its acquaintances, and adjusts its greeting on subsequent encounters.

class Greeter {
    private var acquaintances: Set<String> = []
    
    func hasMet(personNamed name: String) -> Bool {
        return acquaintances.contains(name)
    }
    
    func greet(personNamed name: String) {
        if hasMet(personNamed: name) {
            print("Hello again, \(name)!")
        } else {
            acquaintances.insert(name)
            print("Hello, \(name). Nice to meet you!")
        }
    }
}
 
let greeter = Greeter()
 
greeter.greet(personNamed: "Anton")
greeter.greet(personNamed: "Mei")
greeter.greet(personNamed: "Anton")

If you create a file named Greeter.swift with the code above and run the swiftc command, passing the filename as a command-line argument along with the -g option to generate debug information, an executable named Greeter is created in the current directory.

$ swiftc -g Greeter.swift
$ ls
Greeter.dSYM
Greeter.swift
Greeter*

Running the Greeter executable produces the following output:

$ ./Greeter
Hello, Anton. Nice to meet you!
Hello, Mei. Nice to meet you!
Hello again, Anton!

To run the Greeter program through the LLDB debugger, pass it as a command-line argument to the lldb command.

$ lldb Greeter
(lldb) target create "Greeter"
Current executable set to 'Greeter' (x86_64).

This command starts an interactive console that allows you to run LLDB commands to interact with the program.

Set a breakpoint on line 18 with the breakpoint set (b) command, passing the --line-number (-l) option with the line number as its value, to have the debugger stop after the Greeter type declaration.

(lldb) breakpoint set --line 18
Breakpoint 1: where = Greeter`main + 70 at Greeter.swift:18, address = 0x0000000100001996

Set another breakpoint using the breakpoint set (b) command, passing the --name (-n) option with the function name greet as its value, to have the debugger stop anytime the greet(personNamed:) method is called.

breakpoint set --name greet
Breakpoint 2: where = Greeter`Greeter.Greeter.greet (personNamed : Swift.String) -> () + 27 at Greeter.swift:9, address = 0x0000000100001bab

If you run the process with the process launch (run or r) command, the process stops at the breakpoint on line 18.

(lldb) process launch
Process 97209 launched: 'Greeter' (x86_64)
Process 97209 stopped
* thread #1: tid = 0x1288be3, 0x0000000100001996 Greeter`main + 70 at Greeter.swift:18, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100001996 Greeter`main + 70 at Greeter.swift:18
   15          }
   16      }
   18
-> 18      let greeter = Greeter()
   19
   20      greeter.greet(personNamed: "Anton")
   21      greeter.greet(personNamed: "Mei")

Enter the thread step-over (next or n) command to advance the process to the the next function call on line 20.

(lldb) thread step-over
Process 97209 stopped
* thread #1: tid = 0x1288be3, 0x00000001000019bd Greeter`main + 109 at Greeter.swift:20, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x00000001000019bd Greeter`main + 109 at Greeter.swift:20
   17
   18      let greeter = Greeter()
   19
-> 20      greeter.greet(personNamed: "Anton")
   21      greeter.greet(personNamed: "Mei")
   22      greeter.greet(personNamed: "Anton")

Use the thread step-in (step or s) command to have the debugger step into the greet(personNamed:) method.

(lldb) thread step-in
Process 97209 stopped
* thread #1: tid = 0x1288be3, 0x0000000100001bab Greeter`Greeter.greet(name="Anton", self=0x0000000100606b10) -> () + 27 at Greeter.swift:9, queue = 'com.apple.main-thread', stop reason = step in
    frame #0: 0x0000000100001bab Greeter`Greeter.greet(name="Anton", self=0x0000000100606b10) -> () + 27 at Greeter.swift:9
   6           }
   7
   8           func greet(personNamed name: String) {
-> 9               if hasMet(personNamed: name) {
   10                  print("Hello again, \(name)!")
   11              } else {
   12                  acquaintances.insert(name)

Enter the thread step-over (next or n) command again, passing the --count (-c) option with the value 4, to skip to the last line of the else branch in the greet(personNamed:) method.

(lldb) thread step-over --count 4
Process 97209 stopped
* thread #1: tid = 0x1288be3, 0x0000000100001e0c Greeter`Greeter.greet(name="Mei", self=0x0000000100606b10) -> () + 636 at Greeter.swift:13, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x0000000100001e0c Greeter`Greeter.greet(name="Mei", self=0x0000000100606b10) -> () + 636 at Greeter.swift:13
   10                  print("Hello again, \(name)!")
   11              } else {
   12                  acquaintances.insert(name)
-> 13                  print("Hello, \(name). Nice to meet you!")
   14              }
   15          }
   16      }

Use the thread backtrace (backtrace or bt) command to show the frames leading to the current function being called.

(lldb) thread backtrace
* thread #1: tid = 0x1288be3, 0x0000000100001a98 Greeter`Greeter.hasMet(name="Anton", self=0x0000000101200190) -> Bool + 24 at Greeter.swift:5, queue = 'com.apple.main-thread', stop reason = step in
    frame #0: 0x0000000100001a98 Greeter`Greeter.hasMet(name="Anton", self=0x0000000101200190) -> Bool + 24 at Greeter.swift:5
  * frame #1: 0x0000000100001be4 Greeter`Greeter.greet(name="Anton", self=0x0000000101200190) -> () + 84 at Greeter.swift:9
    frame #2: 0x00000001000019eb Greeter`main + 155 at Greeter.swift:20
    frame #3: 0x00007fff949d05ad libdyld.dylib`start + 1
    frame #4: 0x00007fff949d05ad libdyld.dylib`start + 1

Use the frame variable (f v) command with no arguments to see all of the variables in the current stack frame.

(lldb) frame variable
(String) name = "Anton"
(Greeter.Greeter) self = 0x0000000100502920 {
  acquaintances = ([0] = "Anton")
}

Notice that the acquaintances property is populated with the name Anton, as it was inserted on the previous line.

Using the expression (e) command, you can change the state of the acquaintances property to alter the final output of the program.

(lldb) expression -- acquaintances.insert("Mei")
(lldb) expression -- acquaintances.remove("Anton")
(String?) $R1 = "Anton"

Disable the breakpoint on the greet(personNamed:) method using the breakpoint disable command, passing the breakpoint ID 2 as the argument. Then, enter the process continue (continue or c) command to resume execution of the process.

(lldb) breakpoint disable 2
1 breakpoints disabled.
(lldb) process continue
Resuming thread 0x12f1d19 in process 97209
Process 97209 resuming
Hello again, Mei!
Hello, Anton. Nice to meet you!
Process 97209 exited with status = 0 (0x00000000)

Notice that the output written to the console is different from the output of the program when run previously, without the debugger.



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