You can use the LLDB debugger to run Swift programs step-by-step, set breakpoints, and inspect and modify program state.
As an example,
consider the following Swift code,
which defines a factorial(n:) function,
and prints the result of calling that function:
func factorial(n: Int) -> Int {
if n <= 1 { return n }
return n * factorial(n: n - 1)
}
let number = 4
print("\(number)! is equal to \(factorial(n: number))")Create a file named Factorial.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.
This will create an executable named Factorial
in the current directory.
$ swiftc -g Factorial.swift
$ ls
Factorial.dSYM
Factorial.swift
Factorial*Instead of running the Factorial program directly,
run it through the LLDB debugger
by passing it as a command line argument to the lldb command.
$ lldb Factorial
(lldb) target create "Factorial"
Current executable set to 'Factorial' (x86_64).
This will start an interactive console that allows you to run LLDB commands.
For more information about LLDB commands, see the LLDB Tutorial.
Set a breakpoint on line 2 of the factorial(n:) function
with the breakpoint set (b) command,
to have the process break each time the function is executed.
(lldb) b 2
Breakpoint 1: where = Factorial`Factorial.factorial (Swift.Int) -> Swift.Int + 12 at Factorial.swift:2, address = 0x0000000100000e7c
Run the process with the run (r) command.
The process will stop at the call site of the factorial(n:) function.
(lldb) r
Process 40246 resuming
Process 40246 stopped
* thread #1: tid = 0x14dfdf, 0x0000000100000e7c Factorial`Factorial.factorial (n=4) -> Swift.Int + 12 at Factorial.swift:2, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
frame #0: 0x0000000100000e7c Factorial`Factorial.factorial (n=4) -> Swift.Int + 12 at Factorial.swift:2
1 func factorial(n: Int) -> Int {
-> 2 if n <= 1 { return n }
3 return n * factorial(n: n - 1)
4 }
5
6 let number = 4
7 print("\(number)! is equal to \(factorial(n: number))")
Use the print (p) command
to inspect the value of the n parameter.
(lldb) p n
(Int) $R0 = 4
The print command can evaluate Swift expressions as well.
(lldb) p n * n
(Int) $R1 = 16
Use the backtrace (bt) command
to show the frames leading to factorial(n:) being called.
(lldb) bt
* thread #1: tid = 0x14e393, 0x0000000100000e7c Factorial`Factorial.factorial (n=4) -> Swift.Int + 12 at Factorial.swift:2, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
* frame #0: 0x0000000100000e7c Factorial`Factorial.factorial (n=4) -> Swift.Int + 12 at Factorial.swift:2
frame #1: 0x0000000100000daf Factorial`main + 287 at Factorial.swift:7
frame #2: 0x00007fff890be5ad libdyld.dylib`start + 1
frame #3: 0x00007fff890be5ad libdyld.dylib`start + 1
Use the continue (c) command
to resume the process until the breakpoint is hit again.
(lldb) c
Process 40246 resuming
Process 40246 stopped
* thread #1: tid = 0x14e393, 0x0000000100000e7c Factorial`Factorial.factorial (n=3) -> Swift.Int + 12 at Factorial.swift:2, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
frame #0: 0x0000000100000e7c Factorial`Factorial.factorial (n=3) -> Swift.Int + 12 at Factorial.swift:2
1 func factorial(n: Int) -> Int {
-> 2 if n <= 1 { return n }
3 return n * factorial(n: n - 1)
4 }
5
6 let number = 4
7 print("\(number)! is equal to \(factorial(n: number))")
Use the print (p) command again
to inspect the value of the n parameter
for the second call to factorial(n:).
(lldb) p n
(Int) $R2 = 3
Use the breakpoint disable (br di) command
to disable all breakpoints
and the continue (c) command
to have the process run until it exits.
(lldb) br di
All breakpoints disabled. (1 breakpoints)
(lldb) c
Process 40246 resuming
4! is equal to 24
Process 40246 exited with status = 0 (0x00000000)