When I use lldb in a swift project, I can't modify the bool value

Question

help me, please, I'm new to Swift.

 override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesBegan(touches, with: event)

        var flag = false     
        print(" current->  \(flag)") /** Add a breakpoint  **/
        if flag == true {
            print(" after  -> true")
        }else {
            print(" after ->   false")
        }
    }

I want to use lldb to modify the value of 'flag', so,

(lldb) po flag
false

(lldb) expression flag = true
(lldb) po flag
true

(lldb) continue
2018-11-24 23:57:05.552804+0800 test_swift_lldb[6806:384106] XPC connection interrupted
Process 6806 resuming
 current->  false
 after ->   false

It doesn't seem to be useful. Please tell me how to use lldb, modify the bool value.

Hi, please enter you code as text and not as an external image. You are trying to use the debugger, correct? See if this answer helps with entering debugger commands: how-to-change-variables-value-while-debugging-with-llvm-in-xcode — Bill
– Bill, Commented Nov 24, 2018 at 7:59
@Bill Do you have any good ideas? I was frustrated by this problem... — zw骑猪o0o找牛
– zw骑猪o0o找牛, Commented Nov 25, 2018 at 0:50

Bill · Accepted Answer · 2018-11-25 06:11:14Z

1

First of all, it seems to be an issue between Swift and LLDB. I suspect that Swift is optimizing the var flag away into a register. There are a couple of other SO questions on a similar problem, for instance: Why is Xcode's Variables View's “Edit Value” not changing the variable value?. Interestingly, Xcode somehow works around this problem. The work-around seems to be to "trick" LLDB into recognizing the the variable is updated. I modified your code as follows:

//
//  main.swift
//  debug_example
//

import Foundation

print("Hello, main")
var flag = false
var debugString = "abcd"
if debugString.count == 0 { flag = true }

print(" current->  \(flag)") /** Add a breakpoint  **/
if flag == true {
    print(" after  -> true")
}else {
    print(" after ->   false")
}

The following are my LLDB commands simplified (output indented):

lldb main
breakpoint set --line 8
process launch
    Process 64052 launched:
po flag
    false
ex flag=true
po flag
    true
s
    Hello, World!
    Target 0: (main) stopped.
po flag
    false
ex flag=true
po flag
    true
thread continue
    Resuming thread
    current->  true
    after  -> true

Perhaps someone else can provide more information or some insight into how Xcode works around this.

answered Nov 25, 2018 at 6:11

Bill

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4 Comments

zw骑猪o0o找牛 Over a year ago

Wow, it works. But,when i am coding, I will not write those codes to 'trick' lldb, Because those codes have no meaning for the project...

Bill Over a year ago

I completely agree. I just cleaned out some older versions of llvm and related components. I have lldb --version showing: lldb-1000.11.38.2 Swift-4.2. Now everything works as you would expect; no tricks required. Check your versions of swiftc and lldb.

zw骑猪o0o找牛 Over a year ago

I am ashamed to ask you, My current xcode configuration is lldb --version showing: lldb-1000.11.38.2 Swift-4.2。 But When I run my initial code for this question, it was not work... What caused it? <code> override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) { super.touchesBegan(touches, with: event) var flag = false print(" current-> (flag)") /** Add a breakpoint **/ if flag == true { print(" after -> true") }else { print(" after -> false") } }</code>

Bill Over a year ago

Perhaps backup a bit. Create a very, very simple project like the sample code above. If you have Xcode installed, try if from there. Xcode uses lldb. Next try it from the command line: swiftc -g main.swift followed by lldb main. Like I mentioned, when I removed old versions of lldb, it worked for me.

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