editing Bool variable in lldb while debugging is not working properly

Question

sujithanto OP

Created May ’20

Replies 1

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i wanted to edit an Bool value while debugging. For this i tried multiple approaches but none of them is working

First, I tried to keep the debug cursor on the variable and right click on the variable and tried to edit value. Although it shows the value had changed to true , the if block is not executed

Another ways i tried are

expression myvariable = true
po myvariable = true
p myvariable = true.

But in all these cases , if i print the variable in xcode lldb debugger, the value shows to be changed, but the execution flow is not changed

var myvariable = some expression
if(variable){
// if block this part doesnt get executed after all these modifications. 
}

I am using swift 4.2 and Xcode 10.3. Is there any other way to accomplish this?

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Answer 1

Developer Tools Engineer OP

Apple

Jun ’20

While this is indeed wrong, it is expected. The technical explanation is that we allocate a stack slot with the value of the variable on the side (which we call a shadow copy) so it is available throughout the function. When the expression is executed, that shadow copy gets updated, but the real value is used in the if-clause.

What we want instead is to change Swift so that behaves like clang and generates code that loads the variable from the stack before each access.

2