[lldb-dev] --top-level doesn't work in lldb for Objective-C or Swift :(

Wed Oct 5 11:29:57 PDT 2016

> On Oct 5, 2016, at 10:14 AM, Rex Fenley via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Maybe I have that incorrectly, this does seem to work when using lldb from Xcode's console. This doesn't work when typing `lldb` into the terminal and using it from there. I assumed the two were the same.

The same underlying debugger is used in both cases.  There can be subtle reasons for differences in behavior in the different contexts, but I don’t see how they’d apply here.  Let’s get to a specific scenario and ensure that we can resolve that for you.

> On a separate note, --top-level doesn't seem to work for swift (from Xcode console lldb). We need it to work with swift for the scripts we'll be writing.
> (lldb) expr -l swift --top-level -- let i = 10
> 
> expr -l swift --top-level -- let a = i
> 
> error: <EXPR>:3:9: error: use of unresolved identifier 'i'
> 
The --top-level option isn’t meaningful for Swift, so it’s completely ignored.  The less ambiguous nature of the language allows us to automatically determine what are top-level constructs and what’s intended to be evaluated in scope.  We introduced --top-level for Objective-C / C++ primarily to enable declaring functions and anything else that literally cannot be written in a local scope.

In your particular example the reason you can’t refer to “i” is completely unrelated.  Conceptually, every expression you evaluate is wrapped in its own scope.  So your two subsequent expressions act like this construct :

do {
    let i = 10
}
do {
    let a = i
}

As you can see, “i” is defined in this expression scope and then goes out of scope immediately after execution.  This is useful when you want to write a multi-line expression that introduces declarations for immediate use without having them cluttering up your namespace afterwards.  The convention used by LLDB for any declaration that you intend to use in later expressions is to prefix the name with a dollar sign.  So you can do the following:

(lldb) expr -l swift -- let $i = 10
(lldb) expr -l swift -- let $a = i

… and both “$a” and “$i" will be available in subsequent expressions.

Kate Stone k8stone at apple.com <mailto:k8stone at apple.com>
 Xcode Low Level Tools

> On Wed, Oct 5, 2016 at 10:06 AM, Rex Fenley <rex at remind101.com <mailto:rex at remind101.com>> wrote:
> Jim,
> 
> That doesn't seem to work for us. We're using lldb packaged with Xcode 8 fyi.
> (lldb) expr --top-level -- NSString *string = [NSString stringWithUTF8String: "This is a string"];
> 
> 
> 
> ; Function Attrs: nounwind
> 
> define internal void @_GLOBAL__sub_I__null_() #0 section "__TEXT,__StaticInit,regular,pure_instructions" {
> 
>   call void @__cxx_global_var_init()
> 
>   ret void
> 
> }
> 
> 
> On Tue, Oct 4, 2016 at 4:09 PM, Jim Ingham <jingham at apple.com <mailto:jingham at apple.com>> wrote:
> This isn't an issue with ObjC support in general, but rather shows that ObjC string constants are odd beasts.  You can work around this pretty easily by making dynamic strings:
> 
> (lldb) expr --top-level -- NSString *string = [NSString stringWithUTF8String: "This is a string"];
> (lldb) expr string
> (__NSCFString *) $0 = 0x00000001002001b0 @"This is a string"
> 
> Please file a bug about the problem with ObjC constant strings.
> 
> Jim
> 
> 
> > On Oct 4, 2016, at 3:57 PM, Rex Fenley via lldb-dev <lldb-dev at lists.llvm.org <mailto:lldb-dev at lists.llvm.org>> wrote:
> >
> > Hey lldb team!
> >
> > I'm trying to use `expr --top-level` from lldb in Xcode but it throws errors like the following:
> >
> > (lldb) expression --top-level -- NSString *str = @"This is a string";
> > Error [IRForTarget]: Couldn't replace an Objective-C constant string with a dynamic string
> > error: cannot import unsupported AST node ObjCStringLiteral
> > error: The expression could not be prepared to run in the target
> >
> > It seems like top-level only supports raw C code and not Objective-C. Is there an option we can set to support this? Is there somewhere in lldb's source code that could help point us to fixing this?
> >
> > Thank you, you guys rule!
> >
> > --
> > Rex Fenley  |  IOS DEVELOPER
> >
> >
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> 
> 
> 
> -- 
> Rex Fenley  |  IOS DEVELOPER
> 
> 
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> 
> -- 
> Rex Fenley  |  IOS DEVELOPER
> 
> 
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