lldb error: variable not available - objective-c

Here are my two lines of code:
NSString *frontFilePath = [[NSBundle mainBundle] pathForResource:[self.bookendFileNames objectAtIndex:self.randomIndex] ofType:#"caf"];
NSLog(#"frontFilePath = %#", frontFilePath );
I put a break point on the second line and when there, I try to print it:
(lldb) po frontFilePath
But I get the following error:
error: variable not available
I'm confused because if I step over the NSLog statement, the variable does indeed print to the console.
For what it's worth, I'm trying to debug the first line since sometimes it returns NULL, tho I can't, as of now, figure out why.

This is an artifact of debugging optimized code. When the compiler's optimization is enabled in your build settings, it moves variables between memory and registers as it decides is best. At the point where you're examining the variable in lldb, it may not exist in registers or memory at all -- even though it looks like it should still be available for display.
It's possible it is a shortcoming of the debug information output by the compiler. Sometimes the compiler will copy a variable into a register for its use and only list that register location in the debug information. Later the register is repurposed for other uses; value is still present on the stack but the compiler hasn't told the debugger that the value can be found there.
The only way to really tell whether it's insufficient debug info or if the value genuinely doesn't exist at that particular instruction is to examine the assembly code by hand. As soon as you turn on optimization with the compiler, the source code becomes a weak view into what's actually being executed in any particular order.
Instead of wandering too far into the wacky world of optimized code debugging, I strongly recommend turning off optimization (Optimization Level in your Build Settings) for your build and debugging it that way, if at all possible. If you do need to debug your app with optimization, make sure you're building with the latest Apple LLVM compiler supported by your Xcode -- there is always work being done to improve optimized code debugging and you want to avail yourself of the most up to date tools you can.

The "Address Sanitizer" in the diagnostics seems to make the values of variables unavailable, too.
Schemes > Run > Diagnostics > Address Sanitizer

In Swift, possibly starting with Xcode 9 and still an issue in Xcode 10, this could even come up when code optimization is turned off in the build settings. As #carlos_ms has pointed out here, a temporary solution is to define the variable as mutable, i.e.
Turn
let foo = Bar().string
into
var foo = Bar().string
in order to cause optimization to skip on this variable. Note that this might not work in all instances.
In this case, a good ol' debugPrint() might help you out.

Related

Xcode Debugging not showing values

I'm completely stumped. I've been debugging for over a year and have only had this problem when the Build Configuration was set to Release. I have the Build Configuration set to Debug and I have checked to be sure I am attaching to the correct process and yet I still cannot see the values while stepping through the code. Has anybody else ran into this issue?
Here is a screen shot:
The value is returning, but I am unable to see the values of ANYTHING in this method or any of the other methods and I cannot figure out why.
Thank you for any hints you can give me.
============================== UPDATE ==================================
I've tried to print out the valued and this is the output I receive:
Notice though, that the value in the Variables view is correct for the result, even though I can't print it out. But the other values, like filePath should not be nil.
This is so weird.
============================== UPDATE ==================================
I put the breakpoint on the return statement and still no luck:
This time I see no value for result:
I was facing the same issue, On mouse hover and watch section for every variable it was showing nil value but while NSLog for those variables it was printing correct value.
now it's fixed. Try this
Go to Product menu > Scheme > Edit scheme or ⌘+<
In Run go to Info tab, change the build configuration to "debug".
Hope it will work.
#Lucy, ideally you shouldn't have to run in Release profile when debugging, Release is meant as an App Distribution profile with a small package size and little / no debug symbols.
But if you must debug on Release (e.g. due to Environment, data reasons), make sure you have the Optimization Level set to 'None' for Release.
You can reach the config above through:
1) Project wide settings - affects all Targets
2) Target specific setting - affects a single Target only
Don't forget to switch this back before App Store distribution. Lower Optimization will generate a larger ipk increasing your users' download time - also the potential dSYM generated.
Background
For context, Debug, Adhoc or Release profiles are purely to id different build configurations that XCode ships with. XCode starts projects with a Debug profile with no Optimization (and various other predefined config related to development & debugging), so symbol loading by the interpreter is possible - you're free to tweak this however you wish.
Theoretically able to establish a Release build profile that's identical to Debug.
I do see part of your problem. notice how self is nil?
That means you are in a method call for a de-allocated object. Turn on NSZombies to debug.
see this SO answer for how to enable NSZombie
Given that it is legal to send messages to nil in objective-C, I would suspect that the object is being deallocated as a side effect of calling doesLicenseFileExist, or by another thread.
You may find it helpful to put a breakpoint in IDLicenseCommands -dealloc method.
I had this problem, and turned "Link-Time Optimization" from "Incremental" to "No" and it resolved the debugging issue.

Is there a way to get/nslog an array of all local variables in a function in objective-c [duplicate]

When attached to the debugger via Xcode, LLDB provides a useful view of local variables (the bottom left of the screenshot):
I found an LLDB command frame variable (and gdb's info locals) that provides a list of the local variables (as seen in the right side of the screenshot above).
My hope is that this functionality is possible to perform on the device at runtime. For example, I can access the stack trace using backtrace_symbols(), the current selector via _cmd, and a few others.
Has anyone had experience in this area? Thanks in advance.
Xcode/LLDB can show you this information because they have access to debug information in the binary, called a symbol table, which help it understand what memory locations correspond to which names in your source code. This is all outside the Objective-C runtime, and there's no interface in the runtime to get at it.
There's another reason why this won't work, though. When you're building code to run in the debugger, compiler optimizations are turned off, so all the variables you reference in your code are there.
When you build for release, though, generally the compiler optimizations get in there and re-arrange all your carefully named local variables to make things run faster. They might not even ever get stored in memory, just in CPU registers. Or they might not exist at all, if the optimizer can prove to itself that it doesn't need them.
My advice is to think again about the larger problem you're trying to solve...

Why sometimes it's very difficult to see the content of variables in Xcode 4.3

Sometimes it show up sometimes it doesn't
I wonder why.
LLDB appears to be not as good as GDB at showing the correct state of variables - e.g. sometimes, often near the start of methods, LLDB fails to figure out the type of self and you cannot see the instance variables; whereas GDB with the same code is fine.
There are sometimes other reasons; to do with accessing memory, locks, etc.; that a debugger cannot show you variables.
Whether your particular case is LLDB failing or one of those cases when GDB also would fail I cannot say, but if this is hitting you a lot in a particular project switching to GDB might be worth it for the project.

Hot Code Swapping in Xcode?

I am doing iOS game development using the cocos2d framework, and I tremendously envy the ability for Eclipse Java programmers to hot swap their code while they are debugging their application (i.e. change out variable values within a method and have the program live update as if you had a REPL without having to recompile).
This seems like something that would be absolutely tremendously helpful when it came to iOS development, where my development environment is (obviously) Xcode and the language I am programming in is Objective C. I have googled around but havent been able to find anything - so I thought I would ask the community.
Does anyone know if there a way to Hot Swap code in Xcode while programming in Objective C?
Thanks in advance
There is a great plugin which allow changing code in live, working APP. It is called InjectionPlugin.
As FAQ says:
How does it work? The Objective-C runtime allows you to load a new version of a class into an application using a bundle even if there is already an implementation linked into the application. Swizzling is used as the bundle is loaded to switch the existing class to use the methods of the new implementation. This can be done repeatedly and works for OSX and iOS applications and on iOS devices.
I made some small video which shows how to install and use this plugin
http://nomtek.com/developers/how-to-install-and-use-injection-plugin-for-xcode/
Hope it helps!
Not possible with the current tools.
Keep in mind that iOS applications are signed -- if you change a single byte you'd have resign the whole thing. One could imagine making it work using runtime's support for dynamically adding and removing methods. But that'd surely require adding some extra stuff to support it on the device, and that's something that malware could easily take advantage of. So it's probably not a feature you'll be likely to see anytime soon.
By the way, Xcode versions 1.x-3.x did have a "Fix and Continue" feature. You could edit as you were debugging, use the Fix and Continue command, and continue running the updated code. I believe it was removed at some point, perhaps due to some combination of: requiring that your project be configured to use "zero link" and perhaps some other things; being less than completely reliable; probably not supporting iOS; the switch to llvm; other changes in Xcode 4. Maybe it'll come back someday -- if you want it back, file a bug -- but again, I think supporting it on iOS would be a challenge.
If you're just talking about changing variable values then you can achieve that surreptitiously via lldb (or, presumably) gdb. Supposing you had:
- (void)uselessMethod
{
NSString *localString = #"I'm some local text";
NSLog(#"%#", localString);
}
And put a breakpoint on the NSLog, at that point you could ask lldb to evaluate a reassignment of localString as a means of performing it. E.g.
po localString = #"Hat"
If you then allow program execution to continue, you should find that the reassignment has stuck. Similarly you can call any method or perform any other sort of assignment.
I've just tested this against Xcode 4.3.2.
You can hot swap a variable value in Xcode by:
expression <variable> = <value>;.
By having a break point in the place where you wanna change the value and doing the command in Xcode console.
Example:
// Messages From Console
(lldb) expression graphFlag = #"X"; // Update variable value
(__NSCFConstantString *) $0 = 0x36f95718 #"X" // Xcode prints the updated value
(lldb) expression graphFlag; // Printing value through expression command
(__NSCFConstantString *) $1 = 0x36f95718 #"X" // Hot Swapped variable value

Can Make undefine a variable?

I'm working in an embedded system (RTXC) where I need to disable the debugger functionality which is enabled through a #define command. However, when I change the #define to undefine, compilation goes off fine, but when the linker runs, it encounters an error about a symbol not existing that belongs to the debug code (which should have been taken care of by the debugger variable not being defined). Is there any way for Make to ensure that a preprocessor variable does not get defined or stays undefined ?
The answer to your question is no, Make can't absolutely prevent a variable from being defined by, say, a #define expression in the code.
You seem to have an elusive problem. It could be a bug in your Makefiles, a misspelled directive, a bad macro (if you'll pardon the tautology) or something trivial. I'd suggest burning the forest: cut out everything until the problem stops, then see where it was hiding. If you get down to HelloWorld and the problem persists, let us know.
No. You will need to fix the bug in your code.
More specifically, there is something that is referencing the debug side of things outside of an #ifdef. Make won't be able to help you there.
Another possibility is that you have a .o or something left over from a previous build; you might want to try cleaning the build tree.