Examining arguments to system calls on the frame stack in lldb - objective-c

Question

I'm working with a X86_64 program in Xcode 4.5. I'm getting warnings to break on _NSWarnForDrawingImageWithNoCurrentContext and stop making a drawing call with no context. The call is happening in a deep set of CALayer/NSView calls. I want to find out what view this relates to.
#0 0x00007fff9033f823 in _NSWarnForDrawingImageWithNoCurrentContext ()
#1 0x00007fff8ffd2cf3 in -[NSImageRep drawInRect:fromRect:operation:fraction:respectFlipped:hints:] ()
#2 0x00007fff901444ea in -[NSImageRep _newCGImageForProposedRect:context:hints:flipped:] ()
#3 0x00007fff8ffd3acb in -[NSImageRep CGImageForProposedRect:context:hints:] ()
#4 0x00007fff9006be1d in -[NSImageRep CGImageForProposedRect:context:hints:flipped:] ()
#5 0x00007fff900f0346 in -[NSImage _newSnapshotRepForRep:rect:context:processedHints:] ()
#6 0x00007fff900923c8 in -[NSImage _snapshotRepForRep:rect:context:processedHints:] ()
#7 0x00007fff9006bd77 in __48-[NSImage CGImageForProposedRect:context:hints:]_block_invoke_0 ()
#8 0x00007fff8ffd14d6 in -[NSImage _usingBestRepresentationForRect:context:hints:body:] ()
#9 0x00007fff9006b9e5 in -[NSImage CGImageForProposedRect:context:hints:] ()
#10 0x00007fff8ff14754 in -[_NSImageLayerContents CA_copyRenderValue] ()
#11 0x00007fff890350c3 in -[CALayer(CALayerPrivate) _copyRenderLayer:layerFlags:commitFlags:] ()
#12 0x00007fff89034a7c in CA::Context::commit_layer(CA::Layer*, unsigned int, unsigned int, void*) ()
#13 0x00007fff890349a4 in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#14 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#15 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#16 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#17 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#18 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#19 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#20 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#21 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#22 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#23 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#24 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#25 0x00007fff8903492f in CA::Layer::commit_if_needed(CA::Transaction*, void (*)(CA::Layer*, unsigned int, unsigned int, void*), void*) ()
#26 0x00007fff890258d6 in CA::Context::commit_transaction(CA::Transaction*) ()
#27 0x00007fff89025423 in CA::Transaction::commit() ()
#28 0x00007fff9010f032 in -[NSView(NSLayerKitGlue) _drawRectAsLayerTree:] ()
#29 0x00007fff8ffbe35a in -[NSView _drawRect:clip:] ()
#30 0x00007fff8ffbb093 in -[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] ()
#31 0x00007fff8ffbbb24 in -[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] ()
#32 0x00007fff8ffba223 in -[NSThemeFrame _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] ()
#33 0x00007fff8ffb5e4d in -[NSView _displayRectIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:] ()
#34 0x00007fff8ff7fd73 in -[NSView displayIfNeeded] ()
#35 0x00007fff8ff7f2ac in _handleWindowNeedsDisplayOrLayoutOrUpdateConstraints ()
#36 0x00007fff9054a971 in __83-[NSWindow _postWindowNeedsDisplayOrLayoutOrUpdateConstraintsUnlessPostingDisabled]_block_invoke_01208 ()
How do I find out what view _drawRect:clip: is being called on?
I've tried switching to frame 29 and then doing a p/x $arg1 but that gives the error
error: Couldn't materialize struct: Couldn't read rdi (materialize)
I'm only finding source code and commit logs related to this error and can't figure out what it means. Any help would be greatly appreciated, since this would also help try to avoid messages about Apple code that is using deprecated calls that give me warnings.

Answer 1

You could try using the "register read" command, like "reg read arg1", but the real problem here is that rdi is a volatile register with the SysV x86_64 ABI. This means that whenever you make a function call, the contents of rdi may have been overwritten without being saved anywhere. As soon as frame 29 called frame 28, the debugger will assume that rdi has been overwritten (frame 29 probably stored something there itself as an arg to frame 28) and will not be able to retrieve the previous contents of rdi that frame 29 had on entry.
The registers called "non-volatile" or "callee preserved" are guaranteed to be retrievable even in the middle of the stack. On x86_64 these include rbx and r12-r15. If frame 29 is using r12 to hold a variable, calls frame 28 and frame 28 wants to use r12, it has to save the old value on to the stack and then restore that value before it returns to frame 29. The debugger knows how to find this save location and can show you the contents of r12 in frame 29--by looking at where frame 28 saved it and retrieving the value.
Your only hope here is to either (1) put a breakpoint on frame 29 and print the value of arg1 at that point, or (2) look at the assembly instructions for frame 29 and see if the function saved its first argument somewhere (on the stack, in a non-volatile register) that you can still examine. Otherwise, by the time you're 28 stack frames deeper, the contents of the argument register are long gone.