[lldb-dev] Debugging std::string is weird...?

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Sun Jan 14 13:40:28 PST 2018

You can do a:

(lldb) frame var --raw temp

It will show you the internals of the std::string. If the string is over 22 characters, one of the items in the raw std::string should point to the buffer and the length should be contained inside there as well. The value you see is due to a special summary plug-in that is extracting the std::string value from the innards of the STL implementation.

So a few things could be wrong:
1 - compiler's location for the variable could be wrong (optimizations?) so we show the wrong value
2 - your STL is newer and might pull some tricks when storing the string and our summary extracting code is doing the wrong thing for your std:string. Many std::string implementations will store up to twenty some character inlined into the std::string itself when the string is short. Maybe we aren't handling that quite correctly. What is the actual length of your std::string named "temp"? 

As a work around, try to run an expression to get the "const char *"
(lldb) p temp.c_str()

See if this pointer is the output of the "frame var --raw temp" command. Also to see if the string value is inlined you can do:

(lldb) p &temp
(lldb) p sizeof(temp)

Then see if the "const char *" value from "p temp.c_str()" falls between the "&temp" and "&temp + sizeof(temp)".

Let me know what you find. My guess is we might not be handling special cases when the string is inlined, but that is just a guess.

Greg Clayton

> On Jan 13, 2018, at 6:29 AM, Gabriel Nützi via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Dear Dev-Team
> Ok this question is not a really dev-question, but I more seeking for some hints:
> I have some troubles getting lldb to show me a correct string output in a CefSchemeHandlerFactory
> https://github.com/gabyx/ExecutionGraph/blob/a24f4c1b538edd6b138503fb96db6fbd71c3b1c5/gui/executionGraphGui/cefapp/FileSchemeHandlerFactory.cpp#L31 <https://github.com/gabyx/ExecutionGraph/blob/a24f4c1b538edd6b138503fb96db6fbd71c3b1c5/gui/executionGraphGui/cefapp/FileSchemeHandlerFactory.cpp#L31>
> When I debug this application built with clang-5.0.1 (llvm install with brew) with the debugger lldb:
> [code]
> [ 96%] Building CXX object gui/executionGraphGui/CMakeFiles/ExecutionGraphGUI.dir/cefapp/FileSchemeHandlerFactory.cpp.o
> /usr/local/opt/llvm/bin/clang++   -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/gui/executionGraphGui -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/include -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/include -I/usr/local/include/eigen3 -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/src/meta/include -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/external/args-src -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/external/cefbinaries-src -I/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/external/cefbinaries-src/include  -g -arch x86_64 -mmacosx-version-min=10.9   -std=c++14 -lc++experimental -ferror-limit=50 -Werror=return-type -g -g3 -fno-omit-frame-pointer -Weverything -Wpedantic -Wno-deprecated-register -Wno-documentation -Wno-old-style-cast -Wno-comment -Wno-float-equal -Wno-deprecated -Wno-c++98-compat-pedantic -Wno-undef -Wno-unused-macros -fsanitize=leak -fsanitize=address -o CMakeFiles/ExecutionGraphGUI.dir/cefapp/FileSchemeHandlerFactory.cpp.o -c /Users/gabrielnuetzi/Desktop/ExecutionGraph/gui/executionGraphGui/cefapp/FileSchemeHandlerFactory.cpp
> [/code]
> I only see really weird output, such as:
> [code]
> Process 3741 stopped
> * thread #22, name = 'Chrome_IOThread', stop reason = breakpoint 1.1
>     frame #0: 0x000000010011eb09 ExecutionGraphGUI`FileSchemeHandlerFactory::Create(this=0x00006060000dc340, scheme_name=0x0000700007ac9c18, request=(ptr_ = 0x0000700007ac9c10)) at FileSchemeHandlerFactory.cpp:37
>    34           {
>    35               ++itC;
>    36           }
> -> 37           std::path url(itC, temp.end());
>    38           // e.g. url : "host/folderA/folderB/file.ext""
>    39
>    40           // Split urlPrefix from front (e.g "host/folderA")
> Process 3741 launched: '/Users/gabrielnuetzi/Desktop/ExecutionGraph/build/gui/executionGraphGui/Debug/ExecutionGraphGUI.app/Contents/MacOS/ExecutionGraphGUI' (x86_64)
> (lldb) fr v temp
> (std::__1::string) temp = "\x85\xac\a\0p\0\0\x10\x85\xac\a\0p\0\0�\a\0p\0\0\xa0\x83\xac\a\0p\0\0`\x83\xac\a\0p\0\0@"
> [/code]
> Why is this, or where could the problem be? I have a really hard time to figure out why I cannot successfully debug these strings in my application.
> The above example has been done by launching lldb from the terminal. (No python formatters have been loaded I think…? at least not by me)
> In a normal main.cpp Application like the one here: https://github.com/gabyx/ExecutionGraph/blob/devGUI/gui/testDebugger/build.sh <https://github.com/gabyx/ExecutionGraph/blob/devGUI/gui/testDebugger/build.sh>
> everything works and the strings are formatted normally.
> Any help really welcome!
> BR 
> Gabriel Nützi
> MSc ETH Mechanical Engineering
> gnuetzi at gmail.com <mailto:gnuetzi at gmail.com>
> +41 (0) 77 424 86 45
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> lldb-dev mailing list
> lldb-dev at lists.llvm.org
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