lldb cannot print values in debug code

Question

When print a value in lldb, I got the following error

error: no member named 'rec' in namespace '$__lldb_local_vars'

My code was compiled by -g. Why is lldb not able to print values?

Answer 1

$__lldb_local_vars was a bit of a hack to work around some problems in the name lookup affordances provided by clang for lldb's expression parser. The hack tries to promote local variables to the head of name lookup (ahead of locally visible class and namespace lookups) by injecting local variables into a namespace that is then imported into the expression. This has some performance issues, and it is also fragile since it requires realizing all visible locals. We did a bunch of work to remove locals that we can tell we won't be able to realize, but it still didn't work very reliably.

This hack is off by default in all the lldb's that Apple releases, and is controlled by setting:

(lldb) set list target.experimental.inject-local-vars
  target.experimental.inject-local-vars -- If true, inject local variables explicitly into the
                                           expression text.  This will fix symbol resolution
                                           when there are name collisions between ivars and
                                           local variables.  But it can make expressions run
                                           much more slowly.

You can use settings show to show the current value of this setting, and settings set to change it.

If you can make available an example showing this failure, it would be helpful to file a bug with the llvm bug reporter: https://bugs.llvm.org.