Introspection in Devel::Trepan

Here are some introspection routines in Devel::Trepan. I’m not aware that these exist in other debuggers, nor as Devel::REPL plugins. But if I’m wrong feel free to correct me in comments. And feel free to take code from Devel::Trepan to rework elsewhere.

Recently Jeffrey Ryan Thalhammer asked about variable, and subroutine completion and this got me thinking.

Info functions and Info packages

When he asked, there was a debugger command info functions which listed functions matching some regular expression. (There is also a gdb command of the same name.) That command accepted both fully-qualified and unqualified function names.

It occurred to me that I could also add an adjunct to that command, info packages. This command takes the data used in info functions, but it reindexes the fully-qualified function names keyed by package.

So here are some examples:

 (trepanpl): info packages Tie::   
 Tie::ExtraHash    Tie::Hash    Tie::StdHash

 (trepanpl): info packages -s -f Tie::Hash
   BEGIN    CLEAR    EXISTS    TIEHASH    new 
 Tie::Hash is in file /usr/share/perl/5.14/Tie/

 (trepanpl): info functions Tie::Hash::new
     Tie::Hash::new is at 8-11
 (trepanpl): list Tie::Hash::new  
 /usr/share/perl/5.14.2/Tie/ [4-13]
 5      use Carp;
 6      use warnings::register;
 8      sub new {
 9          my $pkg = shift;
10          $pkg->TIEHASH(@_);
11      }
13      # Grandfather "new"

For lexical variables, there is also info variables lexicals.

Completion and Complete

Well, now that that’s there, what about subroutine, package and filename completion on those above commands? That’s in there too. But wait, there’s more! Being something of a geek, I created internal routines for those types of completions. There are also specialized completion routines for my and our variables.

But to give you access to those from the debugger command line, I extended the debugger command complete. This is also a gdb command, although those specific options are not in gdb’s version. That format of giving options, though, is how gdb would do something like this.

The intent in adding that to the debugger command was not just for interactive use, but to provide something that more sophisticated front-ends could use. Or at least show the way.

In fact, the out-of-process client, --client does use this debugger “complete” command under the covers in its readline completion.

So although I don’t have a complete completion package when you are typing an expression, a number of the underlying components are there should someone want to take this and extend it.


In Perl Tricks, I read about B::Deparse. I found that so interesting and useful that, starting with release 0.57, it has added to Devel::Trepan as debugger command deparse. And we can even use Perl::Syntax::Highlight to colorize the output.


At the opposite end of the spectrum, there is a Devel::Trepan plugin to disassemble Perl code. Right now disassembly is at the file or subroutine level. Since I have the actual stopped OP address via Devel::Callsite, it would be nice to be able to allow a narrower range. However I haven’t figured out exactly how to do that even though I have some hints. See Getting a B::OP from an address?

And finally on this topic of low-level information, I should mention David Golden’s suggestion for hooking into Devel::Peek. For more information and a workaround, see this issue.


Here, I introduced you into some of the introspection aspects of Devel::Trepan. By the way, all of pod documentation for debugger commands given above, can also be gotten inside the debugger itself, with its help command.

In a sense there’s nothing here that really isn’t in Perl itself. One can think of the debugger commands and internal routines, merely as a wrapper around existing modules and existing Perl features. (That’s where all of the real heavy lifting is done.) Given that, there is no reason why this couldn’t be added to other debuggers, or REPLs, if it isn’t there already.

Go forth and multiply! There is more than one way to do it.

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user-pic I blog about Perl.