The Whiteboard System

The BSD debugger is build around a whiteboard system (WBS). A WBS is closely related to a blackboard system used in artificial intelligence applications.


A group of specialists are seated in a room with a large blackboard. They work as a team to brainstorm a solution to a problem, using the blackboard as the workplace for cooperatively developing the solution. The session begins when the problem specifications are written onto the blackboard. The specialists all watch the blackboard, looking for an opportunity to apply their expertise to the developing solution. When someone writes something on the blackboard that allows another specialist to apply their expertise, the second specialist records their contribution on the blackboard, hopefully enabling other specialists to then apply their expertise. This process of adding contributions to the blackboard continues until the problem has been solved.

Why a WBS?

A debugger needs to collect a lot of information and while information is structured in most cases, there's a lot of variation.

The WBS is an attempt to avoid having to know everything up-front and also to avoid that everything is entangled and thus that everything is hard-coded in software. Instead, the WBS records objects and associations and pieces of code get called whenever a change is made that is of interest to that code.


The user opens the debugger and specifies "helloworld" as the program to debug. The WBS records the arrival of a new debuggee. The ELF file handler has previously announced it is interested in new debuggees, and as such gets called. The ELF file handler checks the target and finds it to be an ELF file and updates the WBS that an ELF file is being used. This triggers the execution of numerous ELF specific handlers, such as symbol table handlers, segment readers, section readers, DWARF parsers, unwind papsers. When a core file is loaded, specific handlers that know about the architecture and OS specifics get to take a look and take appropriate action by creating thread and context information, adding new ELF object files, etc...

None of this is explicitly coded. It's all based on updates to the WBS and having those updates trigger calls to functions/routines that can further update the WBS. With nothing hardcoded, everything can be supported by using plugins or extensions.

WhiteBoardSystem (last edited 2009-06-27 20:04:02 by MarcelMoolenaar)