Overview

Reversible debugging (the ability to "step backwards" through a program) is an obviously powerful tool. GDB does not support it today, but the foundations have been laid, and the GDB maintainers are looking for contributors interested in expanding those foundations.

Prior work

• Michael Snyder (while at Red Hat) contributed a checkpoint/restart implementation, which works on native GNU/Linux systems for limited single-threaded programs. It uses fork and other system calls directly in the debuggee to save state. This is included in GDB 6.5.
• Michael also implemented remote protocol support for communication with a reversible simulator. These patches have not been merged, but they should be fairly easy to merge; someone interested needs to restart the discussion. Some of the messages from the previous discussion are:
  • http://sourceware.org/ml/gdb-patches/2006-03/msg00388.html
  • http://sourceware.org/ml/gdb-patches/2006-04/msg00381.html
  • http://sourceware.org/ml/gdb-patches/2006-03/msg00389.html
  • http://sourceware.org/ml/gdb-patches/2006-03/msg00390.html
• Dave Brolley has implemented support for reverse debugging in the Red Hat "sid" simulator for the xstormy16 architecture:
  http://sourceware.org/ml/sid/2006-q3/msg00047.html
  (Note that sid is not a GNU project, but it is licensed under the GPL.)

Still to do

Once the basic commands are in GDB, it should be a simple matter of programming to add support for this to various existing GNU simulators. It could also be added either to native targets (perhaps based on the checkpoint / restart functionality) or to other third-party simulators for GNU/Linux and other platforms (such as valgrind, sid, or qemu).

For the highest fidelity of reversible userspace debugging, some work on Linux and other kernels might also go a long way.

The FSF and the GDB maintainers are interested in GDB contributions in this area. If anyone reading this is likewise interested in contributing to it, we would welcome your help!