Process Record and Replay
Process record and replay is a gdb feature first appearing in the 7.0 release (September 2009).
For supported architectures and OS/ABIs, this feature allows the user to record the execution of a program being debugged by gdb, and then "play back" the recorded execution, deterministicly and repeatedly if desired.
Process record and replay also supports gdb's [ReverseDebugging | reverse debugging] commands, so that during replay it is possible to debug the program backward as well as forward.
Process record and replay is currently supported for the following gdb targets:
How it works
Process record and replay works by logging the execution of each machine instruction in the child process (the program being debugged), together with each corresponding change in machine state (the values of memory and registers). By successively "undoing" each change in machine state, in reverse order, it is possible to revert the state of the program to an arbitrary point earlier in the execution. Then, by "redoing" the changes in the original order, the program state can be moved forward again.
The following gdb commands are defined for process record / replay:
- "target record" (or simply "record", for short)
Start process record/replay (ie. start recording the subsequent execution of the child process). You must start debugging the program (with the "run" command) before using this command to start recording. You can start recording at any point after the child process has been started (eg. at a breakpoint).
- "record stop"
Stop process record/replay (ie. cease recording the program execution), and discard any existing execution log. The child process is not terminated, and you may continue to debug it normally.
- "record delete"
Discard the existing execution log, and begin recording a new log.
- "set record insn-number-max"
Set the maximum number of instruction executions that will be recorded (ie. the size of the process record log buffer). Zero means unlimited. Default is 200,000.
- "set record stop-at-limit"
Controls the behavior when the buffer becomes full. If "on", gdb will stop and ask the user what to do. If "off", the buffer acts as a circular buffer, deleting the oldest records to make room for new ones. Default is "on".
- "info record insn-number"
Show the current number of instructions in the record/replay buffer.
To run the gdb reverse-debugging tests with process record and replay, you need a board description file "precord.exp", which should look like this:
# Testing programs using process record/replay (precord) load_base_board_description "unix" set_board_info gdb,can_reverse 1 set_board_info gdb,use_precord 1
Then the "make check" command will look like this:
make check RUNTESTFLAGS="--target_board precord (test file or files)
At the time of this writing, the reverse debugging tests include: