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7. Recording Inferior's Execution and Replaying It

On some platforms, GDB provides a special process record and replay target that can record a log of the process execution, and replay it later with both forward and reverse execution commands.

When this target is in use, if the execution log includes the record for the next instruction, GDB will debug in replay mode. In the replay mode, the inferior does not really execute code instructions. Instead, all the events that normally happen during code execution are taken from the execution log. While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would. Their contents are taken from the execution log.

If the record for the next instruction is not in the execution log, GDB will debug in record mode. In this mode, the inferior executes normally, and GDB records the execution log for future replay.

The process record and replay target supports reverse execution (see section Running programs backward), even if the platform on which the inferior runs does not. However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log. In other words, reverse execution on platforms that don't support it directly can only be done in the replay mode.

When debugging in the reverse direction, GDB will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not.

For architecture environments that support process record and replay, GDB provides the following commands:

target record

This command starts the process record and replay target. The process record and replay target can only debug a process that is already running. Therefore, you need first to start the process with the run or start commands, and then start the recording with the target record command.

Both record and rec are aliases of target record.

Displaced stepping (see section displaced stepping) will be automatically disabled when process record and replay target is started. That's because the process record and replay target doesn't support displaced stepping.

If the inferior is in the non-stop mode (see section Non-Stop Mode) or in the asynchronous execution mode (see section Background Execution), the process record and replay target cannot be started because it doesn't support these two modes.

record stop

Stop the process record and replay target. When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state.

When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded. In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened.

On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become "live" at that earlier state, and it will then be possible to continue the usual "live" debugging of the process from that state.

When the inferior process exits, or GDB detaches from it, process record and replay target will automatically stop itself.

set record insn-number-max limit

Set the limit of instructions to be recorded. Default value is 200000.

If limit is a positive number, then GDB will start deleting instructions from the log once the number of the record instructions becomes greater than limit. For every new recorded instruction, GDB will delete the earliest recorded instruction to keep the number of recorded instructions at the limit. (Since deleting recorded instructions loses information, GDB lets you control what happens when the limit is reached, by means of the stop-at-limit option, described below.)

If limit is zero, GDB will never delete recorded instructions from the execution log. The number of recorded instructions is unlimited in this case.

show record insn-number-max

Show the limit of instructions to be recorded.

set record stop-at-limit

Control the behavior when the number of recorded instructions reaches the limit. If ON (the default), GDB will stop when the limit is reached for the first time and ask you whether you want to stop the inferior or continue running it and recording the execution log. If you decide to continue recording, each new recorded instruction will cause the oldest one to be deleted.

If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking.

show record stop-at-limit

Show the current setting of stop-at-limit.

info record insn-number

Show the current number of recorded instructions.

record delete

When record target runs in replay mode ("in the past"), delete the subsequent execution log and begin to record a new execution log starting from the current address. This means you will abandon the previously recorded "future" and begin recording a new "future".


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