"Undoing" Program Execution

If you take a look at the Edit => Undo menu item after an execution command, you'll find that DDD offers you to undo execution commands just as other commands. Does this mean that DDD allows you to go backwards in time, undoing program execution as well as undoing any side-effects of your program?

Sorry--we must disappoint you. DDD cannot undo what your program did. (After a little bit of thought, you'll find that this would be impossible in general.) However, DDD can do something different: it can show previously recorded states of your program.

After "undoing" an execution command (via Edit => Undo, or the Undo button), the execution position moves back to the earlier position and displayed variables take their earlier values. Your program state is in fact unchanged, but DDD gives you a view on the earlier state as recorded by DDD.

In this so-called Illegal HTML tag removed : historic mode, most normal DDD commands that would query further information from the program are disabled, since the debugger cannot be queried for the earlier state. However, you can examine the current execution position, or the displayed variables. Using Undo and Redo, you can move back and forward in time to examine how your program got into the present state.

To let you know that you are operating in historic mode, the execution arrow gets a dashed-line appearance (indicating a past position); variable displays also come with dashed lines. Furthermore, the status line informs you that you are seeing an earlier program state.

Here's how historic mode works: each time your program stops, DDD collects the current execution position and the values of displayed variables. Backtrace, thread, and register information is also collected if the corresponding dialogs are open. When "undoing" an execution command, DDD updates its view from this collected state instead of querying the program.

If you want to collect this information without interrupting your program--within a loop, for instance--you can place a breakpoint with an associated cont command (see Breakpoint Commands). When the breakpoint is hit, DDD will stop, collect the data, and execute the cont command, resuming execution. Using a later Undo, you can step back and look at every single loop iteration.

To leave historic mode, you can use Redo until you are back in the current program state. However, any DDD command that refers to program state will also leave historic mode immediately by applying to the current program state instead. For instance, Up leaves historic mode immediately and selects an alternate frame in the restored current program state.

If you want to see the history of a specific variable, as recorded during program stops, you can enter the DDD command

     graph history 

**Illegal HTML tag removed :** name

     

This returns a list of all previously recorded values of the variable Illegal HTML tag removed : name, using array syntax. Note that Illegal HTML tag removed : name must have been displayed at earlier program stops in order to record values.

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