7.4 Controlling the Execution

When an application is being debugged by Valhalla, it is executed as a separate process, under the control of Valhalla. This gives the user of Valhalla extensive control over the debugged process.

7.4.1 Interrupting the debugged process

If the debugged process executes, and you wish to interrupt the execution, you can always hit the Stop button in the Buttons Area of the Universe (or select the Stop item in the Control menu of the Universe). This will immediately interrupt the debugged process, making it possible to inspect it (the state of objects, the current stack, etc.).

7.4.2 Setting Breakpoints

Breakpoints are naturally associated with the imperatives of the BETA program, and acts as points in the debugged process, where Valhalla is given some control over the execution of the debugged process. Valhalla supports three types of break points:

• Break: If a Break point is reached during the execution of the debugged process, the process will be interrupted, and Valhalla is now able to inspect the current state of the application.

• Oneshot: which is a Break point, that is only effective once (will automatically be removed, when it have been reached the first time). Otherwise it functions exactly as a Break point.

• Trace: is used to trace the execution without actually interrupting. A trace point have associated a text string. Each time a trace point is reached during the execution of the debugged process, the associated text will be printed on standard output, and the execution will be continued immediately.

Valhalla supports that break points are positioned in two positions, relative to an imperative:

• Before: when a break point is positioned before an imperative, it will be activated immediately before the imperative will be executed.
  A Before break is shown visually in the code views by >>n>>, where n is a sequence number of the break point.

• After: when a break point is positioned after an imperative, it will be activated immediately after the imperative have been executed. The only places, where using After position is the only possible solution, is when one wish to interrupt the application immediately before it reaches the terminating #) of a descriptor. In all other cases, we could just as well have placed the break point before the imperative following the one selected.
  An After break is shown visually in the code views by <<n<<, where n is a sequence number of the break point.

An important aspect of Oneshot and Break break points is, that the object- and stack views that are visible in the Valhalla Universe, are automatically updated to reflect the current state of the objects and the stack at the time of the break. Should the object shown in an object view have become inaccessible (and therefore reclaimed by the garbage collector), this fact is reflected in the object view by clearing the object view, and changing the label of the view.

7.4.3 Unsetting Breakpoints

Breakpoints may be unset essentially in the same way as they have been set.