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Control of Execution

The debug engine (DE) typically sends one of the following events as the last startup event:

  • The entry point event, if attaching to a newly launched program

  • The load complete event, if attaching to a program that is already running

Both these events are stopping events, meaning that the DE waits for a response from the user by means of the IDE. For more information, see Operational Modes.

When a stopping event is sent to the debug session:

  1. The program and thread that contain the current instruction pointer can be obtained from the event interface.

  2. The IDE determines the current source code file and position, which it displays highlighted in the editor.

  3. The debug session typically responds to this first stopping event by calling the program's Continue method.

  4. The program then runs until it encounters a stopping condition, such as hitting a breakpoint, in which case the DE sends a breakpoint event to the debug session. The breakpoint event is a stopping event, and the DE again waits for a user response.

  5. If the user elects to step into, over, or out of a function, the IDE prompts the debug session to call the program's Step method, passing it the unit of step (instruction, statement, or line) and the kind of step—that is, whether to step into, over, or out of the function. When the step is complete, the DE sends a step complete event to the debug session, which is a stopping event.

    -or-

    If the user elects to continue executing from the current instruction pointer, the IDE prompts the debug session to call the program's Execute method. The program resumes execution until it encounters the next stopping condition.

    -or-

    If the debug session is to ignore a particular stopping event, the debug session calls the program's Continue method. If the program was stepping into, over, or out of a function when it encountered the stopping condition, then it continues the step.

Programmatically, when the DE encounters a stopping condition, it sends such stopping events as IDebugLoadCompleteEvent2 or IDebugEntryPointEvent2 to the session debug manager (SDM) by means of an IDebugEventCallback2 interface. The DE passes the IDebugProgram2 and IDebugThread2 interfaces that represent the program and the thread containing the current instruction pointer. The SDM calls IDebugThread2::EnumFrameInfo to get the top stack frame and calls IDebugStackFrame2::GetDocumentContext to get the document context associated with the current instruction pointer. This document context is typically a source code file name, line, and column number. The IDE uses these to highlight the source code that contains the current instruction pointer.

The SDM typically responds to this first stopping event by calling IDebugProgram2::Continue. The program then runs until it encounters a stopping condition, such as hitting a breakpoint, in which case the DE sends an IDebugBreakpointEvent2 Interface to the SDM. The breakpoint event is a stopping event, and the DE again waits for a user response.

If the user elects to step into, over, or out of a function, the IDE prompts the SDM to call IDebugProgram2::Step, passing it the STEPUNIT (instruction, statement, or line) and the STEPKIND, that is, whether to step into, over, or out of the function. When the step is complete, the DE sends an IDebugStepCompleteEvent2 interface to the SDM, which is a stopping event.

If the user elects to continue executing from the current instruction pointer, the IDE asks the SDM to call IDebugProgram2::Execute. The program resumes execution until it encounters the next stopping condition.

If the debug package is to ignore a particular stopping event, the debug package calls the SDM, which calls IDebugProgram2::Continue. If the program was stepping into, over, or out of a function when it encountered the stopping condition, then it continues the step. This implies that the program maintains a stepping state, so that it knows how to continue.

The calls the SDM makes to Step, Execute, and Continue are asynchronous, which means that the SDM expects the call to return quickly. If the DE sends the SDM a stopping event on the same thread before Step, Execute, or Continue returns, the SDM hangs.

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