Document revision date: 19 July 1999
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OpenVMS Debugger Manual


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10.3.3 Executing Your Program One Source Line at a Time

To execute one source line of your program, click on the STEP button in the push button view or enter the STEP command in the command view. This debugging technique (called stepping) is one of the most commonly used.

After the line executes, the source view is updated and the current-location pointer marks which line of code will execute next.

Note the following points about source lines and the stepping behavior:

Keep in mind that if you optimized your code at compilation time, the source code displayed might not reflect the code that is actually executing (see Section 1.2).

10.3.4 Stepping into a Called Routine

When program execution is paused at a routine call statement, clicking on the STEP button typically executes the called routine in one step (depending on the coding style used), and the debugger suspends execution at the next source line in the calling routine (assuming no breakpoint was set within the called routine). This enables you to step through the code quickly without having to trace execution through any called routines (some of which might be system or library routines). This is called stepping over called routines.

To step into a called routine so that you can execute it one line at a time:

  1. Suspend execution at the routine call statement, for example, by setting a breakpoint (see Section 10.4) and then clicking on the Go button in the push button view.
  2. When execution is paused at the call statement, click on the S/in button in the push button view, or enter the STEP/INTO command at the DBG> prompt. This moves execution just past the start of the called routine.

Once execution is within the called routine, click on the STEP button to execute the routine line by line.

Clicking on the S/in button when execution is not paused at a routine call statement is the same as clicking on the STEP button.

10.3.5 Returning from a Called Routine

When execution is suspended within a called routine, you can execute your program directly to the end of that routine by clicking on the S/ret button in the push button view, or enter the STEP/RETURN command at the DBG> prompt.

The debugger suspends execution just before the routine's return instruction executes. At that point, the routine's call frame has not been deleted from the call stack, so you can still get the values of variables local to that routine, and so on.

You can also use the S/call button in the push button view (or enter the STEP/CALL command at the DBG> prompt) to execute the program directly to the next Return or Call instruction.

The S/ret button is particularly useful if you have inadvertently stepped into a system or library routine (see Section 10.1.3).

10.4 Suspending Execution by Setting Breakpoints

A breakpoint is a location in your program at which you want execution to stop so that you can check the current value of a variable, step into a routine, and so on.

When using the debugger's DECwindows Motif interface, you can set breakpoints on:

Note

If you are stopped at a breakpoint in a routine that has control of the mouse pointer by a PointerGrab or a KeyboardGrab, your workstation will hang.

To work around this problem, debug your program using two workstations. For more information, see Section 9.7.3.1.

The debugger provides two ways to qualify breakpoints:

You can set a breakpoint that is both a conditional and action breakpoint.

The following sections explain these breakpoint options.

10.4.1 Setting Breakpoints on Source Lines

You can set a breakpoint on any source line that has a button to its left in the source display. These are the lines for which the compiler has generated executable code (routine declarations, assignment statements, and so on).

To set a breakpoint on a source line:

  1. Find the source line on which you want to set a breakpoint (see Section 10.1).
  2. Click on the button to the left of that line. (The breakpoint is set when the button is filled in.) The breakpoint is set at the start of the source line---that is, on the first machine-code instruction associated with that line.

Figure 10-4 shows that a breakpoint has been set on the start of line 37.

Figure 10-4 Setting a Breakpoint on a Source Line


10.4.2 Setting Breakpoints on Routines with Source Browser

Setting a breakpoint on a routine enables you to move execution directly to the routine and inspect the local environment.

To set a breakpoint on a routine:

  1. Choose Browse Sources from the File menu on the main window (see Figure 10-2).
    Select SYMBOLIC display the names of all modules linked in the image. Select ALL to display the names of only those modules for which the debugger has symbolic information.
    The Source Browser dialog box displays the name of your executable image, which is highlighted, and the class of shareable images linked with it (SYMBOLIC or ALL). The name of a linked image is dimmed if no symbolic information is available for that image.
  2. Double click on the name of the executable image. The names of the modules in that image are displayed (indented) under the image name.
  3. Double click on the name of the target module. The names of the routines in that module are displayed (indented) under the module name (see Figure 10-5).
  4. Double click on the name of the routine on which to set a breakpoint. The debugger echoes the results of your SET BREAKPOINT command on the command line in the command view.
    Alternatively, click once on the name of the routine, then click the Set Breakpoint button in the Source Browser view. The debugger echoes the results of your SET BREAKPOINT command on the command line in the command view.

Figure 10-5 Setting a Breakpoint on a Routine


10.4.3 Setting an Exception Breakpoint

An exception breakpoint suspends execution when an exception is signaled and before any exception handler declared by your program executes. This enables you to step into the exception handler (if one is available) to check the flow of control.

To set an exception breakpoint, choose On Exception from the Break menu on the main window or the optional views window.

10.4.4 Identifying the Currently Set Breakpoints

There are three ways to determine which breakpoints are currently set:

10.4.5 Deactivating, Activating, and Canceling Breakpoints

After a breakpoint is set, you can deactivate, activate, or delete it.

Deactivating a breakpoint causes the debugger to ignore the breakpoint during program execution. However, the debugger keeps the breakpoint listed in the breakpoint view so that you can activate it at a later time, for example, when you rerun the program (see Section 9.3). Note the following points:

Activating a breakpoint causes it to take effect during program execution:

When you cancel a breakpoint, it is no longer listed in the breakpoint view so that later you cannot activate it from that list. You have to reset the breakpoint as explained in Section 10.4.1 and Section 10.4.2. Note the following points:

10.4.6 Setting a Conditional Breakpoint

The debugger suspends execution of the program at a conditional breakpoint only when a specified expression is evaluated as true. The debugger evaluates the conditional expression when program execution reaches the breakpoint and ignores the breakpoint if the expression is not true.

The following procedure sets a conditional breakpoint, whether or not a breakpoint was previously set at that location:

  1. Display the source line on which you want to set the conditional breakpoint (see Section 10.1).
  2. Do one of the following:
  3. Enter a relational expression in the Condition: field of the dialog box. The expression must be valid in the source language. For example, a[3] :=,= 0 is a valid relational expression in the C language.
  4. Click on OK. The conditional breakpoint is now set. The debugger indicates that a breakpoint is conditional by changing the shape of the breakpoint's button from a square to a diamond.

Figure 10-6 Setting a Conditional Breakpoint


The following procedure modifies a conditional breakpoint; that is, it can be used to change the location or condition associated with an existing conditional breakpoint, or to change an unqualified breakpoint into a conditional breakpoint:

  1. Choose Views... from the Options menu on the main window or optional views window. When the Views dialog box appears, click on Breakpoint View to display the breakpoint view.
  2. From the breakpoint view, do one of the following:
  3. Follow steps 3 and 4 of the previous procedure, as appropriate.

10.4.7 Setting an Action Breakpoint

When a program reaches an action breakpoint, the debugger suspends execution of the program and executes a specified list of commands.

To set an action breakpoint, whether or not a breakpoint was previously set at that location:

  1. Display the source line on which you want to set the action breakpoint (see Section 10.1).
  2. Do one of the following:
  3. Enter one or more debugger commands in the Action: field of the dialog box. For example: DEPOSIT x[j] = 3; STEP; EXAMINE a
  4. Click on OK. The action breakpoint is now set (see Figure 10-7.)

Figure 10-7 Setting an Action Breakpoint


The following procedure modifies an action breakpoint; that is, it can be used to change the location or command associated with an existing action breakpoint, or to change an unqualified breakpoint into an action breakpoint:

  1. Choose Views... from the Options menu on the main window or optional views window, then click on Breakpoint View when the Views dialog box appears.
  2. From the breakpoint view, do one of the following:
  3. Follow steps 3 and 4 of the previous procedure, as appropriate.

10.5 Examining and Manipulating Variables

This section explains how to:

See Section 10.6, which also applies to all operations on variables.

10.5.1 Selecting Variable Names from Windows

Use the following techniques to select variable names from windows for the operations described in the sections that follow (see Section 10.5.2 for examples).

When selecting names, follow the syntax of the source programming language:

Select character strings from windows as follows:

10.5.2 Displaying the Current Value of a Variable

To display the current value of a variable:

  1. Find and select the variable name in a window as explained in Section 10.5.1.
  2. Click on the EX button in the push button view. The debugger displays the variable and its current value in the command view. The debugger displays the value of a variable in the current scope, which might not be the same as the source location you were intending.

Figure 10-8, Figure 10-9, and Figure 10-10 show how to display the value of an integer variable, array aggregate, and array element, respectively.

Figure 10-8 Displaying the Value of an Integer Variable


Figure 10-9 Displaying the Value of an Array Aggregate


Figure 10-10 Displaying the Value of an Array Element


To display the current value in a different type or radix, use the following alternative method:

  1. Find and select the variable name in a window as explained in Section 10.5.1.
  2. Choose Examine... in the Commands menu in the main window. The Examine dialog box appears with the name selected in the Variable/Expression field.
  3. Choose the default, int, long, quad, short, or char* item from the Typecast menu within the dialog box.
  4. Choose the default, hex, octal, decimal, or binary item from the Output Radix menu within the dialog box.
  5. Click on OK.

The value, altered to your specification, appears in the command view.

Figure 10-11 shows that the variable j has been typecast as long.

Figure 10-11 Typecasting the Value of a Variable


10.5.3 Changing the Current Value of a Variable

To change the current value of a variable:

The new value, altered to your specification, appears in the command view and is assigned to the variable.

Figure 10-12 shows a new value for the variable safe.

Figure 10-12 Changing the Value of a Variable


10.5.4 Monitoring a Variable

When you monitor a variable, the debugger displays the value in the monitor view and checks and updates the displayed value whenever the debugger regains control from your program (for example, after a step or at a breakpoint).

Note

You can monitor only a variable, including an aggregate such as an array or structure (record). You cannot monitor a composite expression or memory address.

To monitor a variable (see Figure 10-13):

  1. Find and select the variable name in a window as explained in Section 10.5.1.
  2. Click on the MON button in the push button view. The debugger:

You can typecast the output value when monitoring variables by choosing the Typecast item in the Monitor menu.

You can change the output radix when monitoring variables as follows:

To remove a monitored element from the monitor view, choose Remove from the Monitor menu.

Figure 10-13 Monitoring a Variable


10.5.4.1 Monitoring an Aggregate (Array or Structure) Variable

If you select the name of an aggregate variable, such as an array or structure (record) and click on the MON button, the debugger displays the word Aggregate in the Value/Deposit column of the monitor view. To display the values of all elements (components) of an aggregate variable, double click on the variable name in the Monitor Expression column (or choose Expand in the Monitor menu). The displayed element names are indented relative to the parent name (see Figure 10-14). If an element is also an aggregate, you can double click on its name to display its elements, and so on.

Figure 10-14 Expanded Aggregate Variable (Array) in Monitor View


To collapse an expanded display so that only the aggregate parent name is shown in the monitor view, double click on the name in the Monitor Expression column (or choose Collapse from the Monitor menu).

If you have selected a component of an aggregate variable, and the component expression is itself a variable, the debugger monitors the component that was active when you made the selection. For example, if you select the array component arr[i] and the current value of i is 9, the debugger monitors arr[9] even if the value of i subsequently changes to 10.


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