Updated: 11 December 1998

OpenVMS Debugger Manual

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:

• A source line can consist of one or more programming language elements depending on the language and coding style used.
• When you click on the STEP button, the debugger executes one executable line and suspends execution at the start of the next executable line, skipping over any intervening nonexecutable lines.
• Executable lines are those for which instructions were generated by the compiler (for example, lines with routine call or assignment statements). Executable lines have a button to their left in the main window.
• Examples of nonexecutable lines are comment lines or lines with variable declarations without value assignments. Nonexecutable lines do not have a button to their left in the main window.

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:

• Specific source lines
• Specific routines (functions, subprograms, and so on)
• Exceptions signaled during the execution of your program

The debugger provides two ways to qualify breakpoints:

• You can set a conditional breakpoint. The debugger suspends execution at a conditional breakpoint only when a specified relational expression is evaluated as true.
• You can set an action breakpoint. The debugger executes one or more specified system-specific commands when it reaches the breakpoint.

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

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

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:

• Scroll through your source code and note the lines whose breakpoint button is filled in. This method can be time consuming and also does not show which breakpoints were set and then deactivated (see Section 10.4.5).
• Choose Views... from the Options menu on the main window or the optional views window. When the Views dialog box appears, click on Breakpoint View to display the breakpoint view (see Figure 8-4).
  The breakpoint view lists a module name and line number for each breakpoint (see Section 10.1). A filled-in button next to the breakpoint identification indicates that the breakpoint is activated. A cleared button indicates that the breakpoint is deactivated.
• Enter the SHOW BREAK command at the DBG> prompt in the command view. The debugger list all the breakpoints that are currently set, including specifications for conditional breakpoints, and commands to be executed at action 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:

• To deactivate a specific breakpoint, clear the button for that breakpoint in the main window or in the breakpoint view.
  In the breakpoint view, you can also choose Toggle from the Break menu, if the breakpoint is currently activated.
• To deactivate all breakpoints, choose Deactivate All from the Break menu.

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

• To activate a breakpoint, fill in the button for that breakpoint in the main window or in the breakpoint view.
  In the breakpoint view, you can also choose Toggle from the Break menu, if the breakpoint is currently deactivated.
• To activate all breakpoints, choose Activate All from the Break menu.

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:

• To cancel a specific breakpoint, choose Cancel from the Break menu on the optional views window.
• To cancel all breakpoints, choose Cancel All from the Break menu.

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:
   • Press Ctrl/MB1 on the button to the left of the source line. This displays the Set/Modify Breakpoint dialog box, showing the source line you selected in the Location: field (see Figure 10-6).
   • Choose the Set or Set/Modify item from the Break menu. When the Set/Modify Breakpoint dialog box displays, enter the source line in the Location: field.
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:
   • Press Ctrl/MB1 on the button to the left of the listed breakpoint.
   • Click on a breakpoint listed in the view, and choose the Set/Modify item from the Break menu.
3. Follow steps 3 and 4 of the previous procedure, as appropriate.

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:
   • Press Ctrl/MB1 on the button to the left of the source line. This displays the Set/Modify Breakpoint dialog box, showing the source line you selected in the Location: field (see Figure 10-6).
   • Choose the Set or Set/Modify item from the Break menu. When the Set/Modify Breakpoint dialog box displays, enter the source line in the Location: field.
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:
   • Press Ctrl/MB1 on the button to the left of the listed breakpoint.
   • Click on a breakpoint listed in the view, and choose the Set/Modify item in the Break menu.
3. Follow steps 3 and 4 of the previous procedure, as appropriate.

This section explains how to:

• Select variable names from windows
• Display the value of a variable
• Monitor a variable
• Watch a variable
• Change the value of a variable

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:

• To specify a scalar (nonaggregate) variable, such as an integer, real, Boolean, or enumeration type, select the variable's name.
• To specify an entire aggregate, such as array or structure (record), select the variable's name.
• To specify a single element of an aggregate variable, select the entity using the language syntax. For example:
  • The string arr2[7] specifies element 7 of array arr2 in the C language.
  • The string employee.address specifies component address of record (structure) employee in the Pascal language.
• To specify the object designated by a pointer variable, select the entity following the language syntax. For example, in the C language, the string *int_point specifies the object designated by pointer int_point.

Select character strings from windows as follows:

• In any window, to select a string delimited by blank spaces, use the standard DECwindows Motif word selection technique: position the pointer on that string and then double click MB1.
• In any window, to select an arbitrary character string, use the standard DECwindows Motif text-selection technique: position the pointer on the first character, press and hold MB1 while dragging the pointer over the string and then release MB1.
• In the debugger source display, you also have the option of using language-sensitive text selection. To select a string delimited by language-dependent identifier boundaries, position the pointer on that string and press
  Ctrl/MB1.
  For example, suppose the source display contains the character string arr2[m], then:
  • To select arr2, position the pointer on arr2 and press Ctrl/MB1.
  • To select m, position the pointer on m and press Ctrl/MB1.
  
  You can change the key sequence for language-sensitive text selection as explained in Section 10.10.4.2.

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

To change the current value of a variable:

• Find and select the variable name in a window as explained in Section 10.5.1.
• Choose Deposit... from the Commands menu in the main window. The Deposit dialog box appears with the name selected in the Variable field.
• Enter the new value in the Value field.
• Choose the default, hex, octal, decimal, or binary item from the Input Radix menu within the dialog box.
• Click on OK.

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

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:
   • Displays the monitor view (if it is not displayed)
   • Puts the selected variable's name, along with its qualifying path name, in the Monitor Expression column
   • Puts the value of the variable in the Value/Deposit column
   • Puts a cleared button in the Watched column (see Section 10.5.5).

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:

• Choose Change Radix in the Monitor menu to change the output radix for a selected monitored element.
• Choose the Change All Radix in the Monitor menu to change the output radix for all subsquently monitored elements.

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

Figure 10-13 Monitoring a 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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