HP OpenVMS Systems Documentation

This chapter explains how to:

• Display the source code of your program ( Section 10.1)
• Edit your program under debugger control ( Section 10.2)
• Execute your program under debugger control ( Section 10.3)
• Suspend execution with breakpoints ( Section 10.4)
• Examine and manipulate program variables ( Section 10.5)
• Access program variables ( Section 10.6)
• Display and modify values stored in registers ( Section 10.7)
• Display the decoded instruction stream of your program ( Section 10.8)
• Debug tasking programs ( Section 10.9)
• Customize the debugger's HP DECwindows Motif for OpenVMS user interface ( Section 10.10)

The chapter describes window actions and window menu choices, but you can perform most common debugger operations by choosing items from context-sensitive pop-up menus. To access these menus, click MB3 while the mouse pointer is in the window area.

You can also enter commands at the HP DECwindows Motif for OpenVMS command prompt. For information about entering debugger commands, see Section 8.3.

For the source code of programs EIGHTQUEENS.EXE and 8QUEENS.EXE, shown in the figures of this chapter, see Appendix D.

10.1 Displaying the Source Code of Your Program

The debugger displays the source code of your program in the main window (see Figure 10-1).

Figure 10-1 Source Display

Whenever execution is suspended (for example, at a breakpoint), the debugger updates the source display by displaying the code surrounding the point at which execution is paused. The current-location pointer, to the left of the source code, marks which line of code will execute next. (A source line corresponds to one or more programming-language statements, depending on the language and coding style.)

By default, the debugger displays compiler-generated line numbers to the left of the source code. These numbers help identify breakpoints, which are listed in the breakpoint view (see Section 10.4.4). You can choose not to display line numbers so that more of the source code can show in the window. To hide or display line numbers, toggle Display Line Numbers from the File menu on the main window.

The Call Stack menu, between the source view and the push button view, shows the name of the routine whose source code is displayed.

The current-location pointer is normally filled in as shown in Figure 10-1. It is cleared if the displayed code is not that of the routine in which execution is paused (see Section 10.1.3 and Section 10.6.2).

You can use the scroll bars to show more of the source code. However, you can scroll vertically through only one module of your program at a time. (A module corresponds generally to a compilation unit. With many programming languages, a module corresponds to the contents of a source file. With some languages, such as Ada, a source file might contain one or more modules.)

The following sections explain how to display source code for other parts of your program so that you can set breakpoints in various modules, and so on. Section 10.1.3 explains what to do if the debugger cannot find source code for display. Section 10.6.2 explains how to display the source code associated with routines that are currently active on the call stack.

After navigating the main window, you can redisplay the location at which execution is paused by clicking on the Call Stack menu.

If your program was optimized during compilation, the source code displayed might not reflect the actual contents of some program locations (see Section 1.2).

10.1.1 Displaying the Source Code of Another Routine

To display source code of another 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 your 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 module containing the routine of interest. The names of the routines in that module are displayed (indented) under the module name, and the Display Source button is now highlighted.
4. Click on the name of the routine whose source code you want to display.
5. Click on the Display Source push button. The debugger displays in the source view the source code of the target routine, along with an empty breakpoint button to the left of the source code. If the instruction view is open, this display is updated to show the machine code of the target routine.

Section 10.6.2 describes an alternative way to display routine source code for routines currently active on the call stack.

Figure 10-2 Displaying Source Code of Another Routine

10.1.2 Displaying the Source Code of Another Module

1. Choose Browse Sources from the File menu on the main window.
   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 names of the shareable images are dimmed if no symbolic information is available for them.
2. Double click on the name of your executable image. The names of the modules in that image are displayed (indented) under the image name.
3. Click on the name of the module whose source code you want to display. The Display Source button is now highlighted.
4. Click on Display Source. The source display in the main window now shows the routine's source code. (If the instruction display in the instruction view is open, this display is updated to show the routine's instruction code.)

In certain cases, the debugger cannot display source code. Possible causes are:

• Execution might be paused within a module of your program that was compiled or linked without the debug option (see Section 1.2).
• Execution might be paused within a system or library routine for which no symbolic information is intended to be available. In such cases you can quickly return execution to the calling routine by clicking one or more times on the S/ret button in the push button view (see Section 10.3.5).
• The source file might have been moved to a different directory after it was compiled. Section 10.1.4 explains how to tell the debugger where to look for source files.

If the debugger cannot find source code for display, it tries to display the source code for the next routine down on the call stack for which source code is available. If the debugger can display source code for such a routine, the current-location pointer is moved to point to the source line to which execution returns in the calling routine.

10.1.4 Specifying the Location of Source Files

Information about the characteristics and the location of source files is embedded in the debug symbol table of your program. If a source file has been moved to a different directory since compile time, the debugger might not find the file. To direct the debugger to your source files, use the SET SOURCE command at the DBG> prompt (see Section 6.2).

10.2 Editing Your Program

The debugger provides a simple text editor you can use to edit your source files while debugging your program (see Figure 10-3).

The text editor available through the debugger's HP DECwindows Motif for OpenVMS menu interface is a simple convenience feature, not intended to replace sophisticated text editors such as the Language-Sensitive Editor (LSE). You cannot substitute a more sophisticated editor for the text editor invoked with the Edit File item in the Commands menu. To use a different editor, enter the EDIT command at the DBG> prompt in the command view (see EDIT in the Command Reference Dictionary of this manual).

Figure 10-3 Editor Window

To invoke the editor, choose the Edit File item in the Commands menu on the main window. By default, the editor opens a buffer and displays the module currently displayed in the source view. The buffer is named with the file specification of the file in the buffer. If no file is displayed in the source view, the editor displays an empty text buffer, called main_buffer. The buffer name appears in the buffer menu, which is just under the menu bar of the editor view.

The editor allows you to create any number of text buffers by choosing New (for empty text buffers) or Open (for existing files) from the File menu. The name of each text buffer appears in the buffer menu. You can cut, copy, and paste text from buffer to buffer by choosing items from the Edit menu and selecting buffers from the buffer menu.

You can perform forward and backward search and replace operations by entering strings in the Find and Replace with fields and clicking on a directional arrow. You can perform a repeated search for the string by continuing to press the Return key. You can also continue a search by choosing the Find/Replace Next or Find/Replace Previous items in the Edit menu.

To save the file, choose the Save or Save As... items from the File menu. If you do not save your corrections before closing a modified buffer or exiting the debugger, the debugger displays a warning message.

To test any changes to the source code:

1. Select a DECterm window separate from that in which the debugger is running.
2. Recompile the program.
3. Relink the program.
4. Return to the debugging session.
5. Choose the Run Image... item in the File menu on the main window.

This section explains how to:

• Determine where execution is currently paused within your program
• Start or resume program execution
• Execute the program one source line at a time, step by step

For information about rerunning your program or running another program from the current debugging session, see Section 9.3 and Section 9.4.

10.3.1 Determining Where Execution Is Currently Paused

To determine where execution is currently paused within your program:

1. If the current-location pointer is not visible in the main window, click on the Call Stack menu of that window to display the pointer (see Figure 10-1).
2. Look at the current-location pointer:
   • If the pointer is filled in, it marks the source line whose code will execute next (see Section 10.1). The Call Stack menu always shows the routine at scope level 0 (where execution is paused) when the pointer is filled in.
   • If the pointer is cleared, the source code displayed is that of a calling routine, and the pointer marks the source line to which execution returns in that routine:
     • If the Call Stack menu shows level 0, source code is not available for display for the routine in which execution is paused (see Section 10.1.3).
     • If the Call Stack menu shows a level other than 0, you are displaying the source code for a calling routine (see Section 10.6.2).

To list the sequence of routine calls that are currently active on the call stack, click on the Call Stack menu. Level 0 denotes the routine in which execution is paused, level 1 denotes the calling routine, and so on.

10.3.2 Starting or Resuming Program Execution

To start program execution or resume execution from the current location, click on the Go button in the push button view (see Figure 8-3).

Letting your program run freely without debugger intervention is useful in situations such as the following:

• To test for an infinite loop. In this case, you start execution; then, if your program does not terminate and you suspect that it is looping, click on the Stop button. The main window will show where you interrupted program execution, and the Call Stack menu will identify the sequence of routine calls at that point (see Section 10.3.1).
• To execute your program directly to a particular location. In this case, you first set a breakpoint at the location (see Section 10.4) and then start execution.

Once started, program execution continues until one of the following events occurs:

• The program completes execution.
• A breakpoint is reached (including a conditional breakpoint whose condition is true).
• A watchpoint is triggered.
• An exception is signaled.
• You click on the Stop button on the push button view.

Whenever the debugger suspends execution of the program, the main window display is updated and the current-location pointer marks which line of code will execute next.

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:

• 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 HP DECwindows Motif for OpenVMS user 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 to 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 lists all the breakpoints that are currently set, including specifications for conditional breakpoints, and commands to be executed at action breakpoints.