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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).

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 .

10.5.4.2 Monitoring a Pointer (Access) Variable

If you select the name of a pointer (access) variable and click on the MON button, the debugger displays the address of the referenced object in the
Value/Deposit column of the monitor view (see the top entry in Figure 10-15).

To monitor the value of the referenced object (to dereference the pointer variable), double click on the pointer name in the Monitor Expression column. This adds an entry for the referenced object in the monitor view, indented under the pointer entry (see the bottom entry in Figure 10-15). If a referenced object is an aggregate, you can double click on its name to display its elements, and so on.

Figure 10-15 Pointer Variable and Referenced Object in Monitor View

Whenever the program changes the value of a watched variable, the debugger suspends execution and displays the old and new values in the command view.

To watch a variable (also known as setting a watchpoint on a variable):

• Monitor the variable as explained in Section 10.5.4. The debugger puts a button in the Watched column of the monitor view whenever you monitor a variable. See Figure 10-16.
• Click on the button in the Watched column. A filled-in button indicates that the watchpoint is set.

Figure 10-16 Watched Variable in Monitor View

To deactivate a watchpoint, clear its Watched button in the monitor view (by clicking on the button) or choose Toggle Watchpoint in the Monitor menu. To activate a watchpoint, fill in its Watched button or choose Toggle Watchpoint in the Monitor menu.

Section 10.6.1 explains static and nonstatic (automatic) variables and how to access them. The debugger deactivates a nonstatic watchpoint when execution moves out of (returns from) the variable's defining routine. When a nonstatic variable is no longer active, its entry is dimmed in the monitor view and its Watched button is cleared.

The debugger does not automatically reactivate nonstatic watchpoints if execution later returns to the variable's defining routine. You must reactivate nonstatic watchpoints explicitly.

10.5.6 Changing the Value of a Monitored Scalar Variable

To change the value of a scalar (nonaggregate) variable, such as an integer or Boolean type (see Figure 10-17):

1. Monitor the variable as explained in Section 10.5.4.
2. Click on the variable's value in the Value/Deposit column of the monitor view. A small dialog box is displayed over that value, which you can now edit.
3. Enter the new value in the dialog box.
4. Click on the check mark (OK) in the dialog box. The dialog box is removed and replaced by the new value, indicating that the variable now has that value. The debugger notifies you if you try to enter a value that is incompatible with the variable's type, range, and so on.

Figure 10-17 Changing the Value of a Monitored Scalar Variable

To cancel a text entry and dismiss the dialog box, click on X (Cancel).

You can change the value of only one component of an aggregate variable (such as an array or structure) at a time. To change the value of an aggregate-variable component (see Figure 10-18):

1. Display the value of the component as explained in Section 10.5.4.1.
2. Click on the variable's value in the Value/Deposit column of the monitor view. A small dialog box is displayed over that value, which you can now edit.
3. Enter the new value in the dialog box.
4. Click on the check mark (OK) in the dialog box. The dialog box is removed and replaced by the new value, indicating that the variable now has that value. The debugger notifies you if you try to enter a value that is incompatible with the variable's type, range, and so on.

Figure 10-18 Changing the Value of a Component of an Aggregate Variable

This section provides some general information about accessing program variables while debugging.

If your program was optimized during compilation, you might not have access to certain variables while debugging. When you compile a program for debugging, it is best to disable optimization, if possible (see Section 1.2.1).

Before you check on the value of a variable, always execute the program beyond the point where the variable is declared and initialized. The value contained in any uninitialized variable should be considered invalid.

10.6.1 Accessing Static and Nonstatic (Automatic) Variables

A static variable is associated with the same memory address throughout execution of the program. You can always access a static variable.

A nonstatic variable is allocated on the stack or in a register and has a value only when its defining routine or block is active (on the call stack). Therefore, you can access a nonstatic variable only when program execution is paused within the scope of its defining routine or block (which includes any routine called by the defining routine).

A common technique for accessing a nonstatic variable is first to set a breakpoint on the defining routine and then to execute the program to the breakpoint.

Whenever the execution of your program makes a nonstatic variable inaccessible, the debugger notifies you as follows:

• If you try to display the value of the variable or monitor the variable (as explained in Section 10.5.2 and Section 10.5.4, respectively), the debugger issues a message that the variable is not active or not in scope.
• If the variable (or an expression that includes the variable) is currently being monitored, its entry becomes dimmed in the monitor view. When the entry is dimmed, the debugger does not check or update the variable's displayed value; also, you cannot change that value as explained in Section 10.5.3. The entry is fully displayed whenever the variable becomes accessible again.
• If the variable is currently being watched (as explained in Section 10.5.5), the watchpoint is deactivated (its Watched button is cleared) and its entry is dimmed in the monitor view. However, note that the watchpoint is not reactivated automatically when the variable becomes accessible again.

While debugging a routine in your program, you can set the current scope to a calling routine (a routine down the stack from the routine in which execution is currently paused). This enables you to:

• Determine where the current routine call originated
• Determine the value of a variable declared in a calling routine
• Determine the value of a variable during a particular invocation of a routine that is called recursively
• Change the value of a variable in the context of a routine call

The Call Stack menu on the main window lists the names of the routines (and, under certain conditions, the images and modules) of your program that are currently active on the stack, up to the maximum number of lines that can be displayed on your screen (see Figure 10-19). The numbers on the left side of the menu indicate the level of each routine on the stack relative to level 0, which denotes the routine in which execution is paused.

To set the current scope to a particular routine on the stack, choose the routine's name from the Call Stack menu (see Figure 10-19). This causes the following to occur:

• The Call Stack menu, when released, shows the name and relative level of the routine that is now the current scope.
• The main window shows that routine's source code.
• The instruction view (if displayed) shows that routine's decoded instructions.
• The register view (if displayed) shows the register values associated with that routine call.
• If the scope is set to a calling routine (a call-stack level other than 0), the debugger clears the current-location pointer, as shown in Figure 10-19.
• The debugger sets the scope for symbol searches to the chosen routine, so that you can examine variables, and so on, in the context of that scope.

Figure 10-19 Current Scope Set to a Calling Routine

When you set the scope to a calling routine, the current-location pointer (which is cleared) marks the source line to which execution will return in that routine. Depending on the source language and coding style used, this might be the line that contains the call statement or some subsequent line.

10.6.3 How the Debugger Searches for Variables and Other Symbols

Symbol ambiguities can occur when a symbol (for example, a variable name X) is defined in more than one routine or other program unit.

In most cases, the debugger automatically resolves symbol ambiguities. First, it uses the scope and visibility rules of the currently set language. In addition, because the debugger permits you to specify symbols in arbitrary modules (to set breakpoints and so on), the debugger uses the ordering of routine calls on the call stack to resolve symbol ambiguities.

In some cases, however, the debugger might respond as follows when you specify a symbol that is defined multiple times:

• It might issue a "symbol not unique" message because it is not able to determine the particular declaration of the symbol that you intended.
• It might reference the symbol declaration that is visible in the current scope, not the one you want.

To resolve such problems, you must specify a scope where the debugger should search for the particular declaration of the symbol:

• If the different declarations of the symbol are within routines that are currently active on the call stack, use the Call Stack menu on the main window to reset the current scope (see Section 10.6.2).
• Otherwise, enter the appropriate command at the command prompt (EXAMINE or MONITOR, for example), specifying a path name prefix with the symbol. For example, if the variable X is defined in two modules named COUNTER and SWAP, the following command uses the path name SWAP\X to specify the declaration of X that is in module SWAP:

  DBG> EXAMINE SWAP\X

The register view displays the current contents of all machine registers (see Figure 10-20).

To display the register view, choose Views... from the Options menu on the main window or the optional views window, then click on Registers when the Views dialog box appears.

By default, the register view automatically displays the register values associated with the routine in which execution is currently paused. Any values that change as your program executes are highlighted whenever the debugger regains control from your program.

To display the register values associated with any routine on the call stack, choose its name from the Call Stack menu on the main window (see Section 10.6.2).

To change the value stored in a register:

1. Click on the register value in the register view. A small dialog box is displayed over the current value, which you can now edit.
2. Enter the new value in the dialog box.
3. Click on the check mark (OK) in the dialog box. The debugger removes the dialog box and displays the new value, indicating that the register now contains that value. To dismiss the dialog box without changing the value in the register, click on X (Cancel).

To change the radix used to display register values:

• Choose Change Radix in the Register menu to change the radix in current and subsequent output for a selected register.
• Choose Change All Radix in the Register menu to change the radix in current and subsequent output for all registers.

Figure 10-20 Register View

The instruction view displays the decoded instruction stream of your program: the code that is actually executing (see Figure 10-21). This is useful if the program you are debugging has been optimized by the compiler so that the information in the main window does not exactly reflect the code that is executing (see Section 1.2).

Figure 10-21 Instruction View

To display the instruction view, choose Views... from the Options menu on the main window or the optional views window, then click on Instructions when the Views dialog box appears.

By default, the instruction view automatically displays the decoded instruction stream of the routine in which execution is currently paused. The current-location pointer, to the left of the instructions, marks the instruction that will execute next.

By default, the debugger displays source code line numbers to the left of the instructions with which they are associated. To hide or display line numbers, toggle Display Line Numbers from the File menu in the instruction view.

By default, the debugger displays memory addresses to the left of the instructions. To hide or display addresses, toggle Show Instruction Addresses from the File menu in the instruction view.

After navigating the instruction view, click on the Call Stack menu to redisplay the location at which execution is paused.

To display the instruction stream of any routine on the call stack, choose the routine's name from the Call Stack menu on the main window (see Section 10.6.2).

10.9 Debugging Tasking (Multithread) Programs

Tasking programs, also called multithreaded programs, have multiple threads of execution within a process and include the following:

• Programs in any language that use HP POSIX Threads Library or POSIX 1003.1b services.
• Programs that use language-specific tasking services (services provided directly by the language). Currently, Ada is the only language with built-in tasking services that the debugger supports.

Within the debugger, the term task or thread denotes such a flow of control, regardless of the language or implementation. The debugger's tasking support applies to all such programs.

The debugger enables you to display task information and modify task characteristics to control task execution, priority, state transitions, and so on.

The following sections summarize the tasking features of the debugger's HP DECwindows Motif for OpenVMS user interface. For more information about the debugger's tasking support, see Chapter 16.

10.9.1 Displaying Information About Tasks (Threads)

To display information about one or more tasks (threads) of your program, choose Views... from the Options menu on the main window or the optional views window, then click on Threads when the Views dialog box appears.

The Threads view gives information about all currently existing (nonterminated) tasks of your program. The information is updated whenever the debugger regains control from the program, as shown in Figure 10-22.

Figure 10-22 Thread View

The displayed information includes:

• The thread ID. The arrow in the left column marks the active task; i.e., the thread that runs when you click on the Go or STEP button.
• The thread priority.
• Whether the task (thread) has been put on hold as explained in Section 10.9.2.
• The current state of the task (thread). The task in the RUN (running) state is the active task.
• The current substate of the task (thread). The substate helps indicate the possible cause of a task's state.
• A debugger path name for the task (thread) object or the address of the task object if the debugger cannot symbolize the task object.

10.9.2 Changing Task (Threads) Characteristics