Document revision date: 19 July 1999

To set a breakpoint or a tracepoint on a particular program location, specify an address expression with the SET BREAK or SET TRACE command.

Fundamentally, an address expression specifies a memory address or a register. Because the debugger understands the symbols associated with your program, the address expressions you typically use with the SET BREAK or SET TRACE command are routine names, labels, or source line numbers rather than memory addresses---the debugger converts these symbols to addresses.

3.3.1.1 Specifying Symbolic Addresses

The following examples show how to set a breakpoint on a routine (SWAP) and a tracepoint on a label (LOOP1):

DBG> SET BREAK SWAP DBG> SET TRACE LOOP1

The next command sets a breakpoint on the return instruction of routine SWAP. Breaking on the return instruction of a routine lets you inspect the local environment (for example, to obtain the values of local variables) while the routine is still active.

DBG> SET BREAK/RETURN SWAP

Some languages, for example Fortran, use numeric labels. To set a breakpoint or a tracepoint on a numeric label, you must precede the number with the built-in symbol %LABEL. Otherwise, the debugger interprets the number as a memory address. For example, the following command sets a tracepoint on label 20:

DBG> SET TRACE %LABEL 20

You can set a breakpoint or a tracepoint on a line of source code by specifying the line number preceded by the built-in symbol %LINE. The following command sets a breakpoint on line 14:

DBG> SET BREAK %LINE 14

The previous breakpoint causes execution to pause on the first instruction of line 14. You can set a breakpoint or a tracepoint only on lines for which the compiler generated instructions (lines that resulted in executable code). If you specify a line number that is not associated with an instruction, such as a comment line or a statement that declares but does not initialize a variable, the debugger issues a diagnostic message. For example:

DBG> SET BREAK %LINE 6 %DEBUG-I-LINEINFO, no line 6, previous line is 5, next line is 8 %DEBUG-E-NOSYMBOL, symbol '%LINE 6' is not in the symbol table DBG>

The previous messages indicate that the compiler did not generate instructions for lines 6 or 7 in this case.

Some languages allow more than one statement on a line. In such cases, you can use statement numbers to differentiate among statements on the same line. A statement number consists of a line number, followed by a period (.), and a number indicating the statement. The syntax is as follows:

%LINE line-number.statement-number

For example, the following command sets a tracepoint on the second statement of line 38:

DBG> SET TRACE %LINE 38.2

When searching for symbols that you reference in commands, the debugger uses the conventions described in Section 5.3.1. That is, it first looks within the module where execution is currently paused, then in other scopes associated with routines on the call stack, and so on. Therefore, to specify a symbol that is defined in more than one module, such as a line number, you might need to use a path name. For example, the following command sets a tracepoint on line 27 of module MOD4:

DBG> SET TRACE MOD4\%LINE 27

Remember the symbol lookup conventions when specifying a line number in debugger commands. If that line number is not defined in the module where execution is paused (because it is not associated with an instruction), the debugger uses the symbol lookup conventions to locate another module where the line number is defined.

When specifying address expressions, you can combine symbolic addresses with byte offsets. Thus, you can set a breakpoint or a tracepoint on a particular instruction by specifying its line number and the byte offset from the beginning of that line to the first byte of the instruction. For example, the next command sets a breakpoint on the address that is five bytes beyond the beginning of line 23:

DBG> SET BREAK %LINE 23+5

3.3.1.2 Specifying Locations in Memory

To set a breakpoint or a tracepoint on a location in memory, specify its numerical address in the currently set radix. The default radix for both data entry and display is decimal for most languages.

On VAX processors, the exceptions are BLISS and MACRO--32, which have a default radix of hexadecimal.

On Alpha processors, the exceptions are BLISS, MACRO--32, and MACRO--64, which have a default radix of hexadecimal.

For example, the following command sets a breakpoint at address 2753, decimal, or at address 2753, hexadecimal:

DBG> SET BREAK 2753

You can specify a radix when you enter an individual integer literal (such as 2753) by using one of the built-in symbols %BIN, %OCT, %DEC, or %HEX. For example, in the following command line the symbol %HEX specifies that 2753 should be treated as a hexadecimal integer:

DBG> SET BREAK %HEX 2753

Note that when specifying a hexadecimal number that starts with a letter rather than a number, you must add a leading 0. Otherwise, the debugger tries to interpret the entity specified as a symbol declared in your program.

For additional information about specifying radixes and about the built-in symbols %BIN, %DEC, %HEX, and %OCT, see Section 4.1.10 and Appendix B.

If a breakpoint or a tracepoint was set on a numerical address that corresponds to a symbol in your program, the SHOW BREAK or SHOW TRACE command identifies the breakpoint symbolically.

3.3.1.3 Obtaining and Symbolizing Memory Addresses

Use the EVALUATE/ADDRESS command to determine the memory address associated with a symbolic address expression, such as a line number, routine name, or label. For example:

DBG> EVALUATE/ADDRESS SWAP 1536 DBG> EVALUATE/ADDRESS %LINE 26 1629 DBG>

The address is displayed in the current radix. You can specify a radix qualifier to display the address in another radix. For example:

DBG> EVALUATE/ADDRESS/HEX %LINE 26 0000065D DBG>

The SYMBOLIZE command does the reverse of EVALUATE/ADDRESS. It converts a memory address into its symbolic representation (including its path name) if such a representation is possible. Chapter 5 explains how to control symbolization. See Section 4.1.11 for more information about obtaining and symbolizing addresses.

3.3.2 Setting Breakpoints or Tracepoints on Lines or Instructions

The following SET BREAK and SET TRACE command qualifiers cause the debugger to break on or trace every source line or every instruction of a particular class:

• /LINE
• /BRANCH
• /CALL
• /INSTRUCTION
• /INSTRUCTION=(opcode[,...]) (VAX only)
• /VECTOR_INSTRUCTION (VAX only)

When using these qualifiers, do not specify an address expression.

For example, the following command causes the debugger to break on the beginning of every source line encountered during execution:

DBG> SET BREAK/LINE

The instruction-related qualifiers are especially useful for opcode tracing, which is the tracing of all instructions or the tracing of a class of instructions. The next command causes the debugger to trace every branch instruction encountered (for example BEQL, BGTR, and so on):

DBG> SET TRACE/BRANCH

Note that opcode tracing slows program execution.

By default, when you use the qualifiers discussed in this section, the debugger breaks or traces within all called routines as well as within the currently executing routine (this is equivalent to specifying SET BREAK/INTO or SET TRACE/INTO). By specifying SET BREAK/OVER or SET TRACE/OVER, you can suppress break or trace action within all called routines. Or, you can use the /[NO]JSB, /[NO]SHARE, or /[NO]SYSTEM qualifiers to specify the kinds of called routines where break or trace action is to be suppressed. For example, the next command causes the debugger to break on every line except within called routines that are in shareable images or system space:

DBG> SET BREAK/LINE/NOSHARE/NOSYSTEM

3.3.3 Setting Breakpoints on Emulated Instructions (Alpha Only)

On Alpha systems, to cause the debugger to suspend program execution when an instruction is emulated, use the command SET BREAK/SYSEMULATE. The syntax of the SET BREAK command when using the /SYSEMULATE qualifier is:

SET BREAK/SYSEMULATE[=mask]

The optional argument mask is a quadword with bits set to specify which instruction groups shall trigger breakpoints. The only emulated instruction group currently defined consists of the BYTE and WORD instructions. Specify this instruction group by setting bit 0 of mask to 1.

If you do not specify mask, or if mask = FFFFFFFFFFFFFFFF, the debugger stops program execution whenever the operating system emulates any instruction.

3.3.4 Controlling Debugger Action at Breakpoints or Tracepoints

The SET BREAK and SET TRACE commands provide several options for controlling the behavior of the debugger at breakpoints and tracepoints---the /AFTER, /[NO]SILENT, /[NO]SOURCE, and /TEMPORARY command qualifiers, and the optional WHEN and DO clauses. The following examples show several of these options.

The following command sets a breakpoint on line 14 and specifies that the breakpoint take effect after the fifth time that line 14 is executed:

DBG> SET BREAK/AFTER:5 %LINE 14

The following command sets a tracepoint that is triggered at every line of execution. The DO clause obtains the value of the variable X when each line is executed:

DBG> SET TRACE/LINE DO (EXAMINE X)

The following example shows how you capture the WHEN and DO clauses together. The command sets a breakpoint at line 27. The breakpoint is triggered (execution is paused) only when the value of SUM is greater than 100 (not each time line 27 is executed). The DO clause causes the value of TEST_RESULT to be examined whenever the breakpoint is triggered---that is, whenever the value of SUM is greater than 100. If the value of SUM is not greater than 100 when execution reaches line 27, the breakpoint is not triggered and the DO clause is not executed.

DBG> SET BREAK %LINE 27 WHEN (SUM > 100) DO (EXAMINE TEST_RESULT)

See Section 4.1.6 and Section 14.3.2.2 for information about evaluating language expressions like SUM > 100.

The /SILENT qualifier suppresses the break or trace message and source code display. This is useful when, for example, you want to use the SET TRACE command only to execute a debugger command at the tracepoint. In the following example, the SET TRACE command is used to examine the value of the Boolean variable STATUS at the tracepoint:

DBG> SET TRACE/SILENT %LINE 83 DO (EXAMINE STATUS) DBG> GO . . . SCREEN_IO\CLEAR\STATUS: OFF . . .

In the next example, the SET TRACE command is used to count the number of times line 12 is executed. The first DEFINE/VALUE command defines a symbol COUNT and initializes its value to 0. The DO clause of the SET TRACE command causes the value of COUNT to be incremented and evaluated whenever the tracepoint is triggered (whenever execution reaches line 12).

DBG> DEFINE/VALUE COUNT=0 DBG> SET TRACE/SILENT %LINE 12 DO (DEF/VAL COUNT=COUNT+1;EVAL COUNT)

Source lines are displayed by default at breakpoints, tracepoints, and watchpoints if they are available for the module being debugged. You can also control their display with the SET STEP [NO]SOURCE command and the /[NO]SOURCE qualifier of the SET BREAK, SET TRACE, and SET WATCH commands. See Chapter 6 for information about how to control the display of source code in general and in particular at breakpoints, tracepoints, and watchpoints.

3.3.5 Setting Breakpoints or Tracepoints on Exceptions

The SET BREAK/EXCEPTION and SET TRACE/EXCEPTION commands direct the debugger to treat any exception generated by your program as a breakpoint or tracepoint, respectively. The breakpoint or tracepoint occurs before any application-declared exception handler is invoked. See Section 14.5 for debugging techniques associated with exceptions and condition handlers.

3.3.6 Setting Breakpoints or Tracepoints on Events

The SET BREAK and SET TRACE commands each have an /EVENT=event-name qualifier. You can use this qualifier to set breakpoints or tracepoints that are triggered by various events (denoted by event-name keywords). Events and their keywords are currently defined for the following event facilities:

• ADA event facility, which defines DEC Ada tasking events. Ada events are defined in Section 17.6.4.
• THREADS event facility, which defines tasking (multithread) events for programs written in any language that uses DECthreads services. Threads events are defined in Section 17.6.4.
• SCAN event facility (VAX only), which defines SCAN pattern-matching events. SCAN events are defined in the debugger's online help (type HELP Language SCAN).

The appropriate facility and event-name keywords are defined when the program is brought under debugger control. Use the SHOW EVENT_FACILITY command to identify the current event facility and the associated event-name keywords. The SET EVENT_FACILITY command enables you to change the event facility and change your debugging context. This is useful if you have a multilanguage program and want to debug a routine that is associated with an event facility but that facility is not currently set.

The following example shows how to set a SCAN event breakpoint. It causes the debugger to break whenever a SCAN token is built, for any value:

DBG> SET BREAK/EVENT=TOKEN

When a breakpoint or tracepoint is triggered, the debugger identifies the event that caused it to be triggered and gives additional information.

3.3.7 Deactivating, Activating, and Canceling Breakpoints or Tracepoints

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

To deactivate a breakpoint or tracepoint, enter the DEACTIVATE BREAK or DEACTIVATE TRACE command. This causes the debugger to ignore the breakpoint or tracepoint during program execution. However, you can activate it at a later time, for example, when you rerun the program (see Section 1.3.3). A deactivated breakpoint or tracepoint is listed as such in a SHOW BREAK display.

To activate a breakpoint or tracepoint, use the ACTIVATE BREAK or ACTIVATE TRACE command. Activating a breakpoint or tracepoint causes it to take effect during program execution.

The commands DEACTIVATE BREAK/ALL and ACTIVATE BREAK/ALL (or DEACTIVATE TRACE/ALL and ACTIVATE TRACE/ALL) operate on all breakpoints or tracepoints and are particularly useful when rerunning a program with the RERUN command.

To cancel a breakpoint or tracepoint, use the CANCEL BREAK or CANCEL TRACE command. A canceled breakpoint or tracepoint is no longer listed in a SHOW BREAK or SHOW TRACE display.

When using any of these commands, specify the address expression and qualifiers (if any) exactly as you did when setting the breakpoint or tracepoint. For example:

DBG> DEACTIVATE TRACE/LINE DBG> CANCEL BREAK SWAP,MOD2\LOOP4,2753

3.4 Monitoring Changes in Variables and Other Program Locations

The SET WATCH command enables you to specify program variables (or arbitrary memory locations) that the debugger monitors as your program executes. This process is called setting watchpoints. If, during execution, the program modifies the value of a watched variable (or memory location), the watchpoint is triggered. The debugger then suspends execution, displays information, and prompts for more commands. The debugger monitors watchpoints continuously during program execution.

This section describes the general use of the SET WATCH command. Section 3.4.3 gives additional information about setting watchpoints on nonstatic variables---variables that are allocated on the call stack or in registers.

The syntax of the SET WATCH command is as follows:

SET WATCH[/qualifier[...]] address-expression[, ...] [WHEN (conditional-expression)] [DO (command[; ...])]

You can specify any valid address expression, but usually you specify the name of a variable. The following example shows a typical use of the SET WATCH command and shows the general default behavior of the debugger at a watchpoint:

DBG> SET WATCH COUNT DBG> GO . . . watch of MOD2\COUNT at MOD2\%LINE 24 24: COUNT := COUNT + 1; old value: 27 new value: 28 break at MOD2\%LINE 25 25: END; DBG>

In this example, the SET WATCH command sets a watchpoint on the variable COUNT, and the GO command starts execution. When the program changes the value of COUNT, execution is paused. The debugger then does the following:

• Announces the event ("watch of MOD2\COUNT..."), identifying the location of the instruction that changed the value of the watched variable ("...at MOD2\%LINE 24")
• Displays the associated source line (24)
• Displays the old and new values of the variable (27 and 28)
• Announces that execution is paused at the beginning of the next line ("break at MOD2\%LINE 25") and displays that source line
• Prompts for another command

When the address of the instruction that modified a watched variable is not at the beginning of a source line, the debugger denotes the instruction's location by displaying the line number plus the byte offset from the beginning of the line. For example:

DBG> SET WATCH K DBG> GO . . . watch of TEST\K at TEST\%LINE 19+5 19: DO 40 K = 1, J old value: 4 new value: 5 break at TEST\%LINE 19+9 19: DO 40 K = 1, J DBG>

In this example, the address of the instruction that modified variable K is 5 bytes beyond the beginning of line 19. The breakpoint is on the instruction that follows the instruction that modified the variable (not on the beginning of the next source line as in the preceding example).

You can set watchpoints on aggregates (that is, entire arrays or records). A watchpoint set on an array or record triggers if any element of the array or record changes. Thus, you do not need to set watchpoints on individual array elements or record components. However, you cannot set an aggregate watchpoint on a variant record. In the following example, the watchpoint is triggered because element 3 of array ARR was modified:

DBG> SET WATCH ARR DBG> GO . . . watch of SUBR\ARR at SUBR\%LINE 12 12: ARR(3) := 28 old value: (1): 7 (2): 12 (3): 3 (4): 0 new value: (1): 7 (2): 12 (3): 28 (4): 0 break at SUBR\%LINE 13 DBG>

You can also set a watchpoint on a record component, on an individual array element, or on an array slice (a range of array elements). A watchpoint set on an array slice triggers if any element within that slice changes. When setting the watchpoint, use the syntax of the current language. For example, the following command sets a watchpoint on element 7 of array CHECK using Pascal syntax:

DBG> SET WATCH CHECK[7]

To identify all of the watchpoints that are currently set, use the SHOW WATCH command.

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