HP OpenVMS Systems Documentation
OpenVMS Debugger Manual
16.5.1 Putting Tasks on Hold to Control Task Switching
Task switching might be confusing when you are debugging a program. Placing a task on hold with the SET TASK/HOLD command restricts the state transitions the task can make once the program is subsequently allowed to execute.
A task placed on hold can enter any state except the RUNNING state. If necessary, you can force it into the RUNNING state by using the SET TASK/ACTIVE command.
The SET TASK/HOLD/ALL command freezes the state of all tasks except the active task. You can use this command in combination with the SET TASK/ACTIVE command, to observe the behavior of one or more specified tasks in isolation, by executing the active task with the STEP or GO command, and then switching execution to another task with the SET TASK/ACTIVE command. For example:
When you no longer want to hold a task, use the SET TASK/NOHOLD command.
The following sections explain how to do the following functions:
16.6.1 Setting Task-Specific and Task-Independent Debugger Eventpoints
An eventpoint is an event that you can use to return control to the debugger. Breakpoints, tracepoints, watchpoints, and the completion of STEP commands are eventpoints.
A task-independent eventpoint can be triggered by the execution of any task in a program, regardless of which task is active when the eventpoint is set. Task-independent eventpoints are generally specified by an address expression such as a line number or a name. All watchpoints are task-independent eventpoints. The following are examples of setting task-independent eventpoints:
A task-specific eventpoint can be set only for the task that is active when the command is entered. A task-specific eventpoint is triggered only when that same task is active. For example, the STEP/LINE command is a task-specific eventpoint: other tasks might execute the same source line and not trigger the event.
If you use the SET BREAK, SET TRACE, or STEP commands with the following qualifiers, the resulting eventpoints are task specific:
Any other eventpoints that you set with those commands and any eventpoints that you set with the SET WATCH command are task independent. The following are examples of setting task-specific eventpoints:
You can conditionalize eventpoints that are normally task-independent to make them task specific. For example:
16.6.2 Setting Breakpoints on POSIX Threads Tasking Constructs
You can set a breakpoint on a thread start routine. The breakpoint will trigger just before the start routine begins execution. In Example 16-1, this type of breakpoint is set as follows:
Unlike Ada tasks, you cannot specify the body of a POSIX Threads task by name but the start routine is similar.
Specifying a WHEN clause with the SET BREAK command ensures that you catch the point at which a particular thread begins execution. For example:
In Example 16-1, this conditional breakpoint will trigger when the second worker thread begins executing its start routine.
Other useful places to set breakpoints are just prior to and
immediately after condition waits, joins, and locking of mutexes. You
can set such breakpoints by specifying either a line number or the
You can set a breakpoint on a task body by using one of the following syntaxes to refer to the task body (see Section 16.3.2):
For example, the following command sets a breakpoint on the body of task CHILD. This breakpoint is triggered just before the elaboration of the task's declarative part (also called the task's activation).
CHILD$TASK_BODY is a name for the address of the first instruction the task will execute. It is meaningful to set a breakpoint on an instruction, and hence on this name. However, you must not name the task object (for example, CHILD) in a SET BREAK command. The task-object name designates the address of a data item (the task value). Just as it is erroneous to set a breakpoint on an integer object, it is erroneous to set a breakpoint on a task object.
You can monitor the execution of communicating tasks by setting breakpoints or tracepoints on entry calls and accept statements.
There are several points in and around an accept statement where you might want to set a breakpoint or tracepoint. For example, consider the following program segment, which has two accept statements for the same entry, RENDEZVOUS:
You can set a breakpoint or tracepoint in the following places in this example:
To set a breakpoint or tracepoint in and around an accept statement, you can specify the associated line number. For example, the following command sets a breakpoint on the start and also on the body of the first accept statement in the preceding example:
To set a breakpoint or a tracepoint on an accept statement body, you can also use the entry name (specifying its expanded name to identify the task body where the entry is declared). For example:
If there is more than one accept statement for an entry, the debugger treats the entry as an overloaded name. The debugger issues a message indicating that the symbol is overloaded, and you must use the SHOW SYMBOL command to identify the overloaded names that have been assigned by the debugger. For example:
Overloaded names have an integer suffix preceded by two underscores. For more information on overloaded names, see the debugger's online help (type Help Language_Support Ada).
In the following example, when the breakpoint triggers, the caller task is evaluated (see Section 16.3.4 for information about the symbol %CALLER_TASK):
The following breakpoint triggers only when the caller task is %TASK 2:
16.6.4 Monitoring Task Events
The SET BREAK/EVENT and SET TRACE/EVENT commands enable you to set breakpoints and tracepoints that are triggered by task and exception events. For example, the following command sets tracepoints that trigger whenever task CHILD or %TASK 2 makes a transition to the RUN state:
When a breakpoint or tracepoint is triggered as a result of an event, the debugger identifies the event and gives additional information.
The following tables list the event-name keywords that you can specify with the SET BREAK/EVENT and SET TRACE/EVENT commands:
1 An unhandled exception is an exception for which there is no handler in the current frame or for which there is a handler that executes a raise statement and propagates the exception to an outer scope.
In the previous tables, the exception-related events are included for completeness. Only the task events are discussed in the following paragraphs. For more information about the exception events, see the debugger's online help (type Help Language_Support Ada).
You can abbreviate an event-name keyword to the minimum number of characters that make it unique.
The event-name keywords that you can specify with the SET BREAK/EVENT or SET TRACE/EVENT command depend on the current event facility, which is either THREADS or ADA in the case of task events. The appropriate event facility is set automatically when the program is brought under debugger control. The SHOW EVENT_FACILITY command identifies the facility that is currently set and lists the valid event name keywords for that facility (including those for the generic events).
Ada examples of the predefined and other types of event breakpoints follow.
When the EXCEPTION_TERMINATED event is triggered, it usually indicates an unanticipated program error. For example:
For Ada programs, the DEPENDENTS_EXCEPTION event often unexpectedly precedes a deadlock. For example:
For Ada programs, the RENDEZVOUS_EXCEPTION event enables you to see an exception before it leaves a rendezvous (before exception information has been lost due to copying the exception into the calling task). For example:
To cancel breakpoints (or tracepoints) set with the /EVENT qualifier, use the CANCEL BREAK/EVENT (or CANCEL TRACE/EVENT) command. Specify the event qualifier and optional task expression in the CANCEL command exactly as you did with the SET command, excluding any WHEN or DO clauses.
You might want to set event breakpoints and tracepoints in a debugger initialization file for your tasking programs. For example:
16.7 Additional Task-Debugging Topics
The following sections discuss additional topics related to task debugging:
16.7.1 Debugging Programs with Deadlock Conditions
A deadlock is an error condition in which each task in a group of tasks is suspended and no task in the group can resume execution until some other task in the group executes. Deadlock is a typical error in tasking programs (in much the same way that infinite loops are typical errors in programs that use WHILE statements).
A deadlock is easy to detect: it causes your program to appear to suspend, or hang, in midexecution. When deadlock occurs in a program that is running under debugger control, press Ctrl/C to interrupt the deadlock and display the debugger prompt.
In general, the SHOW TASK/ALL command (see Section 16.4) or the SHOW TASK/STATE=SUSPENDED command is useful because it shows which tasks are suspended in your program and why. The command SET TASK/VISIBLE %NEXT_TASK is particularly useful when debugging in screen mode. It enables you to cycle through all tasks and display the code that each task is executing, including the code in which execution is stopped.
The SHOW TASK/FULL command gives detailed task state information, including information about rendezvous, entry calls, and entry index values. The SET BREAK/EVENT or SET TRACE/EVENT command (see Section 16.6.4) enables you to set breakpoints or tracepoints at or near locations that might lead to deadlock. The SET TASK/PRIORITY and SET TASK/RESTORE commands enable you to see if a low-priority task that never runs is causing the deadlock.
Table 16-9 lists a number of tasking deadlock conditions and suggests debugger commands that are useful in diagnosing the cause.