Updated: 11 December 1998

OpenVMS Debugger Manual

A task is an entity that executes in parallel with other tasks. A task is characterized by a unique task ID (see Section 17.3.3), a separate stack, and a separate register set.

The current definition of the active task and the visible task determine the context for manipulating tasks. See Section 17.3.1.

When specifying tasks in debugger commands, you can use any of the following forms:

• A task (thread) name as declared in the program (for example, FATHER in Section 17.2.2) or a language expression that yields a task value. Section 17.3.2 describes Ada language expressions for tasks.
• A task ID (for example, %TASK 2). See Section 17.3.3.
• A task built-in symbol (for example, %ACTIVE_TASK). See Section 17.3.4.

The active task is the task that runs when a STEP, GO, CALL, or EXIT command executes. Initially, it is the task in which execution is suspended when the program is brought under debugger control. To change the active task during a debugging session, use the SET TASK/ACTIVE command.

The following command makes the task named CHILD the active task:

DBG> SET TASK/ACTIVE CHILD

The visible task is the task whose stack and register set are the current context that the debugger uses when looking up symbols, register values, routine calls, breakpoints, and so on. For example, the following command displays the value of the variable KEEP_COUNT in the context of the visible task:

DBG> EXAMINE KEEP_COUNT

Initially, the visible task is the active task. To change the visible task, use the SET TASK/VISIBLE command. This enables you to look at the state of other tasks without affecting the active task.

You can specify the active and visible tasks in debugger commands by using the built-in symbols %ACTIVE_TASK and %VISIBLE_TASK, respectively (see Section 17.3.4).

See Section 17.5 for more information about using the SET TASK command to modify task characteristics.

17.3.2 Ada Tasking Syntax

You declare a task either by declaring a single task or by declaring an object of a task type. For example:

-- TASK TYPE declaration. -- task type FATHER_TYPE is ... end FATHER_TYPE; task body FATHER_TYPE is ... end FATHER_TYPE; -- A single task. -- task MOTHER is ... end MOTHER; task body MOTHER is ... end MOTHER;

A task object is a data item that contains a task value. A task object is created when the program elaborates a single task or task object, when you declare a record or array containing a task component, or when a task allocator is evaluated. For example:

-- Task object declaration. -- FATHER : FATHER_TYPE; -- Task object (T) as a component of a record. -- type SOME_RECORD_TYPE is record A, B: INTEGER; T : FATHER_TYPE; end record; HAS_TASK : SOME_RECORD_TYPE; -- Task object (POINTER1) via allocator. -- type A is access FATHER_TYPE; POINTER1 : A := new FATHER_TYPE;

A task object is comparable to any other object. You refer to a task object in debugger commands either by name or by path name. For example:

DBG> EXAMINE FATHER DBG> EXAMINE FATHER_TYPE$TASK_BODY.CHILD

When a task object is elaborated, a task is created by the DEC Ada Run-Time Library, and the task object is assigned its task value. As with other Ada objects, the value of a task object is undefined before the object is initialized, and the results of using an uninitialized value are unpredictable.

The task body of a task type or single task is implemented in DEC Ada as a procedure. This procedure is called by the DEC Ada Run-Time Library when a task of that type is activated. A task body is treated by the debugger as a normal Ada procedure, except that it has a specially constructed name.

To specify the task body in a debugger command, use the following syntax to refer to tasks declared as task types:

task-type-identifier$TASK_BODY

Use the following syntax to refer to single tasks:

task-identifier$TASK_BODY

For example:

DBG> SET BREAK FATHER_TYPE$TASK_BODY

The debugger does not support the task-specific Ada attributes T'CALLABLE, E'COUNT, T'STORAGE_SIZE, and T'TERMINATED, where T is a task type and E is a task entry (see the DEC Ada documentation for more information on these attributes). You cannot enter commands such as EVALUATE CHILD'CALLABLE. However, you can get the information provided by each of these attributes with the debugger SHOW TASK command. For more information, see Section 17.4.

17.3.3 Task ID

A task ID is the number assigned to a task when it is created by the tasking system. The task ID uniquely identifies a task during the entire execution of a program.

A task ID has the following syntax, where n is a positive decimal integer:

%TASK n

You can determine the task ID of a task object by evaluating or examining the task object. For example (using Ada path-name syntax):

DBG> EVALUATE FATHER %TASK 2 DBG> EXAMINE FATHER TASK_EXAMPLE.FATHER: %TASK 2

If the programming language does not have built-in tasking services, you must use the EXAMINE/TASK command to obtain the task ID of a task.

Note that the EXAMINE/TASK/HEXADECIMAL command, when applied to a task object, yields the hexadecimal task value. The task value is the address of the task (or thread) control block of that task. For example (Ada example):

DBG> EXAMINE/HEXADECIMAL FATHER TASK_EXAMPLE.FATHER: 0015AD00 DBG>

The SHOW TASK/ALL command enables you to identify the task IDs that have been assigned to all currently existing tasks. Some of these existing tasks may not be immediately familiar to you for the following reasons:

• A SHOW TASK/ALL display includes tasks created by subsystems like DECthreads, Remote Procedure Call services, and the C Run-Time Library, not just the tasks associated with your application.
• A SHOW TASK/ALL display includes task ID assignments that depend on your operating system, your tasking service, and the generating subsystem. The same tasking program, run on different systems or adjusted for different services, will not identify tasks with the same decimal integer. The only exception is %TASK 1, which all systems and services assign to the task that executes the main program.

The following examples are derived from Example 17-1 and Example 17-2, respectively:

DBG> SHOW TASK/ALL task id state hold pri substate thread_object %TASK 1 READY HOLD 12 Initial thread %TASK 2 SUSP 12 Condition Wait THREAD_EX1\main\threads[0].field1 %TASK 3 SUSP 12 Condition Wait THREAD_EX1\main\threads[1].field1 DBG>

DBG> SHOW TASK/ALL task id pri hold state substate task object * %TASK 1 7 RUN SHARE$ADARTL+130428 %TASK 2 7 SUSP Accept TASK_EXAMPLE.MOTHER+4 %TASK 4 7 SUSP Entry call TASK_EXAMPLE.FATHER_TYPE$TASK_BODY.CHILD+4 %TASK 3 6 READY TASK_EXAMPLE.MOTHER+4 DBG>

You can use task IDs to refer to nonexistent tasks in debugger conditional statements. For example, if you ran your program once, and you discovered that %TASK 2 and 3 were of interest, you could enter the following commands at the beginning of your next debugging session before %TASK 2 or 3 was created:

DBG> SET BREAK %LINE 60 WHEN (%ACTIVE_TASK=%TASK 2) DBG> IF (%CALLER=%TASK 3) THEN (SHOW TASK/FULL)

You can use a task ID in certain debugger commands before the task has been created without the debugger reporting an error (as it would if you used a task object name before the task object came into existence). A task does not exist until the task is created. Later the task becomes nonexistent sometime after it terminates. A nonexistent task never appears in a debugger SHOW TASK display.

Each time a program runs, the same task IDs are assigned to the same tasks so long as the program statements are executed in the same order. Different execution orders can result from ASTs (caused by delay statement expiration or I/O completion) being delivered in a different order. Different execution orders can also result from time slicing being enabled. A given task ID is never reassigned during the execution of the program.

17.3.4 Task Built-In Symbols

The debugger built-in symbols defined in Table 17-2 enable you to specify tasks in command procedures and command constructs.

Table 17-2 Task Built-In Symbols

Built-in Symbol	Description
%ACTIVE_TASK	The task that runs when a GO, STEP, CALL, or EXIT command executes.
%CALLER_TASK	(Applies only to Ada programs.) When an accept statement executes, the task that called the entry associated with the accept statement.
%NEXT_TASK	The task after the visible task in the debugger's task list. The ordering of tasks is arbitrary but consistent within a single run of a program.
%PREVIOUS_TASK	The task previous to the visible task in the debugger's task list.
%VISIBLE_TASK	The task whose call stack and register set are the current context for looking up symbols, register values, routine calls, breakpoints, and so on.

Examples using these task built-in symbols follow.

The following command displays the task ID of the visible task:

DBG> EVALUATE %VISIBLE_TASK

The following command places the active task on hold:

DBG> SET TASK/HOLD %ACTIVE_TASK

The following command sets a breakpoint on line 38 that triggers only when task CHILD executes that line:

DBG> SET BREAK %LINE 38 WHEN (%ACTIVE_TASK=CHILD)

The symbols %NEXT_TASK and %PREVIOUS_TASK enable you to cycle through the total set of tasks that currently exist. For example:

DBG> SHOW TASK %VISIBLE_TASK; SET TASK/VISIBLE %NEXT_TASK DBG> SHOW TASK %VISIBLE_TASK; SET TASK/VISIBLE %NEXT_TASK . . . DBG> EXAMINE MONITOR_TASK MOD\MONITOR_TASK: %TASK 2 DBG> WHILE %NEXT_TASK NEQ %ACTIVE DO (SET TASK %NEXT_TASK; SHOW CALLS)

17.3.4.1 Caller Task Symbol (Ada Only)

The symbol %CALLER_TASK is specific to Ada tasks. It evaluates to the task ID of the task that called the entry associated with the accept statement. Otherwise, it evaluates to %TASK 0. For example, %CALLER_TASK evaluates to %TASK 0 if the active task is not currently executing the sequence of statements associated with the accept statement.

For example, suppose a breakpoint has been set on line 61 of Example 17-2 (within an accept statement). The accept statement in this case is executed by task FATHER (%TASK 2) in response to a call of entry RENDEZVOUS by the main program (%TASK 1). Thus, when an EVALUATE %CALLER_TASK command is entered at this point, the result is the task ID of the calling task, the main program:

DBG> EVALUATE %CALLER_TASK %TASK 1 DBG>

When the rendezvous is the result of an AST entry call, %CALLER_TASK evaluates to %TASK 0 because the caller is not a task.

17.4 Displaying Information About Tasks

To display information about one or more tasks of your program, use the SHOW TASK command.

The SHOW TASK command displays information about existing (nonterminated) tasks. By default, the command displays one line of information about the visible task.

Section 17.4.1 and Section 17.4.2 describe the information displayed by a SHOW TASK command for DECthreads and Ada tasks, respectively.

17.4.1 Displaying Information About DECthreads Tasks

The command SHOW TASK displays information about all of the tasks of the program that currently exist (see Example 17-3).

Example 17-3 Sample SHOW TASK/ALL Display for DECthreads Tasks

(1) (2) (3) (4) (5) (6) task id state hold pri substate thread_object %TASK 1 SUSP 12 Condition Wait Initial thread %TASK 2 SUSP 12 Mutex Wait T_EXAMP\main\threads[0].field1 %TASK 3 SUSP 12 Delay T_EXAMP\main\threads[1].field1 %TASK 4 SUSP 12 Mutex Wait T_EXAMP\main\threads[2].field1 * %TASK 5 RUN 12 T_EXAMP\main\threads[3].field1 %TASK 6 READY 12 T_EXAMP\main\threads[4].field1 %TASK 7 SUSP 12 Mutex Wait T_EXAMP\main\threads[5].field1 %TASK 8 READY 12 T_EXAMP\main\threads[6].field1 %TASK 9 TERM 12 Term. by alert T_EXAMP\main\threads[7].field1 DBG>

Key to Example 17-3:

1. The task ID (see Section 17.3.3). The active task is marked with an asterisk (*) in the leftmost column.
2. The current state of the task (see Table 17-3). The task in the RUN (RUNNING) state is the active task. Table 17-3 lists the state transitions possible during program execution.
3. Whether the task has been put on hold with a SET TASK/HOLD command as explained in Section 17.5.1.
4. The task priority.
5. The current substate of the task. The substate helps indicate the possible cause of a task's state. See Table 17-4.
6. A debugger path name for the task (thread) object or the address of the task object if the debugger cannot symbolize the task object.

Table 17-3 Generic Task States

Task State	Description
RUNNING	Task is currently running on the processor. This is the active task. A task in this state can make a transition to the READY, SUSPENDED, or TERMINATED state.
READY	Task is eligible to execute and waiting for the processor to be made available. A task in this state can make a transition only to the RUNNING state.
SUSPENDED	Task is suspended, that is, waiting for an event rather than for the availability of the processor. For example, when a task is created, it remains in the suspended state until it is activated. A task in this state can make a transition only to the READY or TERMINATED state.
TERMINATED	Task is terminated. A task in this state cannot make a transition to another state.

Table 17-4 DECthreads Task Substates

Task Substate	Description
Condition Wait	Task is waiting on a DECthreads condition variable.
Delay	Task is waiting at a call to a DECthreads delay.
Mutex Wait	Task is waiting on a DECthreads mutex.
Not yet started	Task has not yet executed its start routine.
Term. by alert	Task has been terminated by an alert operation.
Term. by exc	Task has been terminated by an exception.
Timed Cond Wait	Task is waiting on a timed DECthreads condition variable.

The SHOW TASK/FULL command provides detailed information about each task selected for display. Example 17-4 shows the output of this command for a sample DECthreads task.

Example 17-4 Sample SHOW TASK/FULL Display for a DECthreads Task

(1) task id state hold pri substate thread_object %TASK 4 SUSP 12 Delay T_EXAMP\main\threads[1].field1 (2) Alert is pending Alerts are deferred (3) Next pc: SHARE$CMA$RTL+46136 Start routine: T_EXAMP\thread_action (4) Scheduling policy: throughput (5) Stack storage: Bytes in use: 1288 (6) Base: 00334C00 Bytes available: 40185 SP: 003346F8 Reserved Bytes: 10752 Top: 00329A00 Guard Bytes: 4095 (7) Thread control block: Size: 293 Address: 00311B78 (8) Total storage: 56613 DBG>

Key to Example 17-4:

1. Identifying information about the task.
2. Bulletin-type information about something unusual.
3. Next execution PC value and start routine.
4. Task scheduling policy.
5. Stack storage information:
   • "Bytes in use:" the number of bytes of stack currently allocated.
   • "Bytes available:" the unused space in bytes.
   • "Reserved Bytes:" the storage allocated for handling stack overflow.
   • "Guard Bytes:" the size of the guard area or unwritable part of the stack.
6. Minimum and maximum addresses of the task stack.
7. Task (thread) control block information. The task value is the address, in hexadecimal notation, of the task control block.
8. The total storage used by the task. Adds together the task control block size, the number of reserved bytes, the top guard size, and the storage size.

Figure 17-1 shows a task stack.

Figure 17-1 Diagram of a Task Stack

The SHOW TASK/STATISTICS command reports some statistics about all tasks in your program. Example 17-5 shows the output of the SHOW TASK/STATISTICS/FULL command for a sample program with DECthreads tasks. This information enables you to measure the performance of your program. The larger the number of total schedulings (also known as context switches), the more tasking overhead there is.

Example 17-5 Sample SHOW TASK/STAT/FULL Display for DECthreads Tasks

task statistics Total context switches: 0 Number of existing threads: 0 Total threads created: 0 DBG>

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