The debugger built-in symbols defined in Table 16-2 enable you to specify tasks in command procedures and command constructs.

Table 16-2 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 that is 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 25 that triggers only when task CHILD executes that line:

DBG> SET BREAK %LINE 25 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)

16.3.4.1 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 46 of Example 16-2 (within an accept statement). The accept statement in this case is executed by task FATHER (%TASK 3) 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.

16.4 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 16.4.1 and Section 16.4.2 describe the information displayed by a SHOW TASK command for POSIX Threads and Ada tasks, respectively.

16.4.1 Displaying Information About POSIX Threads Tasks

The command SHOW TASK displays information about all of the tasks of the program that currently exist (see Example 16-3).

Example 16-3 Sample SHOW TASK/ALL Display for POSIX Threads 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 16-3:

1. The task ID (see Section 16.3.3). The active task is marked with an asterisk (*) in the leftmost column.
2. The current state of the task (see Table 16-3). The task in the RUN (RUNNING) state is the active task. Table 16-3 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 16.5.1.
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 16-4.
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 16-3 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 16-4 POSIX Threads Task Substates

Task Substate	Description
Condition Wait	Task is waiting on a POSIX Threads condition variable.
Delay	Task is waiting at a call to a POSIX Threads delay.
Mutex Wait	Task is waiting on a POSIX Threads 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 POSIX Threads condition variable.

The SHOW TASK/FULL command provides detailed information about each task selected for display. Example 16-4 shows the output of this command for a sample POSIX Threads task.

Example 16-4 Sample SHOW TASK/FULL Display for a POSIX Threads 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 16-4:

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 16-1 shows a task stack.

Figure 16-1 Diagram of a Task Stack

The SHOW TASK/STATISTICS command reports some statistics about all tasks in your program. Example 16-5 shows the output of the SHOW TASK/STATISTICS/FULL command for a sample program with POSIX Threads 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 16-5 Sample SHOW TASK/STAT/FULL Display for POSIX Threads Tasks

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

16.4.2 Displaying Task Information About Ada Tasks

The SHOW TASK/ALL command displays information about all of the tasks of the program that currently exist---namely, tasks that have been created and whose master has not yet terminated (see Example 16-6).

Example 16-6 Sample SHOW TASK/ALL Display for Ada Tasks

DBG> SHOW TASK/ALL (1) (2) (3) (4) (5) (6) task id state hold pri substate thread_object %TASK 1 READY 24 main thread * %TASK 2 RUN 24 1515464 %TASK 3 READY 19 1519768 %TASK 4 SUSP 24 Timed Condition Wait 4932680 DBG>

Key to Example 16-6:

1. The task ID (see Section 16.3.3). An asterisk indicates that the task is a visible task.
2. The current state of the task (see Table 16-3). The task that is in the RUN (RUNNING) state is the active task. Table 16-3 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 16.5.1. Placing a task on HOLD restricts the state transitions it can make after the program is subsequently allowed to execute.
4. The task priority. Ada priorities range from 0 to 15. On Alpha systems, a task with an undefined priority competes with other tasks, as if having a mid-range priority (between 7 and 8). Ada provides the following two mechanisms for controlling task priorities:
   • Pragma PRIORITY A priority is associated with a task if a pragma PRIORITY appears in the corresponding task specification or in the outermost declarative part of a main subprogram.
   • Types and operations in the Ada package SET_TASK_PRIORITY. Ada allows task priorities to be changed dynamically at run-time to values of the subtype PRIORITY.
5. The current substate of the task. The substate helps indicate the possible cause of a task's state. See Table 16-5.
6. A debugger path name for the task object or the address of the task object if the debugger cannot symbolize the task object.

Table 16-5 Ada Task Substates

Task Substate	Description
Abnormal	Task has been aborted.
Accept	Task is waiting at an accept statement that is not inside a select statement.
Activating	Task is elaborating its declarative part.
Activating tasks	Task is waiting for tasks it has created to finish activating.
Completed [abn]	Task is completed due to an abort statement but is not yet terminated. In Ada, a completed task is one that is waiting for dependent tasks at its end statement. After the dependent tasks are terminated, the state changes to terminated.
Completed [exc]	Task is completed due to an unhandled exception 1 but is not yet terminated. In Ada, a completed task is one that is waiting for dependent tasks at its end statement. After the dependent tasks are terminated, the state changes to terminated.
Completed	Task is completed. No abort statement was issued and no unhandled exception 1 occurred.
Delay	Task is waiting at a delay statement.
Dependents	Task is waiting for dependent tasks to terminate.
Dependents [exc]	Task is waiting for dependent tasks to allow an unhandled exception 1 to propagate.
Entry call	Task is waiting for its entry call to be accepted.
Invalid state	There is an error in the Compaq Ada Run-Time Library.
I/O or AST	Task is waiting for I/O completion or some AST.
Not yet activated	Task is waiting to be activated by the task that created it.
Select or delay	Task is waiting at a select statement with a delay alternative.
Select or terminate	Task is waiting at a select statement with a terminate alternative.
Select	Task is waiting at a select statement with no else, delay, or terminate alternative.
Shared resource	Task is waiting for an internal shared resource.
Terminated [abn]	Task was terminated by an abort statement.
Terminated [exc]	Task was terminated because of an unhandled exception. 1
Terminated	Task terminated normally.
Timed entry call	Task is waiting in a timed entry call.

Figure 16-1 shows a task stack.

The SHOW TASK/FULL command provides detailed information about each task selected for display. Example 16-7 shows the output of this command for a sample Ada task.

Example 16-7 Sample SHOW TASK/FULL Display for an Ada Task

(1) task id state hold pri substate thread_object %TASK 1 RUN 24 1515464 (2) Cancellation is enabled Next pc: (unknown) Start routine: 1380128 Scheduling policy: fifo (real-time) (3) Stack storage: Bytes in use: 7069696 (4) Base: 00000000006BE000 Bytes available: -4972544 SP: 0000000000000000 Reserved Bytes: 0 Top: 00000000004BE000 Guard Bytes: 0 (5) Thread control block: Size: 5856 Address: 00000000006BF280 (6) Total storage: 2103008 DBG>

Key to Example 16-7:

1. Identifying information about the task.
2. Rendezvous information. If the task is a caller task, it lists the entries for which it is queued. If the task is to be called, it gives information about the kind of rendezvous that will take place and lists the callers that are currently queued for any of the task's entries.
3. 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.
4. Minimum and maximum addresses of the task stack.
   • "SP:" Stack Pointer indicates a location in the stack memory.
   • "TOP:" indicates the last allocated stack location.
5. Task (thread) control block information. The task value is the address, in hexadecimal notation, of the task control block.
6. 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.

The SHOW TASK/STATISTICS command reports some statistics about all tasks in your program. Example 16-8 shows the output of the SHOW TASK/STATISTICS/FULL command for a sample Ada tasking program on a VAX system. 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 16-8 Sample SHOW TASK/STATISTICS/FULL Display for Ada Tasks

DBG> SHOW TASK/STATISTICS/FULL task statistics Total context switches: 0 Number of existing threads: 6 Total threads created: 4 DBG>

16.5 Changing Task Characteristics

For more information, see the SET TASK command description.