HP OpenVMS Debugger Manual

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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 61 of Example 16-2 (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.

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:

The task ID (see Section 16.3.3). The active task is marked with an asterisk (*) in the leftmost column.
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.
Whether the task has been put on hold with a SET TASK/HOLD command as explained in Section 16.5.1.
The task priority.
The current substate of the task. The substate helps indicate the possible cause of a task's state. See Table 16-4.
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 16-3 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.

**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:

Identifying information about the task.
Bulletin-type information about something unusual.
Next execution PC value and start routine.
Task scheduling policy.
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.
Minimum and maximum addresses of the task stack.
Task (thread) control block information. The task value is the address, in hexadecimal notation, of the task control block.
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

(1) (2) (3) (4) (5) (6) task id pri hold state substate task object * %TASK 1 7 RUN SHARE$ADARTL+130428 %TASK 2 7 HOLD SUSP Accept TASK_EXAMPLE.MOTHER+4 %TASK 4 7 SUSP Entry call TASK_EXAMPLE.FATHER_TYPE$TASK_BODY.CHILD+4 %TASK 3 6 READY TASK_EXAMPLE.MOTHER) DBG>

Key to Example 16-6:

The task ID (see Section 16.3.3). An asterisk indicates that the task is a visible task.
The task priority. Ada priorities range from 0 to 15.
On VAX processors, a task is created with a default priority of 7 unless another value is specified with the pragma PRIORITY.
On Alpha processors, if time slicing is disabled, a task is created with a default prioity of 7; if time slicing is enabled, then a task is created with an approximate midrange value (unless the pragma PRIORITY is specified).
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.
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.
The current substate of the task. The substate helps indicate the possible cause of a task's state. See Table 16-5.
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.

**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.

¹ An unhandled exception is one 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.

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 pri hold state substate task object * %TASK 2 7 RUN TASK_EXAMPLE.MOTHER+4 (2) Waiting entry callers: Waiters for entry BOGUS: %TASK 4, type: CHILD (3) Task type: FATHER_TYPE Created at PC: TASK_EXAMPLE.%LINE 14+22 Parent task: %TASK 1 Start PC: TASK_EXAMPLE.FATHER_TYPE$TASK_BODY (4) Task control block: (5) Stack storage (bytes): Task value: 490816 RESERVED_BYTES: 10640 Entries: 3 TOP_GUARD_SIZE: 5120 Size: 1488 STORAGE_SIZE: 30720 (6) Stack addresses: Bytes in use: 456 Top address: 001EB600 Base address: 001F2DFC (7) Total storage: 47968 DBG>

Key to Example 16-7:

Identifying information about the task.
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.
Task context information.
Task control block information. The task value is the address, in decimal notation, of the task control block.
Stack storage information:
- RESERVED_BYTES gives the storage allocated by the Ada Run-Time Library for handling stack overflow.
- TOP_GUARD_SIZE gives the storage allocated for guard pages, which provide protection against storage overflow during task execution.
  On VAX processors, you can specify the number of bytes to be allocated as guard pages with the Compaq Ada pragmas TASK_STORAGE and MAIN_STORAGE; the number shown by the debugger is the number of bytes allocated (the pragma value is rounded up to an integral number of pages, as necessary). For more information about these pragmas and the top guard storage area, see the Compaq Ada documentation.
  On Alpha processors, you can specify the number of bytes to be allocated as guard pages with the Compaq Ada pragmas TASK_STORAGE; the number shown by the debugger is the number of bytes allocated (the pragma value is rounded up to an integral number of pages, as necessary). For more information about these pragmas and the top guard storage area, see the Compaq Ada documentation.
- STORAGE_SIZE gives the storage allocated for the task activation.
  On VAX processors, you can specify the number of bytes to be allocated with the T'STORAGE_SIZE representation clause or in the Ada pragma MAIN_STORAGE; the number shown by the debugger is the number of bytes allocated (the value specified is rounded up to an integral number of pages, as necessary). For more information about this representation clause and pragma and about the task activation (working) storage area, see the Compaq Ada documentation.
  On Alpha processors, you can specify the number of bytes to be allocated with the T'STORAGE_SIZE representation clause; the number shown by the debugger is the number of bytes allocated (the value specified is rounded up to an integral number of pages, as necessary). For more information about this representation clause and pragma and about the task activation (working) storage area, see the Compaq Ada documentation.
- "Bytes in use:" gives the number of bytes of stack currently allocated.
Stack addresses of the task stack.
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 (VAX Example)

task statistics Entry calls = 4 Accepts = 1 Selects = 2 Tasks activated = 3 Tasks terminated = 0 ASTs delivered = 4 Hibernations = 0 Total schedulings = 15 Due to readying a higher priority task = 1 Due to task activations = 3 Due to suspended entry calls = 4 Due to suspended accepts = 1 Due to suspended selects = 2 Due to waiting for a DELAY = 0 Due to scope exit awaiting dependents = 0 Due to exception awaiting dependents = 0 Due to waiting for I/O to complete = 0 Due to delivery of an AST = 4 Due to task terminations = 0 Due to shared resource lock contention = 0 DBG>

16.5 Changing Task Characteristics

To modify a task's characteristics or the tasking environment while debugging, use the SET TASK command, as shown in the following table:

Command	Description
SET TASK/ACTIVE	Makes a specified task the active task; not for POSIX Threads (on OpenVMS Alpha, VAX, or I64) or Ada on OpenVMS Alpha and I64 (see Section 16.3.1).
SET TASK/VISIBLE	Makes a specified task the visible task (see Section 16.3.1).
SET TASK/ABORT	Requests that a task be terminated at the next allowed opportunity. The exact effect depends on the current event facility (language dependent). For Ada tasks, this is equivalent to executing an abort statement.
SET TASK/PRIORITY	Sets a task's priority. The exact effect depends on the current event facility (language dependent).
SET TASK/RESTORE	Restores a task's priority. The exact effect depends on the current event facility (language dependent).
SET TASK/[NO]HOLD	Controls task switching (task state transitions, see Section 16.5.1).
SET TASK/TIME_SLICE	Controls the time slice value or disable time slicing (see Section 16.5.2); supported for VAX Ada only, and not with POSIX Threads.

For more information, see the SET TASK command description.

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