|Example 16-2 Sample Ada Tasking Program
1 -- Tasking program that demonstrates various tasking conditions.
3 package TASK_EXAMPLE_PKG is
4 procedure BREAK;
7 package body TASK_EXAMPLE_PKG is
8 procedure BREAK is
15 with TEXT_IO; use TEXT_IO;
16 with TASK_EXAMPLE_PKG; use TASK_EXAMPLE_PKG;
17 procedure TASK_EXAMPLE is (1)
19 pragma TIME_SLICE(0.0); -- Disable time slicing. (2)
21 task type FATHER_TYPE is
22 entry START;
23 entry RENDEZVOUS;
24 entry BOGUS; -- Never accepted, caller deadlocks.
25 end FATHER_TYPE;
27 FATHER : FATHER_TYPE; (3)
29 task body FATHER_TYPE is
30 SOME_ERROR : exception;
32 task CHILD is (4)
33 entry E;
34 end CHILD;
36 task body CHILD is
38 FATHER_TYPE.BOGUS; -- Deadlocks on call to its parent
39 end CHILD; -- (parent does not have an accept
40 -- statement for entry BOGUS). Whenever
41 -- a task-type name (here, FATHER_TYPE)
42 -- is used within a task body, the
43 -- name designates the task currently
44 -- executing the body.
45 begin -- (of FATHER_TYPE body)
47 accept START do
48 BREAK; -- Main program is waiting for this rendezvous to
49 -- complete; CHILD is suspended when it calls the
50 -- entry BOGUS.
52 end START;
54 PUT_LINE("FATHER is now active and"); (5)
55 PUT_LINE("is going to rendezvous with main program.");
57 for I in 1..2 loop
59 accept RENDEZVOUS do
60 PUT_LINE("FATHER now in rendezvous with main program");
61 end RENDEZVOUS;
64 end select;
66 if I = 2 then
67 raise SOME_ERROR;
68 end if;
69 end loop;
72 when OTHERS =>
73 BREAK; -- CHILD is suspended on entry call to BOGUS.
74 -- Main program is going to delay while FATHER
75 -- terminates.
76 -- MOTHER is ready to begin executing.
77 abort CHILD;
78 BREAK; -- CHILD is now abnormal due to the abort statement.
80 raise; -- SOME_ERROR exception terminates FATHER.
81 end FATHER_TYPE;
83 begin -- (of TASK_EXAMPLE) (6)
86 task MOTHER is (7)
87 entry START;
88 pragma PRIORITY (6);
89 end MOTHER;
91 task body MOTHER is
93 accept START;
94 BREAK; -- At this point, the main program is waiting for
95 -- its dependents (FATHER and MOTHER) to terminate.
96 -- FATHER is terminated.
98 end MOTHER;
99 begin (8)
102 BREAK; -- FATHER is suspended at accept start.
103 -- CHILD is suspended in its deadlock.
104 -- MOTHER has activated and ready to begin executing.
105 FATHER.START; (9)
106 BREAK; -- FATHER is suspended at its 'select or terminate'
107 -- statement.
111 FATHER.RENDEZVOUS; (10)
112 loop (11)
113 -- This loop causes the main program to busy wait
114 -- for the termination of FATHER, so that FATHER
115 -- can be observed in its terminated state.
116 if FATHER'TERMINATED then
118 end if;
119 delay 1.0;
120 end loop;
122 BREAK; -- FATHER has terminated by now with the unhandled
123 -- exception SOME_ERROR. CHILD no longer exists
124 -- because its master (FATHER) has terminated. Task
125 -- MOTHER is ready.
126 MOTHER.START; (12)
127 -- The main program enters a wait-for-dependents state,
128 -- so that MOTHER can finish executing.
130 end TASK_EXAMPLE; (13)
Key to Example 16-2:
- After all of the Ada library packages are
elaborated (in this case, TEXT_IO), the main program is automatically
called and begins to elaborate its declarative part (lines 18 through
- To ensure repeatability from run to run, the
example uses no time slicing (see Section 16.5.2). The 0.0 value for the
pragma TIME_SLICE documents that the procedure TASK_EXAMPLE needs to
have time slicing disabled.
On VAX processors, time slicing is
disabled if the pragma TIME_SLICE is omitted or is specified with a
value of 0.0.
On Alpha processors, pragma TIME_SLICE (0.0) must be
used to disable time slicing.
- Task object FATHER is elaborated, and a task
designated %TASK 2 is created. FATHER has no pragma PRIORITY, and thus
assumes a default priority. FATHER (%TASK 2) is created in a suspended
state and is not activated until the beginning of the statement part of
the main program (line 83), in accordance with Ada rules. The
elaboration of the task body on lines 29 through 81 defines the
statements that tasks of type FATHER_TYPE will execute.
- Task FATHER declares a single task named
CHILD (line 32). A single task represents both a task object and an
anonymous task type. Task CHILD is not created or activated until
FATHER is activated.
- The only source of asynchronous system traps
(ASTs) is this series of TEXT_IO.PUT_LINE statements (I/O completion
- The task FATHER is activated while the main
program waits. FATHER has no pragma PRIORITY and this assumes a default
priority of 7. (See the DEC Ada Language Reference Manual for the rules about default
priorities.) FATHER's activation consists of the elaboration of lines
29 through 44.
When task FATHER is activated, it waits while its
task CHILD is activated and a task designated %TASK 3 is created. CHILD
executes one entry call on line 38, and then deadlocks because the
entry is never accepted (see Section 16.7.1).
Because time slicing
is disabled and there are no higher priority tasks to be run, FATHER
will continue to execute past its activation until it is blocked at the
ACCEPT statement at line 47.
- A single task, MOTHER, is defined, and a task
designated %TASK 4 is created. The pragma PRIORITY gives MOTHER a
priority of 6.
- The task MOTHER begins its activation and
executes line 91. After MOTHER is activated, the main program (%TASK 1)
is eligible to resume its execution. Because %TASK 1 has the default
priority 7, which is higher than MOTHER's priority, the main program
- This is the first rendezvous the main program
makes with task FATHER. After the rendezvous FATHER will suspend at the
SELECT with TERMINATE statement at line 58.
- At the third rendezvous with FATHER, FATHER
raises the exception SOME_ERROR on line 67. The handler on line 72
catches the exception, aborts the suspended CHILD task, and then
reraises the exception; FATHER then terminates.
- A loop with a delay statement ensures that
when control reaches line 122, FATHER has executed far enough to be
- This entry call ensures that MOTHER does not
wait forever for its rendezvous on line 93. MOTHER executes the accept
statement (which involves no other statements), the rendezvous is
completed, and MOTHER is immediately switched off the processor at line
94 because its priority is only 6.
- After its rendezvous with MOTHER, the main
program (%TASK 1) executes lines 127 through 129. At line 129, the main
program must wait for all its dependent tasks to terminate. When the
main program reaches line 129, the only nonterminated task is MOTHER
(MOTHER cannot terminate until the null statement at
line 97 has been executed). MOTHER finally executes to its completion
at line 98. Now that all tasks are terminated, the main program
completes its execution. The main program then returns and execution
resumes with the command line interpreter.
16.3 Specifying Tasks in Debugger Commands
A task is an entity that executes in parallel with
other tasks. A task is characterized by a unique task ID (see
Section 16.3.3), 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 16.3.1.
When specifying tasks in debugger commands, you can use any of the
- A task (thread) name as declared in the program (for example,
FATHER in Section 16.2.2) or a language expression that yields a task
value. Section 16.3.2 describes Ada language expressions for tasks.
- A task ID (for example, %TASK 2). See Section 16.3.3.
- A task built-in symbol (for example, %ACTIVE_TASK). See
16.3.1 Definition of Active Task and Visible Task
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
The SET TASK/ACTIVE command does not work for POSIX Threads (on OpenVMS
VAX, Alpha, and I64 systems) or for Ada on OpenVMS Alpha and I64
systems, the tasking for which is implemented via POSIX Threads.
Instead of SET TASK/ACTIVE, use the SET TASK/VISIBLE command on
POSIX Threads for query-type actions. Or, to gain control to step
through a particular thread, use a strategic placement of breakpoints.
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:
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 16.3.4).
See Section 16.5 for more information about using the SET TASK command
to modify task characteristics.
16.3.2 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
task body FATHER_TYPE is
-- A single task.
task MOTHER is
task body MOTHER is
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
A, B: INTEGER;
T : FATHER_TYPE;
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$TASK_BODY.CHILD
When a task object is elaborated, a task is created by the Compaq Ada
Run-Time Library, and the task object is assigned its task value. As
with other Ada objects, the value of a task object is undefined before
the object is initialized, and the results of using an uninitialized
value are unpredictable.
The task body of a task type or single task is
implemented in Compaq Ada as a procedure. This procedure is called by
the Compaq Ada Run-Time Library when a task of that type is activated.
A task body is treated by the debugger as a normal Ada procedure,
except that it has a specially constructed name.
To specify the task body in a debugger command, use the following
syntax to refer to tasks declared as task types:
Use the following syntax to refer to single tasks:
DBG> SET BREAK FATHER_TYPE$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 Compaq 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 16.4.
16.3.3 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
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
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
DBG> EXAMINE/HEXADECIMAL FATHER
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
- A SHOW TASK/ALL display includes tasks created by subsystems such
as POSIX Threads, 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 16-1 and
Example 16-2, 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.field1
%TASK 3 SUSP 12 Condition Wait THREAD_EX1\main\threads.field1
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
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.
16.3.4 Task Built-In Symbols
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
The task that runs when a GO, STEP, CALL, or EXIT command executes.
(Applies only to Ada programs.) When an accept statement executes, the
task that called the entry that is associated with the accept statement.
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
The task previous to the visible task in the debugger's task list.
The task whose call stack and register set are the current context for
looking up symbols, register values, routine calls, breakpoints, and so
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)