Section author: Gustavo A. Hoffmann
Tasks and protected objects allow the implementation of concurrency in Ada. The following sections explain these concepts in more details.
Tasks may synchronize with the main application but may also process information completely independent from the main application. Here we show how this is accomplished.
Tasks are declared using the keyword
task. The task implementation
is specified in a
task body block. For example:
Here, we're declaring and implementing the task
T. As soon as the main
application starts, task
T starts automatically --- it's not necessary
to manually start this task. By running the application above, we can see
that both calls to
Put_Line are performed.
The main application is itself a task (the main task).
- In this example, the subprogram
Show_Simple_Taskis the main task of the application.
- In this example, the subprogram
Tis a subtask.
- Each subtask has a master task.
- Therefore the main task is also the master task of task
The number of tasks is not limited to one: we could include a task
T2in the example above.
This task also starts automatically and runs concurrently with both task
Tand the main task. For example:
As we've just seen, as soon as the main task starts, its subtasks also start automatically. The main task continues its processing until it has nothing more to do. At that point, however, it will not terminate. Instead, the task waits until its subtasks have finished before it allows itself to terminate. In other words, this waiting process provides synchronization between the main task and its subtasks. After this synchronization, the main task will terminate. For example:
The same mechanism is used for other subprograms that contain subtasks: the subprogram's master task will wait for its subtasks to finish. So this mechanism is not limited to the main application and also applies to any subprogram called by the main application or its subprograms.
Synchronization also occurs if we move the task to a separate package. In
the example below, we declare a task
T in the package
This is the corresponding package body:
Because the package is
with'ed by the main procedure, the task
defined in the package is part of the main task. For example:
Again, as soon as the main task reaches its end, it synchronizes with task
Simple_Sync_Pkg before terminating.
We can introduce a delay by using the keyword
delay. This puts the
task to sleep for the length of time (in seconds) specified in the delay
statement. For example:
In this example, we're making the task
T wait one second after each
time it displays the "hello" message. In addition, the main task is waiting
1.5 seconds before displaying its own "hello" message
The only type of synchronization we've seen so far is the one that happens
automatically at the end of the main task. You can also define custom
synchronization points using the keyword
entry. An entry can be
viewed as a special kind of subprogram, which is called by the master task
using a similar syntax, as we will see later.
In the task definition, you define which part of the task will accept the
entries by using the keyword
accept. A task proceeds until it
accept statement and then waits for the master task to
synchronize with it. Specifically,
- The subtask waits at that point (in the
acceptstatement), ready to accept a call to the corresponding entry from the master task.
- The master task calls the task entry, in a manner similar to a procedure call, to synchronize with the subtask.
This synchronization between tasks is called rendez-vous. Let's see an example:
In this example, we declare an entry
Start for task
T. In the task
body, we implement this entry using
accept Start. When task
reaches this point, it waits for the master task. This synchronization
occurs in the
T.Start statement. After the synchronization completes,
the main task and task
T again run concurrently until they synchronize
one final time when the main task finishes.
An entry may be used to perform more than a simple task synchronization: it
also may perform multiple statements during the time both tasks are
synchronized. We do this with a
do ... end block. For the previous
example, we would simply write
accept Start do <statements>;
end;. We use this kind of block in the next example.
There's no limit to the number of times an entry can be accepted. We could
even create an infinite loop in the task and accept calls to the same entry
over and over again. An infinite loop, however, prevents the subtask from
finishing, so it blocks the master task when it reaches the end of its
processing. Therefore, a loop containing
accept statements in a task
body is normally used in conjunction with a
select ... or terminate
statement. In simple terms, this statement allows the master task to
automatically terminate the subtask when the master task finishes. For
In this example, the task body implements an infinite loop that accepts
calls to the
Increment entry. We make the following
accept E do ... endblock is used to increment a counter.
- As long as task
Tis performing the
do ... endblock, the main task waits for the block to complete.
- As long as task
The main task is calling the
Incremententry multiple times in the loop from
1 .. 4. It is also calling the
Resetentry before and the loop.
- Because task
Tcontains an infinite loop, it always accepts calls to the
- When the main task finishes, it checks the status of the
Ttask. Even though task
Tcould accept new calls to the
Incremententries, the master task is allowed to terminate task
Tdue to the
or terminatepart of the
- Because task
In a previous example, we saw how to delay a task a specified time by using
delay keyword. However, using delay statements in a loop is not
enough to guarantee regular intervals between those delay statements. For
example, we may have a call to a computationally intensive procedure
between executions of successive delay statements:
while True loop delay 1.0; -- ^ Wait 1.0 seconds Computational_Intensive_App; end loop;
In this case, we can't guarantee that exactly 10 seconds have elapsed after
10 calls to the delay statement because a time drift may be introduced by
Computational_Intensive_App procedure. In many cases, this time
drift is not relevant, so using the
delay keyword is good enough.
However, there are situations where a time drift isn't acceptable. In those
cases, we need to use the
delay until statement, which accepts a
precise time for the end of the delay, allowing us to define a regular
interval. This is useful, for example, in real-time applications.
We will soon see an example of how this time drift may be introduced and
delay until statement circumvents the problem. But before we
do that, we look at a package containing a procedure allowing us to measure
the elapsed time (
Show_Elapsed_Time) and a dummy
Computational_Intensive_App procedure which is simulated by using a
simple delay. This is the package specification:
And this is the package body:
Using this auxiliary package, we're now ready to write our time-drifting application:
We can see by running the application that we already have a time
difference of about four seconds after three iterations of the loop due to
the drift introduced by
Computational_Intensive_App. Using the
delay until statement, however, we're able to avoid this time drift
and have a regular interval of exactly one second:
Now, as we can see by running the application, the
statement ensures that the
Computational_Intensive_App doesn't disturb
the regular interval of one second between iterations.
When multiple tasks are accessing shared data, corruption of that data may occur. For example, data may be inconsistent if one task overwrites parts of the information that's being read by another task at the same time. In order to avoid these kinds of problems and ensure information is accessed in a coordinated way, we use protected objects.
Protected objects encapsulate data and provide access to that data by means of protected operations, which may be subprograms or protected entries. Using protected objects ensures that data is not corrupted by race conditions or other simultaneous access.
Protected objects can be implemented using Ada tasks. In fact, this was the only possible way of implementing them in Ada 83 (the first version of the Ada language). However, the use of protected objects is much simpler than using similar mechanisms implemented using only tasks. Therefore, you should use protected objects when your main goal is only to protect data.
You declare a protected object with the
protected keyword. The
syntax is similar to that used for packages: you can declare operations
(e.g., procedures and functions) in the public part and data in the private
part. The corresponding implementation of the operations is included in the
protected body of the object. For example:
In this example, we define two operations for
Get. The implementation of these operations is in the
Obj body. The
syntax used for writing these operations is the same as that for normal
procedures and functions. The implementation of protected objects is
straightforward --- we simply access and update
Local in these
subprograms. To call these operations in the main application, we use
prefixed notation, e.g.,
In addition to protected procedures and functions, you can also define
protected entry points. Do this using the
entry keyword. Protected
entry points allow you to define barriers using the
keyword. Barriers are conditions that must be fulfilled before the entry
can start performing its actual processing --- we speak of releasing the
barrier when the condition is fulfilled.
The previous example used procedures and functions to define operations on
the protected objects. However, doing so permits reading protected
Obj.Get) before it's set (via
Obj.Set). To allow
that to be a defined operation, we specified a default value (0). Instead,
Obj.Get using an entry instead of a function, we
implement a barrier, ensuring no task can read the information before it's
The following example implements the barrier for the
operation. It also contains two concurrent subprograms (main task and task
T) that try to access the protected object.
As we see by running it, the main application waits until the protected
object is set (by the call to
Obj.Set in task
T) before it reads
the information (via
Obj.Get). Because a 4-second delay has been added
T, the main application is also delayed by 4 seconds. Only
after this delay does task
T set the object and release the barrier in
Obj.Get so that the main application can then resume processing (after
the information is retrieved from the protected object).
Task and protected types¶
In the previous examples, we defined single tasks and protected objects. We can, however, generalize tasks and protected objects using type definitions. This allows us, for example, to create multiple tasks based on just a single task type.
A task type is a generalization of a task. The declaration is similar to
simple tasks: you replace
task type. The
difference between simple tasks and task types is that task types don't
create actual tasks that automatically start. Instead, a task declaration
is needed. This is exactly the way normal variables and types work:
objects are only created by variable definitions, not type definitions.
To illustrate this, we repeat our first example:
We now rewrite it by replacing
task T with
task type TT. We
declare a task (
A_Task) based on the task type
TT after its
We can extend this example and create an array of tasks. Since we're using
the same syntax as for variable declarations, we use a similar syntax for
array (<>) of Task_Type. Also, we can pass information
to the individual tasks by defining a
Start entry. Here's the updated
In this example, we're declaring five tasks in the array
pass the array index to the individual tasks in the entry point
Start). After the synchronization between the individual subtasks and
the main task, each subtask calls
A protected type is a generalization of a protected object. The
declaration is similar to that for protected objects: you replace
protected type. Like task types,
protected types require an object declaration to create actual
objects. Again, this is similar to variable declarations and allows
for creating arrays (or other composite objects) of protected objects.
We can reuse a previous example and rewrite it to use a protected type:
In this example, instead of directly defining the protected object
Obj, we first define a protected type
Obj_Type and then
Obj as an object of that protected type. Note that the
main application hasn't changed: we still use
Obj.Get to access the protected object, just like in the original