Section 9: Tasks and Synchronization
1
The execution of an Ada program
consists of the execution of one or more
tasks.
Each
task represents a separate thread of control that proceeds independently
and concurrently between the points where it
interacts with other
tasks. The various forms of task interaction are described in this section,
and include:
2
- the activation and termination of
a task;
3
- a call on a protected
subprogram of a protected object, providing exclusive read-write
access, or concurrent read-only access to shared data;
4
- a call on an entry, either of another
task, allowing for synchronous communication with that task, or of a
protected object, allowing for asynchronous communication with one or
more other tasks using that same protected object;
5
- a timed operation, including a simple
delay statement, a timed entry call or accept, or a timed asynchronous
select statement (see next item);
6
- an asynchronous transfer of control
as part of an asynchronous select statement, where a task stops what
it is doing and begins execution at a different point in response to
the completion of an entry call or the expiration of a delay;
7
- an abort statement, allowing one task
to cause the termination of another task.
8
In addition, tasks can communicate indirectly by
reading and updating (unprotected) shared variables, presuming the access
is properly synchronized through some other kind of task interaction.
Static Semantics
9
The properties of a task are
defined by a corresponding task declaration and
task_body,
which together define a program unit called a
task unit.
Dynamic Semantics
10
Over time, tasks proceed through various
states.
A
task is initially
inactive; upon activation, and prior to its
termination it is either
blocked (as part of some task
interaction) or
ready to run.
While ready,
a task competes for the available
execution resources that it
requires to run.
11
1 Concurrent task execution may be implemented
on multicomputers, multiprocessors, or with interleaved execution on
a single physical processor. On the other hand, whenever an implementation
can determine that the required semantic effects can be achieved when
parts of the execution of a given task are performed by different physical
processors acting in parallel, it may choose to perform them in this
way.