9 Tasks and Synchronization
The execution of an Ada program
consists of the execution of one or more
tasks.
Each
task represents a separable activity that proceeds independently and
concurrently between the points where it
interacts with other
tasks. A single task, when within the context of a parallel construct,
can represent multiple
logical threads of control which can proceed
in parallel; in other contexts, each task represents one logical thread
of control.
The various forms
of task interaction are described in this clause, and include:
the activation and termination of a task;
a call on a protected subprogram
of a
protected object, providing exclusive read-write access,
or concurrent read-only access to shared data;
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;
a timed operation, including a simple delay statement,
a timed entry call or accept, or a timed asynchronous select statement
(see next item);
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;
an abort statement, allowing one task to cause
the termination of another task.
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
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
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. In the context of a parallel construct, a single task
can utilize multiple processing resources simultaneously.
NOTE Concurrent task execution can
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 single logical thread of
control are performed by different physical processors acting in parallel,
it can choose to perform them in this way.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
