Ada Reference Manual (Ada 2022) — Legal Information

# 9 Tasks and Synchronization

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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.
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The various forms of task interaction are described in this clause, and include:
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the activation and termination of a task;
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a call on a protected subprogram of a protected object, providing exclusive read-write access, or concurrent read-only access to shared data;
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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;
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a timed operation, including a simple delay statement, a timed entry call or accept, or a timed asynchronous select statement (see next item);
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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;
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an abort statement, allowing one task to cause the termination of another task.
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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

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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

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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.
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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.

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