9.3. Task Execution - Task Activation

A task body defines the execution of any task that is designated by a task object of the corresponding task type. The initial part of this execution is called the activation of the task object, and also that of the designated task; it consists of the elaboration of the declarative part, if any, of the task body. The execution of different tasks, in particular their activation, proceeds in parallel.

If an object declaration that declares a task object occurs immediately within a declarative part, then the activation of the task object starts after the elaboration of the declarative part (that is, after passing the reserved word begin following the declarative part); similarly if such a declaration occurs immediately within a package specification, the activation starts after the elaboration of the declarative part of the package body. The same holds for the activation of a task object that is a subcomponent of an object declared immediately within a declarative part or package specification. The first statement following the declarative part is executed only after conclusion of the activation of these task objects.

Should an exception be raised by the activation of one of these tasks, that task becomes a completed task (see 9.4); other tasks are not directly affected. Should one of these tasks thus become completed during its activation, the exception TASKING_ERROR is raised upon conclusion of the activation of all of these tasks (whether successfully or not); the exception is raised at a place that is immediately before the first statement following the declarative part (immediately after the reserved word begin). Should several of these tasks thus become completed during their activation, the exception TASKING_ERROR is raised only once.

Should an exception be raised by the elaboration of a declarative part or package specification, then any task that is created (directly or indirectly) by this elaboration and that is not yet activated becomes terminated and is therefore never activated (see section 9.4 for the definition of a terminated task).

For the above rules, in any package body without statements, a null statement is assumed. For any package without a package body, an implicit package body containing a single null statement is assumed. If a package without a package body is declared immediately within some program unit or block statement, the implicit package body occurs at the end of the declarative part of the program unit or block statement; if there are several such packages, the order of the implicit package bodies is undefined.

A task object that is the object, or a subcomponent of the object, created by the evaluation of an allocator is activated by this evaluation. The activation starts after any initialization for the object created by the allocator; if several subcomponents are task objects, they are activated in parallel. The access value designating such an object is returned by the allocator only after the conclusion of these activations.

Should an exception be raised by the activation of one of these tasks, that task becomes a completed task; other tasks are not directly affected. Should one of these tasks thus become completed during its activation, the exception TASKING_ERROR is raised upon conclusion of the activation of all of these tasks (whether successfully or not); the exception is raised at the place where the allocator is evaluated. Should several of these tasks thus become completed during their activation, the exception TASKING_ERROR is raised only once.

Should an exception be raised by the initialization of the object created by an allocator (hence before the start of any activation), any task designated by a subcomponent of this object becomes terminated and is therefore never activated.

Example:

    procedure P is
       A, B : RESOURCE;  --  elaborate the task objects A, B
       C    : RESOURCE;  --  elaborate the task object C
    begin
       --  the tasks A, B, C are activated in parallel before the first statement
       ...
    end; 

Notes:

An entry of a task can be called before the task has been activated. If several tasks are activated in parallel, the execution of any of these tasks need not await the end of the activation of the other tasks. A task may become completed during its activation either because of an exception or because it is aborted (see 9.10).

References: allocator, completed task, declarative part, elaboration, entry, exception, handling an exception, package body, parallel execution, statement, subcomponent, task body, task object, task termination, task type, tasking_error exception.