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9.8 Abort of a Task - Abort of a Sequence of Statements

An abort_statement causes one or more tasks to become abnormal, thus preventing any further interaction with such tasks. The completion of the triggering_statement of an asynchronous_select causes a sequence_of_statements to be aborted.

Syntax

   
       abort_statement ::= abort task_name {, task_name};

Name Resolution Rules

Each task_name is expected to be of any task type; they need not all be of the same task type.

Dynamic Semantics

For the execution of an abort_statement, the given task_names are evaluated in an arbitrary order. Each named task is then aborted, which consists of making the task abnormal and aborting the execution of the corresponding task_body, unless it is already completed.

When the execution of a construct is aborted (including that of a task_body or of a sequence_of_statements), the execution of every construct included within the aborted execution is also aborted, except for executions included within the execution of an abort-deferred operation; the execution of an abort-deferred operation continues to completion without being affected by the abort; the following are the abort-deferred operations:

• a protected action;

• waiting for an entry call to complete (after having initiated the attempt to cancel it -- see below);

• waiting for the termination of dependent tasks;

• the execution of an Initialize procedure as the last step of the default initialization of a controlled object;

• the execution of a Finalize procedure as part of the finalization of a controlled object;

• an assignment operation to an object with a controlled part.

The last three of these are discussed further in 7.6.

When a master is aborted, all tasks that depend on that master are aborted.

The order in which tasks become abnormal as the result of an abort_statement or the abort of a sequence_of_statements is not specified by the language.

If the execution of an entry call is aborted, an immediate attempt is made to cancel the entry call (see 9.5.3). If the execution of a construct is aborted at a time when the execution is blocked, other than for an entry call, at a point that is outside the execution of an abort-deferred operation, then the execution of the construct completes immediately. For an abort due to an abort_statement, these immediate effects occur before the execution of the abort_statement completes. Other than for these immediate cases, the execution of a construct that is aborted does not necessarily complete before the abort_statement completes. However, the execution of the aborted construct completes no later than its next abort completion point (if any) that occurs outside of an abort-deferred operation; the following are abort completion points for an execution:

• the point where the execution initiates the activation of another task;

• the end of the activation of a task;

• the start or end of the execution of an entry call, accept_statement, delay_statement, or abort_statement;

• the start of the execution of a select_statement, or of the sequence_of_statements of an exception_handler.

Bounded (Run-Time) Errors

An attempt to execute an asynchronous_select as part of the execution of an abort-deferred operation is a bounded error. Similarly, an attempt to create a task that depends on a master that is included entirely within the execution of an abort-deferred operation is a bounded error. In both cases, Program_Error is raised if the error is detected by the implementation; otherwise the operations proceed as they would outside an abort-deferred operation, except that an abort of the abortable_part or the created task might or might not have an effect.

Erroneous Execution

If an assignment operation completes prematurely due to an abort, the assignment is said to be disrupted; the target of the assignment or its parts can become abnormal, and certain subsequent uses of the object can be erroneous, as explained in 13.9.1.

NOTES

(38) An abort_statement should be used only in situations requiring unconditional termination.

(39) A task is allowed to abort any task it can name, including itself.

(40) Additional requirements associated with abort are given in D.6, ``Preemptive Abort''.

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