9.5 Intertask Communication

From OC Systems Wiki!
Jump to: navigation, search

The primary means for intertask communication is provided by calls on entries and protected subprograms. Calls on protected subprograms allow coordinated access to shared data objects. Entry calls allow for blocking the caller until a given condition is satisfied (namely, that the corresponding entry is open -- see 9.5.3), and then communicating data or control information directly with another task or indirectly via a shared protected object.

Static Semantics

Any call on an entry or on a protected subprogram identifies a target object for the operation, which is either a task (for an entry call) or a protected object (for an entry call or a protected subprogram call). The target object is considered an implicit parameter to the operation, and is determined by the operation name (or prefix) used in the call on the operation, as follows:

  • If it is a direct_name or expanded name that denotes the declaration (or body) of the operation, then the target object is implicitly specified to be the current instance of the task or protected unit immediately enclosing the operation; such a call is defined to be an internal call;
  • If it is a selected_component that is not an expanded name, then the target object is explicitly specified to be the task or protected object denoted by the prefix of the name; such a call is defined to be an external call;
  • If the name or prefix is a dereference (implicit or explicit) of an access-to-protected-subprogram value, then the target object is determined by the prefix of the Access attribute_reference that produced the access value originally, and the call is defined to be an external call;
  • If the name or prefix denotes a subprogram_renaming_declaration, then the target object is as determined by the name of the renamed entity.

A corresponding definition of target object applies to a requeue_statement (see 9.5.4), with a corresponding distinction between an internal requeue and an external requeue.

Dynamic Semantics

Within the body of a protected operation, the current instance (see 8.6) of the immediately enclosing protected unit is determined by the target object specified (implicitly or explicitly) in the call (or requeue) on the protected operation.

Any call on a protected procedure or entry of a target protected object is defined to be an update to the object, as is a requeue on such an entry.

Copyright © 1992,1993,1994,1995 Intermetrics, Inc.
Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual