Mailboxes (MIPC)

The optional MIPC feature is designed to allow an application composed of one or multiple actors to create a shared communication environment (or message space) within which these actors can exchange messages efficiently. In particular, supervisor and user actors of the same application can exchange messages with the MIPC service. Furthermore, these messages can be allocated initially and sent by interrupt handlers for later processing in the context of threads.

The MIPC option supports the following:

• Multiple sets of dynamically allocated messages

• Messages of configurable size

Message spaces

The MIPC service is designed around the concept of message spaces, that encapsulates, within a single entity, both a set of message pools shared by all the actors of the application and a set of message queues through which these actors exchange messages allocated from the shared message pools.

Each message pool is defined by a pair of characteristics (message size, number of messages) provided by the application when it creates the message space. The configuration of the set of message pools depends on the communication needs of the application. From the point of view of the application, message pool requirements depend on the size range of the messages exchanged by the application, and the distribution of messages within the size range.

A message space is a temporary resource that must be created explicitly by the application. Once created, a message space can be opened by other actors of the application. A message space is bound to the actor that creates it, and it is deleted automatically when its creating actor and all actors that opened it have been deleted.

When an actor opens a message space, the system first assigns a private identifier to the message space. This identifier is returned to the calling actor and is used to designate the message space in all functions of the interface. The shared message pools are then mapped in the address space of the actor, at an address chosen automatically by the system.

Messages and queues

A message is simply an array of bytes that can be structured and manipulated at application level through any appropriate convention. Messages are presented to actors as pointers in their addressing spaces.

Messages are posted to message queues. Inside a message space, a queue is designated by an unsigned integer that corresponds to its index in the set of queues. Messages can also have priorities.

All resources of a message space are shared without any restriction by all actors of the application that open it. Any of these actors can allocate messages from the shared message pools. In the same way, all actors have both send and receive rights on each queue of the message space. Most applications only need to create a single message space. However, the MIPC service is designed to allow an application to create or open multiple message spaces. Inside these types of applications, messages cannot be exchanged directly across different message spaces. In other words, a message allocated from a message pool of one message space cannot be sent to a queue of another message space.

For details of the MIPC feature, see the MIPC(5FEA) man page.

The MIPC API is summarized in the following table: