T-Kernel Specification (1.B0.02)

4.5. EXTENDED SYNCHRONIZATION AND COMMUNICATION FUNCTIONS 127
4.5.3 Rendezvous Port
Rendezvous is a function for synchronization and communication between tasks, supporting the procedures
for making processing requests by one task to another and for returning the processing result to
the requesting task. The object for which both of these tasks wait is called a rendezvous port. The
rendezvous function is typically used to realize task communication in a client/server model, but can
also support more flexible synchronization and communication models.
Functions are provided for creating and deleting a rendezvous port, issuing a processing request to a
rendezvous port (call rendezvous), accepting a processing request from a rendezvous port (accept rendezvous),
returning the processing result (reply rendezvous), forwarding an accepted processing request
to another rendezvous port (forward rendezvous to other port), and referencing rendezvous port status
and rendezvous status. A rendezvous port is identified by an ID number called a rendezvous port ID.
A task issuing a processing request to a rendezvous port (the client-side task) calls a rendezvous, designating
a message (called a call message) with information about the rendezvous port, the rendezvous
conditions, and the processing being requested. The task accepting a processing request on a rendezvous
port (the server-side task) accepts the rendezvous, designating the rendezvous port and rendezvous conditions.
The rendezvous conditions are indicated in a bit pattern. If the bitwise logical AND of the bit patterns
on both sides (the rendezvous conditions bit pattern of the task calling a rendezvous for a rendezvous
port and the rendezvous conditions bit pattern of the accepting task) is not 0, the rendezvous is established.
The state of the task calling the rendezvous is WAIT on rendezvous call until the rendezvous is
established. The state of the task accepting a rendezvous is WAIT on rendezvous acceptance until the
rendezvous is established.
When a rendezvous is established, a call message is passed from the task that called the rendezvous to
the accepting task. The state of the task calling the rendezvous goes to WAIT for rendezvous completion
until the requested processing is completed. The task accepting the rendezvous is released from WAIT
state and it performs the requested processing. Upon completion of the requested processing, the task
accepting the rendezvous passes the result of the processing in a reply message to the calling task and
ends the rendezvous. At this point the WAIT state of the task that called the rendezvous is released.
A rendezvous port has separate queues for tasks waiting on rendezvous call (call queue) and tasks waiting
on rendezvous acceptance (accept queue). Note, however, that after a rendezvous is established, both
tasks that formed the rendezvous are detached from the rendezvous port. In other words, a rendezvous
port does not have a queue for tasks waiting for rendezvous completion. Nor does it keep information
about the task performing the requested processing.
T-Kernel assigns an object number to identify each rendezvous when more than one is established at the
same time. The rendezvous object number is called the rendezvous number. The method of assigning
rendezvous numbers is implementation-dependent, but at a minimum information must be included for
designating the task that called the rendezvous. Numbers must also be uniquely assigned to the extent
possible; for example, even if the same task makes multiple rendezvous calls, the first rendezvous and
second rendezvous must have different rendezvous numbers assigned.
[Additional Notes]
Rendezvous is a synchronization and communication function introduced in the ADA programming
language, based on Communicating Sequential Processes (CSP). In ADA, however, the rendezvous
function is part of the language specification and therefore has a different role than in T-Kernel, which is
a real-time kernel specification. The rendezvous ports provided by the real-time kernel are OS primitives
by which an ADA rendezvous capability is implemented. Since the ADA rendezvous function differs
from that in the T-Kernel specification in a number of ways, the T-Kernel-specification rendezvous
functions cannot necessarily be used to implement the ADA rendezvous.
Copyright c○ 2002, 2003 by T-Engine Forum
T-Kernel 1.B0.02