Echelon Neuron C - Manual (Page 138)

126 How Devices Communicate Using Application Messages
void msg_cancel(void);
This function cancels the message being built for the msg_out object and frees the
associated buffer, allowing another message to be constructed. It has no
parameters, and has no return value.
If a message is constructed but not sent before the task is exited, the message is
automatically canceled.
The first write operation to the msg_out object triggers automatic, implicit, buffer
allocation. Because all buffers could be busy at that moment, the time taken to
complete the first assignment is non-deterministic. For applications where a
non-deterministic waiting period is not acceptable, the msg_alloc( ) function is
supported and allows for explicit message allocation. The msg_alloc( ) function
does not wait for any pending transaction to complete, but returns immediately
with success or failure. See the Neuron C Reference Guide for more information
about this function.
Receiving a Message
You typically receive a message using the msg_arrives predefined event. You can
also use the msg_receive( ) function to receive a message.
The msg_arrives Event
The predefined event for receiving a message is msg_arrives. Its syntax is:
msg_arrives [( message-code )]
If a message arrives, this event evaluates to TRUE. You can optionally qualify
the event using a message code. In this case, the event is TRUE only when a
message arrives containing the specified code.
When mixing unqualified msg_arrives events with qualified msg_arrives events,
the #pragma scheduler_reset directive must be specified so that the unqualified
event when clause is processed after all the qualified event when clauses.
It is essential that your program contain a default case as shown in the example
below, to prevent an event queue lockup. This issue is explained in detail in
Importance of a Default When Clause on page 128.
A sample use of this event is shown in Listing 3 below.
Listing 3. Use of msg_arrives Event
#pragma scheduler_reset
when (msg_arrives(1))
{
io_out(sprinkler,
ON);
}
when (msg_arrives(2))
{
io_out(sprinkler,
OFF);
}
// default case for handling unexpected message codes
when (msg_arrives)