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Section H3)

Implementing abort/cancel for outstanding messages (abort)

The -core v1.0 language and implementation do not support the cancellation of outstanding messages. We denote a message send by an asynchronous call with an explicit after to be outstanding, unless it has already started to execute. I.e., a released but not yet running message may be aborted/cancelled.
(Implementing abort is yet another assignment for the students in Compiler Construction at LTU, fall 2014.)

Let us first have a look at an example program using both pend and abort

// TestAbort.core
// Per Lindgren (C) 2014
// Example showing the use of both pend and abort
Task task_to_abort() {
	#> printf("Task to abort, not aborted\n");<#

Task task_print(char c) {
	#> printf("Char entered %c, Current time !!!! %f\n", c, RT_time_to_float(RT_get_bl()));<#

Idle {
	#> printf("User Idle\n"); <#
	m := async after 5s task_to_abort();
	char c = getc(stdin);
	abort (m);
	pend before 1s task_print(c);

We see that, in order to later abort a message, we need a "hook" to the message/task instance.
m := async after 5s task_to_abort();

To this end we introduce the assignment operator := and the additional syntax tin the language:

  | #> C-code <#                                  
  | "async" after? before? ID ( C-params ) ;  
  | ID ":=" "async" after? before? ID ( C-params ) ;  

(Un-hooked async should still be possible. alternatively we could write:)
  | #> C-code <#                                  
  | (ID ":=")? "async" after? before? ID ( C-params ) ;  

You may start by altering the Parser (add ASSIGN) and give a rule for it in the
Second thing is to add an additional field (string) to the statement constructor Async.

Similar to the halt (Section H.1) you will need to adopt a few more files in the compiler.
After that you should be able to get the above TestAbort.core to be accepted.

What about code generation?

Assume there is a primitive in the run-time that aborts a message (given a hook). (We will provide you with PTCORE/WINCORE doing exactly that.)

"RT_abort(" ^ par ^ ");" 

Also you may assume there is a type for the hook, given in RTFM-RT.h

The prototype for the RTFM_async is given as:
RTFM_msg RTFM_async(RTFM_time, RTFM_time, int, int);

Your assignment is now to look at the code generation for the Async statement, and adopt it such that it:
  1. defines the C-variable (given as an ID from the grammar) as a variable of type RTFM_msg
  2. sets the message parameters (look at the existing code)
  3. assigns the message hook to the result of RTFM_async

After doing this successfully, the example program should compile and run according to the semantics given for abort.

Per Lindgren, September 8th.

Last edited Sep 7, 2014 at 11:58 PM by RTFMPerLindgren, version 6