4.4.3 Receiving

Sending, mentioned the <send_stmt>’s UNSUSPECTING keyword, which specifies that the targets should not implicitly perform a receive operation. Because every send must have a matching receive, coNCePTuaL offers a RECEIVE statement that explicitly receives a set of messages. A <receive_stmt> is much like a <send_stmt> (see Sending) with the <source_task> and <target_tasks> in the reverse order:

<target_tasks> is described in Target tasks; <message_spec> is described in Message specifications; and, <source_task> is described in Source tasks.

Like other statements that contain both a <source_task> and a <target_tasks>, the RECEIVE statement propagates all variables declared by the <source_task> to the <target_tasks>, as in

TASKS src-1 RECEIVE A 1 MEGABYTE MESSAGE FROM ALL TASKS src.

However, the RECEIVE statement is unique among coNCePTuaL statements in that it can alternatively propagate all variables declared by the <target_tasks> to the <source_task>, as in

ALL TASKS dst RECEIVE A 1 MEGABYTE MESSAGE FROM TASKS dst+1.

The RECEIVE statement automatically determines the direction in which variables should be propagated. If the direction cannot unambiguously be determined, the coNCePTuaL compiler aborts with an error message. For example, the RECEIVE statement in the following code is ambiguous because the ‘bbb’ in the expression ‘aaa < bbb’ can refer to either the one in the LET statement or the one in the ‘TASKS bbb SUCH THAT bbb > 3’ expression:

LET bbb BE 5 WHILE {
  TASKS aaa SUCH THAT aaa < bbb RECEIVE A MESSAGE FROM TASKS bbb SUCH
  THAT bbb > 3 THEN
  TASKS ccc SUCH THAT ccc > 3 SEND A MESSAGE TO UNSUSPECTING TASKS
  ddd SUCH THAT ddd < ccc
}

For each message sent via a SEND… TO UNSUSPECTING statement there must be a RECEIVE statement that receives a message of the same size. The <target_tasks>’s <message_spec> can, however, specify different values for message uniqueness, message alignment, and data touching. In addition, the source and target do not need to agree on the use of the ASYNCHRONOUSLY keyword. The only restriction is that WITH VERIFICATION will return spurious results if used by the target but not by the source. Hence, the following <send_stmt> and <receive_stmt> correctly match each other:

TASK 0 SENDS 3 4 KILOBYTE MESSAGES TO UNSUSPECTING TASK 1

TASK 1 ASYNCHRONOUSLY RECEIVES 3 UNIQUE 4 KILOBYTE 48 BYTE ALIGNED
MESSAGES WITH DATA TOUCHING FROM TASK 0.

In general, it is better to use a single SEND statement with a WHO RECEIVES IT clause (see Sending) than a RECEIVE plus a matching SEND… TO UNSUSPECTING; the former is less error-prone than the latter. However, the latter is useful for programs in which a set of receives is posted, then the tasks perform various communication, computation, and synchronization operations, and—towards the end of the program—the matching sends are posted. That sort of split-phase structure requires separate SEND and RECEIVE statements.