= Message Passing =

First attempt:

{{{
$input int NPROCS;
$assume NPROCS >= 1;
$scope top;
$heap mp_heap;
$proc procs[NPROCS];

/* There will be one message queue for each pair of prods (i,j).
 * Each queue is a doubly-linked list of Message objects.
 */
typedef struct Message {
  struct Message *<top> next;
  struct Message *<top> prev;
  int tag;
  int size;
  void *<top> data;
} Message;

typedef struct Comm_struct {
  Message *<top> buf_front[NPROCS][NPROCS];
  Message *<top> buf_back[NPROCS][NPROCS];
} Comm_struct;

typedef Comm_struct *Comm;

/* As in MPI, when a receive returns, this structure
 * tells you the source and tag of received message,
 * which you need if you used wildcards,  Also size.
 */
typedef struct Status {
  int source;
  int tag;
  int size;
} Status;

Comm _Comm_world;

/* The user will use COMM_WORLD */
Comm *<top> COMM_WORLD = &_Comm_world;



void init() {
  for (int i=0; i<NPROCS; i++)
    procs[i] = $spawn proc(i);
  for (int i=0; i<NPROCS; i++)
    for (int j=0; j<NPROCS; j++) {
      COMM_WORLD->buf_front[i][j] = NULL;
      COMM_WORLD->buf_back[i][j] = NULL;
    }
}    

void send(int source, void *buf, int size, int tag, int dest, Comm comm) {
  // create a message
  Message message;

  message.tag = tag;
  message.size = size;
  message.data = $malloc<top>(&mp_heap, size);
  memcpy(message.data, buf, size);
  // enqueue on comm->buf_front[i][j] …
  // update  message.next, message.prev, buf_back[i][j]
}

// can I use a function as a guard?  not now
// can always use busy-wait loop?
boolean probe(…) {

}

void recv(int dest, void *buf, int size, int tag, int source, Comm comm, Status *status) {
  Message *message;  
  int message_source;
  int message_dest;
  // search the queue looking for the message
  // also set message_source, message_dest;
  // and remove message from the queue
  
  if (message.size > size) error…;
  memcpy(buf, message.data, message.size);
  status->size = message.size;
  status->source = message_source;
  status->dest = message_dest;
  $free(message->data);
}

}}}