Experimental Physics and
Industrial Control System

Till Straumann <[email protected]> · Fri, 28 Jan 2011 19:38:27 -0600

On 01/28/2011 03:34 PM, Andrew Johnson wrote:

On Friday 28 January 2011 12:30:59 Eric Norum wrote:

 From the indenting style I infer that Marty Kraimer wrote the code and had
me commit it......

Marty *never* uses GNU-style function definitions (putting the routine name at
the beginning of a new line after the return type and any decorations), so I
still think this had to be your code.  He might have written the vxWorks
implementation, but I doubt that you would have listed yourself as the Author
if that was the case.

The vxWorks implementation doesn't appear to need anything like an eventNode,
and I'm wondering if the free-list is even necessary — couldn't the eventNode
just be allocated on the stack of the thread that's waiting?  Every call to
getEventNode() is followed after the epicsEventWait*() by a line that puts it
back on the eventFreeList.  I guess the important question is how much work it
would be to create a new epicsEventId every time.

Not sure an eventNode is necessary at all (see my earlier message).

This example is incomplete (only receiver blocks, queue length a power

of two etc.)

#define MOD_Q_LEN(x)  ((x) & ((1<<LD_MQ_LEN)-1))

struct MQ {
  epicsMutexId   mtx;
  void          *buf[ (1<<LD_MQ_LEN) ];
  unsigned       hd, tl, n_waiting;
  epicsEventId   sync;
};

int
mqSend( struct MQ *q, void *msg )
{
  epicsMutexLock( q->mtx );
  if ( (q->tl - q->hd) >= (1<<LD_MQ_LEN) ) {
    /* full */
    epicsMutexUnlock( q->mtx );
    return -1;
  }

  /* store message */
  q->buf[ MOD_Q_LEN( q->tl ) ];
  q->tl++;

  if ( q->n_waiting ) {
    /* blocked threads */
    q->n_waiting--
    epicsEventSignal( q->sync );
  }

  epicsMutexUnlock( q->mtx );

  return 0;
}

void *
mqRecv( struct MQ *q )
{
void *msg;

  epicsMutexLock( q->mtx );

  /* Current implementation of epicsEventWait() using
   * pthread_cond_wait() may spuriously unblock multiple
   * threads as a result of a single epicsEventSignal().
   * See pthread_cond_signal(3)
   */
  while ( q->hd == q->tl ) {
    q->n_waiting++; /* increment count of blocked threads */
    epicsMutexUnlock( q->mtx );
    epicsEventWait( q->sync );
  }

  msg = q->buf[ MOD_Q_LEN( q->hd ) ];

  q->hd++;

  epicsMutexUnlock( q->mtx );

  return msg;
}

FWIW
-- Till

- Andrew

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System