Announcement

**Michael Mattias** · 5 Oct 2021, 09:31 AM

You may have collisions in the Queue between queuing and dequeueing. I had asked PB about this a number of years ago and they told me if I wanted it thread-safe I would have to code it myself. So I came up with ...

Code:

...


#IF %DEF(%UNICODE)
       MACRO zSTR = WSTRINGZ
        MACRO bSTR = WSTRING
#ELSE
       MACRO zSTR = ASCIIZ
       MACRO bSTR = STRING
#ENDIF
...
...
' -----------------------------------
' QUEUE RELATED ITEMS
' -----------------------------------
' queue access

ENUM QueueAction
    CREATE
    CLEAR
     enqueue
    dequeue
    COUNT
    DESTROY
END ENUM
.....
' ===================
' Access the Queue
' --------------------
'  Written as separate function to ensure it is threadsafe. AS we are enqueueing and dequeueing from
'  multiple threads of execution it would be really easy to get "crossed up"  if we did not use
'  sme kind of syncronization
' ACTION CODE RETURNS
' on DEQUUE, returns the String representation of the Queue Data
' on ENQUEU, accepts QD AS THe Data to be queued
' other actions do not use the second param
' String return:
'   DEQUEUE ==> QUEUEDATATYPE as a character string (may be TYPE SET or LSET directory into a UDT)
'   ENQUEUE ==> ???
'   CLEAR   ==> ????
'   COUNT  ==> FORMAT$(count) (use VAL() to get number)
'   DESTROY ==> empties and then destroys returns "0"


' 6/15/21 I could keep a count as I enquee and dequue  but method count is always available.
' 6/18/21 I could just make this THREADSAFE (even though I do not ike how it's done)


FUNCTION Queue_Action (ActionCode AS LONG, OPT QD AS QUEUEDATATYPE ) AS BSTR

STATIC oQ  AS IQUEUECOLLECTION
STATIC CS   AS CRITICAL_SECTION, bCS  AS LONG
LOCAL vUDT AS VARIANT,s AS BSTR, n AS LONG

' if first call to this function, set up the CRITCAL_SECTION

 ON ERROR GOTO ERRORTRAPPER_QUEUE_ACTION

   IF ISFALSE bCS THEN
       InitializeCriticalSection CS
       bCS = %TRUE
   END IF


  IF ActionCode = %QUeueAction.Destroy THEN
      oq = NOTHING
      'DeleteCriticalSection CS  NOPE CAUSES PROBLEM IF CS is currently owned. Accept overhead.
      'bCS = %FALSE
  ELSE
      EnterCriticalSection CS

      SELECT CASE AS LONG ActionCode
         CASE %QueueAction.Create
               oq = CLASS "QueueCollection"
         CASE %QueueAction.CLEAR
             oq.Clear
         CASE %QueueAction.ENQUEUE
             oq.Enqueue QD AS STRING
         CASE %QueueAction.Dequeue
             vUDt = OQ.Dequeue
             S    = VARIANT$(BYTE,vUDT)
             FUNCTION = S
         CASE %QUEUEACTION.Count
                n = OQ.Count
                FUNCTION = FORMAT$(n)
         CASE %QUEUEACTION.Destroy
             OQ.Clear                ' empty it out
             oQ = NOTHING            ' release object

      END SELECT
      LeaveCriticalSection CS
  END IF

  'function return assigned if used above

  EXIT FUNCTION

ERRORTRAPPER_QUEUE_ACTION:
  MSGBOX USING$("ActionCode # ErrNo # &", ActionCode, ERR, ERROR$(ERRCLEAR)),,"Queue_Action_error"
  RESUME  NEXT


END FUNCTION  ' Queue_Action

.. which is called from multiple threads of execution. By forcing all queue actions to use the same function, I can control the queue access with a single CRITICAL_SECTION object and guarantee myself no collisions... and to date, no data corruption.

**Ganeshan Kalmane** · 6 Oct 2021, 12:41 AM

Yes. I too figured that this might be the case and wrote and used the following functions.
But problem persisted.

Code:

'-----------------------------------------------------------------------------------------
'FUNCTION DataPacket1QueueCOunt () THREADSAFE AS LONG
'Returns count of elements in Queue
'-----------------------------------------------------------------------------------------
FUNCTION DataPacket1QueueCOunt () THREADSAFE AS LONG

LOCAL L1 AS LONG
EnterCriticalSection g_CSDatapacket
L1=DataPacket1Queue.Count
LeaveCriticalSection g_CSDatapacket
FUNCTION=L1
END FUNCTION

'-------------------------------------------------------------------------------------------------------------------
'FUNCTION Datapacket1ReadWrite(sReadWrite AS STRING) THREADSAFE AS LONG
'Call with "R" or "W"
'Takes/returns queuelement in global variable
'Returns 1 if success, 0 if fail
'-------------------------------------------------------------------------------------------------------------------
FUNCTION Datapacket1ReadWrite(sReadWrite AS STRING) THREADSAFE AS LONG

LOCAL v1 ,v2 AS VARIANT
LOCAL s1 AS STRING
LOCAL L1 AS LONG
v1=0
FUNCTION=0
s1=UCASE$(sReadWrite)

EnterCriticalSection g_CSDatapacket

SELECT CASE s1
      CASE "W": 'Write
                  LET v1=PacketToQue AS STRING
                  DataPacket1Queue.Enqueue(v1)
                  FUNCTION=1
                  EXIT SELECT

      CASE "R": 'read
                  L1=DataPacket1QueueCount
                  IF L1<1 THEN
                      FUNCTION=0
                      EXIT SELECT
                  END IF
                  v1=Datapacket1Queue.Dequeue
                  IF OBJRESULT=%S_FALSE OR VARIANTVT(v1)=%VT_EMPTY THEN
                       FUNCTION=0
                       EXIT SELECT
                  END IF
                  PacketFromQue=VARIANT$(BYTE,v1)
                  FUNCTION=1
                  EXIT SELECT
 END SELECT

LeaveCriticalSection g_CSDatapacket

END FUNCTION

Announcement

Probelm with QueueCollection?

Probelm with QueueCollection?

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