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| Old data in message buffer on MQGET on z/OS |
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| Alyindar |
 Posted: Mon Jan 05, 2015 1:32 pm Post subject: Old data in message buffer on MQGET on z/OS |
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Newbie
Joined: 13 Mar 2008
Posts: 5
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I did some searching but did not see a related topic, my apologies if I missed one.
I'm working with an ISPF panel that displays a list of messages on the queue from a program written in COBOL. It strips away any extra headers and displays the first 40 characters of the message buffer. Unfortunately, if subsequent messages are shorter than the previous, the data spills into the next one. For example, I sent the following messages to a queue:
| Code: |
The quick red fox
jumped over the
lazy brown dog |
And here's the display:
| Code: |
Format Message Message
Name Length Contents
MQSTR 17 The quick red fox
MQSTR 15 jumped over theox
MQSTR 14 lazy brown dogeox |
The impression I'm getting is that the MSGBUFFER is not being flushed between subsequent MQGETs, but this is not a problem I've had in writing batch applications. Here's snippets of code where the buffer is defined and the MQGETs occur:
Working storage:
| Code: |
01 W01-FORMATNAME PIC X(8) VALUE SPACES.
01 W01-MSGLEN PIC Z(7)9 VALUE SPACES.
01 W01-MSGBUFF PIC X(40) VALUE SPACES.
01 W02-MSGBUFFER PIC X(600) VALUE SPACES.
01 W02-DATALENGTH PIC S9(09) BINARY.
01 W02-MAXMSGLENGTH PIC 9(6) VALUE 600.
01 MQM-MESSAGE-DESCRIPTOR.
COPY CMQMDV.
01 MQM-GET-MESSAGE-OPTIONS.
COPY CMQGMOV. |
Procedure division (first get happens outside of the loop with MQGMO-BROWSE-FIRST):
| Code: |
MOVE MQMD-FORMAT TO W01-FORMATNAME
MOVE W02-DATALENGTH TO W01-MSGLEN
MOVE W02-MSGBUFFER TO W01-MSGBUFF
CALL 'ISPLINK' USING ITBADD IMSG-TABLE
COMPUTE MQGMO-OPTIONS = MQGMO-BROWSE-NEXT +
MQGMO-NO-WAIT +
MQGMO-ACCEPT-TRUNCATED-MSG +
MQGMO-CONVERT +
MQGMO-NO-SYNCPOINT
MOVE MQMI-NONE TO MQMD-MSGID
MOVE MQCI-NONE TO MQMD-CORRELID
CALL 'MQGET' USING W01-HCONN
W01-HOBJ
MQMD
MQGMO
W02-MAXMSGLENGTH
W02-MSGBUFFER
W02-DATALENGTH
W02-COMPCODE
W02-REASON |
W01-MSGBUFF, W01-MSGLEN AND W01-FORMATNAME are VDEFINE'd to the correlating fields in the panel:
| Code: |
CALL 'ISPLINK' USING
IVDEFINE IFORMATNAME W01-FORMATNAME ICHAR I8.
CALL 'ISPLINK' USING
IVDEFINE IMSGLEN W01-MSGLEN ICHAR I8.
CALL 'ISPLINK' USING
IVDEFINE IMSGBUFF W01-MSGBUFF ICHAR I36.
CALL 'ISPLINK' USING ITBCREATE
IMSG-TABLE
IMSG-TABLE-KEY IMSG-TABLE-FIELDS
INOWRITE IREPLACE. |
I can prevent the problem by adding a line "MOVE SPACES TO W02-MSGBUFFER" before the "CALL 'MQGET'" line, but I'm trying to understand why this step is necessary when I've not had these problems with batch programs (for example, the CSQ4BVJ1 sample program in SCSQCOBS). Is there something specific to the MQ-ISPF relationship that I'm missing here? Any advice is greatly appreciated! |
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| tczielke |
| Posted: Mon Jan 05, 2015 1:42 pm Post subject: |
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Guardian
Joined: 08 Jul 2010
Posts: 941
Location: Illinois, USA
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You should take into account the length of the message returned in the DATALENGTH field of the MQGET, and only print those number of bytes in your buffer. Notice that the CSQ4BVJ1 sample program that you mentioned does that with reference modification:
| Code: |
DISPLAY W00-LOOP, ' : ', W00-DATALENGTH, ' : ',
W00-MSGBUFFER(1:W00-DATALENGTH)
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| bruce2359 |
| Posted: Mon Jan 05, 2015 4:13 pm Post subject: Re: Old data in message buffer on MQGET on z/OS |
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Poobah
Joined: 05 Jan 2008
Posts: 9469
Location: US: west coast, almost. Otherwise, enroute.
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| Alyindar wrote: |
The impression I'm getting is that the MSGBUFFER is not being flushed between subsequent MQGETs, but this is not a problem I've had in writing batch applications. |
You are correct in that MQ internals do NOT initialize (flush or clear) your MSGBUFFER - this is a programmer responsibility.
A best-practice is for your app to have a virgin MQMD (from COPYBOOK) that you use to initialize (clear) your working MQMD before the next MQGET. MOVE VIRGIN-MQMD TO MQM-MESSAGE-DESCRIPTOR.
_________________
I like deadlines. I like to wave as they pass by.
ב''ה
Lex Orandi, Lex Credendi, Lex Vivendi. As we Worship, So we Believe, So we Live. |
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| PaulClarke |
| Posted: Mon Jan 05, 2015 9:49 pm Post subject: |
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Grand Master
Joined: 17 Nov 2005
Posts: 1002
Location: New Zealand
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When MQ gives you a message it always tells you how big it is. In other words, how many of the bytes in the message buffer are relevant. It would be unnecessary and woefully inefficient if MQ initialised your message buffer (lets say set them all to zeroes or blanks) for every MQGET call. Suppose the buffer you give is 100MB long - would you really want MQ to set 100 million bytes to zero each time you retrieved a single 10 byte message?
Cheers,
Paul.
_________________
Paul Clarke
MQGem Software
www.mqgem.com |
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| Alyindar |
| Posted: Thu Jan 08, 2015 7:58 am Post subject: |
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Newbie
Joined: 13 Mar 2008
Posts: 5
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I missed that in the example, thanks for point that out tczielke.
I modified my program to use reference modification and everything is working perfectly. Thanks for the explanations folks! It's good to take off the sysadmin hat and don the programmer hat once in awhile, you just might learn something!  |
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