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A fix is available


You can track all active APARs for this component.


APAR status

  • Closed as program error.

Error description

  • The channel which encounters a problem is a multiplexed SVRCONN
    with SSL enabled.
    The following processing occurred in the run-up to the 0C4
    - The channel has several conversations active, but none of
    them have been actively doing any MQ work for some time.
    - As there has been no proper activity on the channel, the last
    few requests are all heartbeat flows which check that the
    channel connection is still ok while no real work is being done.
    - Because the channel is using SSL, and the qmgr is configured
    with an SSL key reset value, the first piece of work after a
    heartbeat results in an SSL key reset being negotiated.
    - The last request from the client before the abend is the
    start of the processing for an SSL key reset, asking the server
    to pause channel activity (e.g. asynchronous consumers) so that
    the key reset can proceed uninterrupted.
    - The failing function, ccxReceiveThreadFn, is responsible for
    processing incoming requests from the client, and has a loop
    which handles one request at a time. While processing the pause
    request, it attempts to obtain a value from a field in the
    session control block (TraceIdentifier). The pointer to the
    session block (psess) is invalid, resulting in the 0C4.
    For the pause request, the psess variable isn't initialized, so
    it would contain a residual value from a previous client
    request. Generally this would still be a valid session for one
    of the conversations on the channel so the pause would work ok
    in most circumstances.
    In this case the last request before the heartbeats was a
    request to end a conversation. This would have resulted in
    psess being set to the session block for the conversation which
    is ending, and the session block would have been freed as part
    of the clean-up of the conversation.

Local fix

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All users of IBM MQ for z/OS Version 9       *
    *                 Release 1 Modification 0 and Release 2       *
    *                 Modification 0.                              *
    * PROBLEM DESCRIPTION: When using a multiplexed SSL channel    *
    *                      with multiple conversations, if an SSL  *
    *                      key reset interval is specified, then a *
    *                      precise FAP flow involving a            *
    *                      conversation disconnecting can result   *
    *                      in an ABEND0C4 in the CHIN.             *
    As part of end of conversation processing, the CHIN frees
    control blocks which it no longer needs. A flaw in the
    multiplexed receiver code can result in one of these freed
    conversation blocks being reused. This code is primarily driven
    when performing an SSL key reset, and depending on whether the
    storage page has been released by RSM, an ABEND0C4 may occur.

Problem conclusion

  • The multiplexed receiver code has been changed to no longer
    reference freed MQ control blocks.

Temporary fix


APAR Information

  • APAR number


  • Reported component name

    IBM MQ Z/OS V9

  • Reported component ID


  • Reported release


  • Status


  • PE




  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date


  • Closed date


  • Last modified date


  • APAR is sysrouted FROM one or more of the following:

  • APAR is sysrouted TO one or more of the following:

    UI72042 UI72043



Fix information

  • Fixed component name

    IBM MQ Z/OS V9

  • Fixed component ID


Applicable component levels

  • R100 PSY UI72042

       UP20/12/04 P F012

  • R200 PSY UI72043

       UP21/01/04 P F012

Fix is available

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Document Information

Modified date:
05 January 2021