IBM Support

IT19194: Core file from SIGSEGV in zswLock under MQCONNX

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APAR status

  • Closed as program error.

Error description

  • After upgrading from MQ V8 to V9.0.0.0 a 32-bit application
    program that had been working without trouble is now
    consistently reporting SIGSEGV in zswLock upon its first MQCONNX
    attempt. No MQ FDCs reported.
    The application had one or both of:
    - relatively small thread stack size
    - relatively high use of thread stack before calling MQCONNX
    (dbx) where
    zswLock() at 0xd210a8cc
    zswCheckAndInit() at 0xd21b0154
    zswGetEntryPointsByName() at 0xd21b5eac
    zswMQCONNX_Client_Call() at 0xd21e1540
    MQCONNX() at 0xd1ae0f4c

Local fix

  • There are two workarounds to this problem:
    1. Tell the system to use bigger thread stack by tuning the
    thread support tuneable parameter AIXTHREAD_STK  as seen in
    example 15 in the AIX 6.1 Knowledge Center.  Example:
    export AIXTHREAD_STK=130000
    2. Build the application as a 64 bit application using -q64
    compile read stacks should be available by default in this case.

Problem summary

  • ****************************************************************
    Users running 32-bit MQ application programs or any MQ
    application program with one or both of:
    - relatively small thread stack size
    - relatively high use of thread stack before calling MQCONNX
    Platforms affected:
    Stack space required by the zswLock() function has grown by
    approximately 45 KB in MQ compared to previous releases.
     When MQCONNX processing called this function, this caused the
    thread stack usage to grow close to its configured limit within
    the application process where the MQ code was running.  The
    SIGSEGV memory exception occurred while zswLock() was increasing
    the stack further as it was preparing to make a call to another
    It is possible to respond to this situation by configuring the
    system to allocate bigger thread stack space for the application
    process (for example by using the AIXTHREAD_STK environment
    variable on AIX).  However, this 45 KB memory block need not be
    allocated from the stack, and instead can easily be allocated
    from the process heap instead, so a change to IBM MQ product
    code is preferable, to avoid the issue occurring.

Problem conclusion

  • IBM MQ has been changed to allocate the 45 KB memory from
    process heap instead of from the thread's stack space.
    The fix is targeted for delivery in the following PTFs:
    Version    Maintenance Level
    v9.0 CD    9.0.3
    v9.0 LTS
    The latest available maintenance can be obtained from
    'WebSphere MQ Recommended Fixes'
    If the maintenance level is not yet available information on
    its planned availability can be found in 'WebSphere MQ
    Planned Maintenance Release Dates'

Temporary fix


APAR Information

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  • Reported component ID


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  • Status


  • PE




  • Special Attention

    NoSpecatt / Xsystem

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

Fix information

  • Fixed component name


  • Fixed component ID


Applicable component levels

  • R900 PSY


[{"Business Unit":{"code":"BU053","label":"Cloud & Data Platform"},"Product":{"code":"SSYHRD","label":"IBM MQ"},"Component":"","ARM Category":[],"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"9.0","Edition":"","Line of Business":{"code":"LOB45","label":"Automation"}}]

Document Information

Modified date:
04 December 2017