Opening distribution lists

Use the MQOPEN call to open a distribution list, and use the options of the call to specify what you want to do with the list.

As input to MQOPEN, you must supply:
  • A connection handle (see Putting messages on a queue for a description)
  • Generic information in the Object Descriptor structure (MQOD)
  • The name of each queue that you want to open, using the Object Record structure (MQOR)
The output from MQOPEN is:
  • An object handle that represents your access to the distribution list
  • A generic completion code
  • A generic reason code
  • Response Records (optional), containing a completion code and reason for each destination

Using the MQOD structure

Use the MQOD structure to identify the queues that you want to open.

To define a distribution list, you must specify MQOD_VERSION_2 in the Version field, a value greater than zero in the RecsPresent field, and MQOT_Q in the ObjectType field. See MQOD for a description of all the fields of the MQOD structure.

Using the MQOR structure

Provide an MQOR structure for each destination.

The structure contains the destination queue and queue manager names. The ObjectName and ObjectQMgrName fields in the MQOD are not used for distribution lists. There must be one or more object records. If the ObjectQMgrName is left blank, the local queue manager is used. See ObjectName and ObjectQMgrName for further information about these fields.

You can specify the destination queues in two ways:
  • By using the offset field ObjectRecOffset.

    In this case, the application must declare its own structure containing an MQOD structure, followed by the array of MQOR records (with as many array elements as are needed), and set ObjectRecOffset to the offset of the first element in the array from the start of the MQOD. Ensure that this offset is correct.

    Use of built-in facilities provided by the programming language is recommended, if these are available in all the environments in which the application runs. The following code illustrates this technique for the COBOL programming language: 
    
 01  MY-OPEN-DATA.
     02 MY-MQOD.
        COPY CMQODV.
     02 MY-MQOR-TABLE OCCURS 100 TIMES.
        COPY CMQORV.
     MOVE LENGTH OF MY-MQOD TO MQOD-OBJECTRECOFFSET.
    Alternatively, use the constant MQOD_CURRENT_LENGTH if the programming language does not support the necessary built-in facilities in all the environments concerned. The following code illustrates this technique: 
    
 01  MY-MQ-CONSTANTS.
     COPY CMQV.
 01  MY-OPEN-DATA.
     02 MY-MQOD.
        COPY CMQODV.
     02 MY-MQOR-TABLE OCCURS 100 TIMES.
        COPY CMQORV.
     MOVE MQOD-CURRENT-LENGTH TO MQOD-OBJECTRECOFFSET.
    However, this works correctly only if the MQOD structure and the array of MQOR records are contiguous; if the compiler inserts skip bytes between the MQOD and the MQOR array, these must be added to the value stored in ObjectRecOffset.

    Using ObjectRecOffset is recommended for programming languages that do not support the pointer data type, or that implement the pointer data type in a way that is not portable to different environments (for example, the COBOL programming language).

  • By using the pointer field ObjectRecPtr.
    In this case, the application can declare the array of MQOR structures separately from the MQOD structure, and set ObjectRecPtr to the address of the array. The following code illustrates this technique for the C programming language: 
    
MQOD MyMqod;
MQOR MyMqor[100];
MyMqod.ObjectRecPtr = MyMqor;

    Using ObjectRecPtr is recommended for programming languages that support the pointer data type in a way that is portable to different environments (for example, the C programming language).

Whichever technique you choose, you must use one of ObjectRecOffset and ObjectRecPtr ; the call fails with reason code MQRC_OBJECT_RECORDS_ERROR if both are zero, or both are nonzero.

Using the MQRR structure

These structures are destination-specific; each Response Record contains a CompCode and Reason field for each queue of a distribution list. You must use this structure to enable you to distinguish where any problems lie.

For example, if you receive a reason code of MQRC_MULTIPLE_REASONS and your distribution list contains five destination queues, you will not know which queues the problems apply to if you do not use this structure. However, if you have a completion code and reason code for each destination, you can locate the errors more easily.

See MQRR for further information about the MQRR structure.

Figure 1 shows how you can open a distribution list in C.
Figure 1. Opening a distribution list in C. The MQOD uses pointers to the MQOR and MQRR structures.
The diagram shows an MQOD (Object Descriptor) data structure. The version number is identified as 2. The structure contains: the number of MQOR and MQRR entries; a pointer to a separate MQOR (Object Record) structure; a pointer to a separate MQRR (Response Record) structure. These structures are also shown, and each contains a list with the same number of entries. The number was given in the MQOD. The MQOR entries have fields for destination queue name and queue manager name. The MQRR entries have fields for completion code and reason code.
Figure 2 shows how you can open a distribution list in COBOL.
Figure 2. Opening a distribution list in COBOL. The MQOD uses offsets in COBOL.
The diagram shows an MQOD (Object Descriptor) data structure. The version number is identified as 2. The structure contains two offsets that identify where in the MQOD the list of MQOR (Object Record) and MQRR (Response Record) values are found. The MQOR entries have fields for destination queue name and queue manager name. The MQRR entries have fields for completion code and reason code.

Using the MQOPEN options

You can specify the following options when opening a distribution list:
  • MQOO_OUTPUT
  • MQOO_FAIL_IF_QUIESCING (optional)
  • MQOO_ALTERNATE_USER_AUTHORITY (optional)
  • MQOO_*_CONTEXT (optional)

See Opening and closing objects for a description of these options.