MDMFL (10-digit signed integer)

This option allows the message to be broken into segments by the queue manager. If specified for a message that is already a segment, this option allows the segment to be broken into smaller segments. MFSEGA can be set without either MFSEG or MFLSEG being set.

When the queue manager segments a message, the queue manager turns on the MFSEG flag in the copy of the MQMD that is sent with each segment, but does not alter the settings of these flags in the MQMD provided by the application on the MQPUT or MQPUT1 call. For the last segment in the logical message, the queue manager also turns on the MFLSEG flag in the MQMD that is sent with the segment.

Note:

Care is needed when messages are put with MFSEGA but without PMLOGO. If the message is:

Not a segment, and
Not in a group, and
Not being forwarded,

the application must remember to reset the MDGID field to GINONE prior to each MQPUT or MQPUT1 call, in order to cause a unique group identifier to be generated by the queue manager for each message. If this is not done, unrelated messages could inadvertently end up with the same group identifier, which might lead to incorrect processing subsequently. See the descriptions of the MDGID field and the PMLOGO option for more information about when the MDGID field must be reset.

The queue manager splits messages into segments as necessary in order to ensure that the segments (plus any header data that may be required) fit on the queue. However, there is a lower limit for the size of a segment generated by the queue manager (see below), and only the last segment created from a message can be smaller than this limit. (The lower limit for the size of an application-generated segment is one byte.) Segments generated by the queue manager may be of unequal length. The queue manager processes the message as follows:

User-defined formats are split on boundaries which are multiples of 16 bytes. This means that the queue manager will not generate segments that are smaller than 16 bytes (other than the last segment).
Built-in formats other than FMSTR are split at points appropriate to the nature of the data present. However, the queue manager never splits a message in the middle of an MQ header structure. This means that a segment containing a single MQ header structure cannot be split further by the queue manager, and as a result the minimum possible segment size for that message is greater than 16 bytes.
The second or later segment generated by the queue manager will begin with one of the following:
- An MQ header structure
- The start of the application message data
- Part-way through the application message data
FMSTR is split without regard for the nature of the data present (SBCS, DBCS, or mixed SBCS/DBCS). When the string is DBCS or mixed SBCS/DBCS, this may result in segments which cannot be converted from one character set to another (see below). The queue manager never splits FMSTR messages into segments that are smaller than 16 bytes (other than the last segment).
The MDFMT, MDCSI, and MDENC fields in the MQMD of each segment are set by the queue manager to describe correctly the data present at the start of the segment; the format name will be either the name of a built-in format, or the name of a user-defined format.
The MDREP field in the MQMD of segments with MDOFF greater than zero are modified as follows:
- For each report type, if the report option is RO*D, but the segment cannot possibly contain any of the first 100 bytes of user data (that is, the data following any MQ header structures that may be present), the report option is changed to RO*.

The queue manager follows the above rules, but otherwise splits messages as it thinks fit; no assumptions should be made about the way that the queue manager will choose to split a particular message.

For persistent messages, the queue manager can perform segmentation only within a unit of work:

If the MQPUT or MQPUT1 call is operating within a user-defined unit of work, that unit of work is used. If the call fails partway through the segmentation process, the queue manager removes any segments that were placed on the queue as a result of the failing call. However, the failure does not prevent the unit of work being committed successfully.
If the call is operating outside a user-defined unit of work, and there is no user-defined unit of work in existence, the queue manager creates a unit of work just for the duration of the call. If the call is successful, the queue manager commits the unit of work automatically (the application does not need to do this). If the call fails, the queue manager backs out the unit of work.
If the call is operating outside a user-defined unit of work, but a user-defined unit of work does exist, the queue manager is unable to perform segmentation. If the message does not require segmentation, the call can still succeed. But if the message does require segmentation, the call fails with reason code RC2255.

For nonpersistent messages, the queue manager does not require a unit of work to be available in order to perform segmentation.

Special consideration must be given to data conversion of messages which may be segmented:

If data conversion is performed only by the receiving application on the MQGET call, and the application specifies the GMCMPM option, the data-conversion exit will be passed the complete message for the exit to convert, and the fact that the message was segmented will not be apparent to the exit.
If the receiving application retrieves one segment at a time, the data-conversion exit will be invoked to convert one segment at a time. The exit must therefore be capable of converting the data in a segment independently of the data in any of the other segments.
If the nature of the data in the message is such that arbitrary segmentation of the data on 16-byte boundaries may result in segments which cannot be converted by the exit, or the format is FMSTR and the character set is DBCS or mixed SBCS/DBCS, the sending application should itself create and put the segments, specifying MFSEGI to suppress further segmentation. In this way, the sending application can ensure that each segment contains sufficient information to allow the data-conversion exit to convert the segment successfully.
If sender conversion is specified for a sending message channel agent (MCA), the MCA converts only messages which are not segments of logical messages; the MCA never attempts to convert messages which are segments.