These are flags that specify attributes of the message, or control its processing. The flags are divided into the following categories:
Segmentation flags: When a message is too big for a queue, an attempt to put the message on the queue usually fails. Segmentation is a technique whereby the queue manager or application splits the message into smaller pieces called segments, and places each segment on the queue as a separate physical message. The application that retrieves the message can either retrieve the segments one by one, or request the queue manager to reassemble the segments into a single message that is returned by the MQGET call. The latter is achieved by specifying the MQGMO_COMPLETE_MSG option on the MQGET call, and supplying a buffer that is big enough to accommodate the complete message. (See MQGMO - Get-message options for details of the MQGMO_COMPLETE_MSG option.) A message can be segmented at the sending queue manager, at an intermediate queue manager, or at the destination queue manager.
You can specify one of the following to control the segmentation of a message:
The value of this flag is binary zero. This is the default.
When the queue manager segments a message, the queue manager turns on the MQMF_SEGMENT 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 MQMF_LAST_SEGMENT flag in the MQMD that is sent with the segment.
The queue manager splits messages into segments as necessary so that the segments (plus any required header data) 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 might be of unequal length. The queue-manager processes the message as follows:
The second or later segment generated by the queue manager begins with one of the following:
The queue manager follows the above rules, but otherwise splits messages as it thinks fit; you cannot assume that the queue manager splits a message in a particular way.
For persistent messages, the queue manager can perform segmentation only within a unit of work:
For nonpersistent messages, the queue manager does not require a unit of work to be available in order to perform segmentation.
Take special care when converting data in messages that might be segmented:
If the nature of the data in the message is such that arbitrary segmentation of the data on 16-byte boundaries might result in segments that cannot be converted by the exit, or the format is MQFMT_STRING and the character set is DBCS or mixed SBCS/DBCS, the sending application must create and put the segments, specifying MQMF_SEGMENTATION_INHIBITED 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.
This flag is an input flag on the MQPUT and MQPUT1 calls, and an output flag on the MQGET call. On the latter call, the queue manager also echoes the value of the flag to the Segmentation field in MQGMO.
The initial value of this flag is MQMF_SEGMENTATION_INHIBITED.
Status flags: These are flags that indicate whether the physical message belongs to a message group, is a segment of a logical message, both, or neither. One or more of the following can be specified on the MQPUT or MQPUT1 call, or returned by the MQGET call:
If this flag is set, the queue manager turns on MQMF_MSG_IN_GROUP in the copy of MQMD that is sent with the message, but does not alter the settings of these flags in the MQMD provided by the application on the MQPUT or MQPUT1 call.
It is valid for a group to consist of only one logical message. If this is the case, MQMF_LAST_MSG_IN_GROUP is set, but the MsgSeqNumber field has the value one.
When MQMF_SEGMENT is specified without MQMF_LAST_SEGMENT, the length of the application message data in the segment (excluding the lengths of any MQ header structures that might be present) must be at least one. If the length is zero, the MQPUT or MQPUT1 call fails with reason code MQRC_SEGMENT_LENGTH_ZERO.
On z/OS, this option is not supported if the message is being put on a queue that has an index type of MQIT_GROUP_ID.
If this flag is set, the queue manager turns on MQMF_SEGMENT in the copy of MQMD that is sent with the message, but does not alter the settings of these flags in the MQMD provided by the application on the MQPUT or MQPUT1 call.
A logical message can consist of only one segment. If this is the case, MQMF_LAST_SEGMENT is set, but the Offset field has the value zero.
When MQMF_LAST_SEGMENT is specified, the length of the application message data in the segment (excluding the lengths of any header structures that might be present) can be zero.
On z/OS, this option is not supported if the message is being put on a queue that has an index type of MQIT_GROUP_ID.
The application must ensure that these flags are set correctly when putting messages. If MQPMO_LOGICAL_ORDER is specified, or was specified on the preceding MQPUT call for the queue handle, the settings of the flags must be consistent with the group and segment information retained by the queue manager for the queue handle. The following conditions apply to successive MQPUT calls for the queue handle when MQPMO_LOGICAL_ORDER is specified:
Table 53 shows the valid combinations of the flags, and the values used for various fields.
These flags are input flags on the MQPUT and MQPUT1 calls, and output flags on the MQGET call. On the latter call, the queue manager also echoes the values of the flags to the GroupStatus and SegmentStatus fields in MQGMO.
Default flags: The following can be specified to indicate that the message has default attributes:
This inhibits segmentation, and indicates that the message is not in a group and is not a segment of a logical message. MQMF_NONE is defined to aid program documentation. It is not intended that this flag be used with any other, but as its value is zero, such use cannot be detected.
The MsgFlags field is partitioned into subfields; for details see Appendix E. Report options and message flags.
The initial value of this field is MQMF_NONE. This field is ignored if Version is less than MQMD_VERSION_2.
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