Message flags.
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 which retrieves the message can either retrieve the segments one by one, or request the queue manager to reassemble the segments into a single message which is returned by the MQGET call. The latter is achieved by specifying the GMCMPM 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 GMCMPM option.) Segmentation of a message can occur 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:
This option prevents the message being broken into segments by the queue manager. If specified for a message that is already a segment, this option prevents the segment being broken into smaller segments.
The value of this flag is binary zero. This is the default.
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.
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:
The second or later segment generated by the queue manager will begin with one of the following:
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:
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 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.
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 GMSEG field in MQGMO.
The initial value of this flag is MFSEGI.
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 MFMIG 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, MFLMIG is set, but the MDSEQ field has the value one.
When MFSEG is specified without MFLSEG, the length of the application message data in the segment (excluding the lengths of any MQ header structures that may be present) must be at least one. If the length is zero, the MQPUT or MQPUT1 call fails with reason code RC2253.
If this flag is set, the queue manager turns on MFSEG 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 logical message to consist of only one segment. If this is the case, MFLSEG is set, but the MDOFF field has the value zero.
When MFLSEG is specified, it is permissible for the length of the application message data in the segment (excluding the lengths of any header structures that may be present) to be zero.
The application must ensure that these flags are set correctly when putting messages. If PMLOGO 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 PMLOGO is specified:
Table 31 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 GMGST and GMSST 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. MFNONE 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 MDMFL field is partitioned into subfields; for details see Appendix D. Report options and message flags.
The initial value of this field is MFNONE. This field is ignored if MDVER is less than MDVER2.
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