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ExitSpace (MQLONG)

 

Number of bytes in transmission buffer reserved for exit to use.

This field is relevant only for a send exit. It specifies the amount of space in bytes that the MCA will reserve in the transmission buffer for the exit to use. This allows the exit to add to the transmission buffer a small amount of data (typically not exceeding a few hundred bytes) for use by a complementary receive exit at the other end. The data added by the send exit must be removed by the receive exit.

The value is always zero on z/OS.

This facility should not be used to send large amounts of data, as this may degrade performance, or even inhibit operation of the channel.

By setting ExitSpace the exit is guaranteed that there will always be at least that number of bytes available in the transmission buffer for the exit to use. However, the exit can use less than the amount reserved, or more than the amount reserved if there is space available in the transmission buffer. The exit space in the buffer is provided following the existing data.

ExitSpace can be set by the exit only when ExitReason has the value MQXR_INIT; in all other cases the value returned by the exit is ignored. On input to the exit, ExitSpace is zero for the MQXR_INIT call, and is the value returned by the MQXR_INIT call in other cases.

If the value returned by the MQXR_INIT call is negative, or there are fewer than 1024 bytes available in the transmission buffer for message data after reserving the requested exit space for all of the send exits in the chain, the MCA outputs an error message and closes the channel. Similarly, if during data transfer the exits in the send exit chain allocate more user space than they reserved such that fewer than 1024 bytes remain in the transmission buffer for message data, the MCA outputs an error message and closes the channel. The limit of 1024 allows the channel’s control and administrative flows to be processed by the chain of send exits, without the need for the flows to be segmented.

This is an input/output field to the exit if ExitReason is MQXR_INIT, and an input field in all other cases. The field is not present if Vis less than MQCXP_VERSION_5.

 

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