If you are using feedback from the APPCCMD macroinstruction, remember that at times this notification is not available. If the application program is not participating in a conversation, or is participating in a conversation but has no APPCCMD macroinstructions outstanding, the application program cannot be informed of the receipt of an FMH-5 through the feedback fields in the RPL.

If you are using the ATTN exit, be aware that the information on the length of the FMH-5, shown in the RPL extension, might not reflect the length of the next FMH-5 to be received. When set by VTAM®, the RPL extension FMH5LEN field contains the length of the longest FMH-5, if FMH-5s are queued. When an ATTN exit is driven, the length of the FMH-5 for which the ATTN exit is driven is found in the same RPL extension feedback field (the exit's parameter list points to a read-only RPL). If a queue of FMH-5s has formed, this will not be the next one to be received. Any problems with the length can be avoided by reserving 255 bytes for any FMH-5 that needs to be received, because an FMH-5 cannot exceed that length. When the application program receives the FMH-5, the RPL's RECLEN field contains the length of the FMH-5.

If you are using the ATTN exit, remember that it is driven only once for each FMH-5. With persistent LU-LU sessions, it might be driven once for each FMH-5 for each application instance. If the application program lacks the resources to start a conversation at that time, the exit can note in any way convenient for the application program that an FMH-5 needs to be received. The mainline program can receive the FMH-5 at a later time.