The begin function is a required element in any session management exit routine. This function is processed only once. It is processed before any of the other functions and is either initialized immediately following VTAM® initialization, or is activated by issuing a MODIFY EXIT,ID=exitname,OPTION=ACT command. See z/OS Communications Server: SNA Operation for more information about this command. The begin function indicates to VTAM the functions for which the exit routine is to be invoked.

The fourth entry in the parameter list passed to the begin function contains the address of the exit routine functions. The session management exit routine must set the proper bit on (B'1') for each function for which VTAM is to call the routine. The bit settings for each of the functions associated with the sample routine follow:

Function

Secondary session authorization

Byte 0, bit 1 or X'40'

Final accounting

Byte 0, bit 2 or X'20'

Gateway path selection

Byte 0, bit 3 or X'10'

Alias selection

Byte 1, bit 2 or X'20'

End function

Byte 0, bit 4 or X'08'

The fifth word in the parameter list passed to the BEGIN function contains the address of a 2-byte length field followed by the character string that was entered on the MODIFY EXIT, ID=ISTEXCAA,OPT=INACT,PARMS= parameter. If the exit is being driven during VTAM initialization, or the PARMS= parameter was not entered, this pointer will be zero.