IBM SC34-6814-04 - Manual (Page 240)

TS_ADDR7
is the address of an area containing the value of SYSID.
Modifying fields in the command-level parameter structure: Some fields that
are passed to temporary storage are used as input to the request, some are used
as output fields, and some are used for both input and output. The method your
user exit program uses to modify a field depends on the usage of the field.
The following are always input fields:
v QUEUE|QNAME
v FROM
v SYSID
The following are always output fields:
v INTO
v NUMITEMS
v SET
LENGTH is an input field on a WRITEQ request, and an output field on a READQ
request that specifies SET. It is both an input and an output field on a READQ
request that specifies INTO.
ITEM is an input field on a READQ request, and on a WRITEQ request that
specifies REWRITE. It is both an input and an output field on a WRITEQ request
that does not specify REWRITE.
Modifying input fields: The correct method of modifying an input field is to create
a new copy of it, and to change the address in the command-level parameter list to
point to your new data.
Note: You must never modify an input field by altering the data that is pointed to by
the command-level parameter list. To do so would corrupt storage belonging
to the application program and would cause a failure when the program
attempted to reuse the field.
Modifying output fields: The technique described in “Modifying input fields” is not
suitable for modifying output fields. (The results would be returned to the new area
instead of the application’s area, and would be invisible to the application.)
An output field is modified by altering the data that is pointed to by the
command-level parameter list. In the case of an output field, you can modify the
application’s data in place, because the application is expecting the field to be
modified anyway.
Modifying fields used for both input and output: An example of a field that is
used for both input and output is LENGTH on a READQ request that specifies
INTO. You can treat such fields in the same way as output fields, and they are
considered to be the same.
Modifying the EID: It is not possible to modify the EID to make major changes to
requests. It is not possible, for example, to change a READQ request to a WRITEQ
request.
218 Customization Guide