IBM AS/400 manual Using Operational Descriptors, Passing Prototyped Parameters

Passing Prototyped Parameters

To pass a parameter by read-only reference, specify the keyword CONST definition specification of the parameter definition in the prototype. page 138 shows an example of a prototype definition for the ILE CEE AP CEETSTA (Test for omitted argument).

*------------------------------------------------------------------
* CEETSTA (Test for omitted argument)					-- ILE CEE API
*	1. Presence flag				Output	Binary(4)
*	2. Argument number				Input	Binary(4)
*------------------------------------------------------------------
D CEETSTA		PR			EXTPROC('CEETSTA')
D	Present		10I	0
D	ArgNum		10I	0	CONST
D	Feedback		12A		OPTIONS(*OMIT)
...
D HaveParm		S	10I	0
...
C		CALLP	CEETSTA(HaveParm : 3 : *OMIT)
C		IF	HaveParm = 1
*	do something with third parameter
C		ENDIF

Figure	65.	Prototype for ILE CEE API CEETSTA with CONST Parameter
The second		parameter	passed to CEETSTA can be any numeric field, a lite
built-in	function, or		expression.

Using Operational Descriptors

Sometimes it is necessary to					pass a	parameter to	a procedure even		t
data	type	is not	precisely		known to the called procedure, (for examp
types	of	strings).	In	these	instancesoperationalyoudescriptorscan use		to
provide descriptive information to the called procedure regarding the
parameter.		The additional information allows the procedure to						properly	i
string. You		should	only	use	operational	descriptors	when they	are expe

called procedure.

Many ILE bindable APIs expect operational descriptorsSystem API. ReferIn the-

ence, if any parameter is defined as 'by descriptor', then you shoul tional descriptors to the API. An example of this is the ILE CEE API (Convert Seconds to Character Timestamp). The second and third parameters require an operational descriptor.

Note: Currently, the ILE RPG compiler only supports operational descriptors character and graphic fields and subfields. Operational descripto available for data structures, arrays, or tables. In addition, o descriptors are not available for data of type numeric, date, basing pointer or procedure pointer.

Operational	descriptors have	no	effect on	the parameters being passed
way that they are passed.		When a procedure is passed			operational
which it	does not expect,	the	operational	descriptors are	simply ignor

You can request operational descriptors for both prototyped and non-pro parameters. For prototyped parameters, you specify the keyword OPDESC prototype definition. For non-prototyped parameters, you specify (D) as tion code extender of the CALLB operation. In either case, operationa

138 ILE RPG for AS/400 Programmer's Guide