-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Larry Ducie
Sent: Thursday, November 01, 2007 7:39 AM
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: Is const really that much better than value?
Also, could anybody take this thread further and explain HOW
the use of varying and/or options(*varsize) affect the
efficiencies in passing large string data. I can see the
value (no pun intended) in using both of these and I use
options(*varsize) frequently in conjunction with opdesc and
CEEDOD calls. But what I'd like to know is "exactly" why
varying is better than non-varying . I can obviously see
there will be efficiencies because only the data defined by
the length is copied to temporaries but I'd like to know a
little more about the mechanics of, say, passing a varying
character string by value.
It's pretty simple really, the key thing to realize is that in both cases not only are you passing the
data in the variable but you're passing the current length of the variable also.
With, *VARSIZE, you have to pass the length explicitly in another parameter.
With VARYING, the length is implicitly defined as the first two bytes of the variable.
Now consider a procedure that takes a 2500a CONST VARYING parm.
You pass a 100a VARYING variable to it, which at run time only contains 50 characters.
Inside the procedure is a 2500a VARYING work field.
if you have an eval WRKFLD = PARM1, at run time, the procedure knows PARM1 only has 50 chars, so the
procedure only accesses the first 50 characters.
if you're dealing with *VARSIZE instead, the code in the procedure instead has to look like
eval WRKFLD = %subst(PARM1:1:PARM2)
where PARM2 contains the length of PARM1.
Works the same way if you pass a literal, except a temporary variable is created during compile to
hold the data. But the temporary variable need not be 2500a, instead it only needs to be big enough
to hold the literal.
AFAIK, passing a VARYING by value is always going to result in a temporary variable (onto the stack?)
of the same max length as the procedure parameter.
If you're dealing with large strings,
1) Use VARYING
2) pass by CONST reference
Passing by value makes sense for small fields, 1-16 bytes, larger than that and CONST is a better
choice.
Here's a related thread from last year (that you participated in BTW ;-)
http://archive.midrange.com/rpg400-l/200605/msg00162.html
HTH,
Charles
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