Re: Poorly documented APIs

Well, i'm not from IBM.
But obviously i understood the manuals.
They could be better, it is not easy, i agree.

You build up the structure for the input DS, that you've got right. So you 
have to know.

The offset is simply of the same kind as offset in the left of the 
structures in the manuals. It's a storage layout. The info you have to 
provide can vary from some bytes to many bytes. It's a self-describing 
structure, designed to be used with pointers.

Solution proposal:

Get yourself some storage. Define a DS with, say, 32767 bytes. You can 
shorten it to a reasonable value later on.
D MyJSLT0200   DS      32767

You build up a DS for all fixed postioned fields of JSLT0200 (you didn't 
tell us which you want to use, i take this one).
These are for JSLT0200 all fields up to CCSID and the reserved 4 bytes. 
This leads us to a DS with a size of 92 bytes, but that is irrelevant.
This DS is based on your storage.
D JSLT0200Hdr    DS       based(%ADDR(MyJSLT0200)
D JSLT0200MaxMsg...
D                                               10I 0
<snip>
D JSLT0200OfsFI...                              offset to field identifiers of 
fields to return
D                                               10I 0
D JSLT0200NofFI...                              number of fields to return 
(better: field identifiers 
of fields to return)
D                                               10I 0

and so on till
D JSLT0200ReservedMustBeZero
D                                               10I 0

The based means that you get no storage for this DS-variables. You have to 
provide a pointer pointing on existing storage to have values in it. In 
this case, as it will never change, the address of the big DS is put into 
the based() and JSLT0200Hdr lies over the very same storage as MyJSLT0200. 
And then you have to
EVAL JSLT0200ReservedMustBeZero = *ZERO. (e.g. *INZSR)

Now build up another DS, again based. This time with a pointer.
D JSLT0200FIDS    DS          based(PtrJSLT0200FIDS)
D JSLT0200FI                          10I 0

This time, a pointer field is in the based(). Its a variable that can only 
hold addresses in storage. Now its *NULL, you've to set it.
It is filled with the pointer of your address PLUS an offset to the 
beginning of this DS.
You need the offset anyway, so let's set and use it.

C       EVAL    JSLT0200OfsFI = %SIZE(JSLT0200Hdr)       the size in byte of 
the 
"Header"-DS.

C       EVAL    PtrJSLT0200FIDS = %ADDR(MyJSLT0200) +
C                                       JSLT0200OfsFI

Now set the first key you want to retrieve, e.g. 0301 Message.

C       EVAL    JSLT0200FI = 0301

Say, you want another key. Now comes the big adavantage, you simply shift 
the DS around in storage to fill the next 4 bytes.

C       EVAL    PtrJSLT0200FIDS = PtrJSLT0200FIDS +
C                                       %SIZE(SLT0200FIDS)

This increases the address and "moves" the DS this way.

C       EVAL    JSLT0200FI = 00603                                      Sending 
program name

Now you have to provide the number of entries you have in the "Number of 
fields to return"-field.

C       EVAL    JSLT0200NofFI = 2

Wen want two keys in this example, 0301 and 00603.

The offset to call message queue is calculated:

C       EVAL    JSLT0200OfsCMQ = JSLT0200OfsFI +
C                                       JSLT0200NofFI * %SIZE(JSLT0200FIDS)

(You might also calculate such things by subtracting two pointers to get 
the difference in numbers of bytes between them.)

And with this info, you build up a based DS (Pointer) and fille the Pointer 
with the address of MYJSLT0200 plus the offset, just like before.

When you debug, look at MyJSLT0200 (ev MyJSLT0200:x 256) and compare it 
with the manual.

You should see the light then.

Download the redbook "Who knew you could that with RPG?" to see that there 
are good books at IBM.
(regardless of how near the writers are or were to IBM.)

It's suddenly needed to register, but i did it and it's fast and does no harm.
http://www.redbooks.ibm.com/abstracts/sg245402.html
right click the link "View online" and save target as.
You'll need Adobe Acrobat Reader. (For free at 
http://www.adobe.com/products/acrobat/readstep1.html )

GET IT, YOU NEED IT!

0.04 Euro

Anton Gombkötö

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