Re: Service Programs 101 -- RPG400-L

On Thu, 2004-07-22 at 18:51, daparnin@xxxxxxxxxxxxxxxxxx wrote:
> 
> This should be a quick one for most of you.  
Ha! 

> What exactly is a service
> program, or rather, how are they implemented?  

The short explanation is that a service program is a collection of
*MODULE objects (1 or more).  Those *MODULE objects contain procedures,
some (or all) are "exported", meaning that they are available to be
called from outside the service program, say from your main program or
another service program.  (The corollary is that procedures in the
service program that are NOT exported cannot be accessed outside of
their *MODULE).  A *MODULE can have one-or-many of these procedures.  

Once a service program is created, it is implemented by "binding" it to
a program.  This is done either by specifying the service program to be
bound on the CRTPGM command or by putting the service program in a
binding directory and referencing the binding directory on the CRTPGM
command.  This can also be accomplished by using an "H-Spec" in the
*MODULE's source code.  

> Is a service program like a
> compiled, standalone subroutine or module of multiple subroutines?

Basically, yes, accept a service program cannot be called.  The exported
procedures of a service program can be called (using callp) from
programs and other service programs (if they are bound as discussed
above), but the service program itself cannot be called directly.  This
makes sense once you realize that a service program has no *ENTRY point.

>   I've
> read the comments people have made in reference to them but haven't had a
> need for them so far.  I'm still getting my feet wet with procedures and
> procedure interfaces.  (I've used them for CALLP's and for replacing *ENTRY
> PLIST).  This may or may not be applicable but I am familiar with creating
> RPG modules and then binding them into a executable program.

OK, creating a service program is simply binding *MODULEs together into
a special kind of a program.  Procedure rles are the same: you need a
prototype and an interface in order to be able to call a procedure in a
service program.  These are typically extrnalized by putting them into
/copy members.

> Specifically, I've been adapting some of Scott Klement's code for reading
> and writing to the IFS in preparation for a project that's coming up.  It's
> been going good so far but one of the examples creates a service program
> and uses it in later programs.  I want to try to use it in my program but
> I'm not sure if I understand the implementation.
> The service program was successfully created in QGPL.
> I'm using a /COPY member of Scott's to define the procedure.
> I get the following error when I try to do a CRTBNDRPG on my program.  Do I
> need to create my program as a module and then bind it to the service
> program?

At CRTxxx time, your program cannot find the procedure 'READLINE', which
I assume is in your service program.  Look at the first paragraph of my
response above: you need to reference the service program one way or the
other in order to use one of its procedures.  Otherwise, the compiler
cannot find the reference to the procedure, which is what it is
reporting below.

> 
> MY ERROR MESSAGE:
> Message . . . . :   Definition not found for symbol 'READLINE'.
> Cause . . . . . :   No definition was found for reference READLINE in
> *MODULE
>   object NISCOTST1 in library QTEMP. The definition either does not exist
> or
>   is not of the same data or procedure type as the reference. Recovery  . .
> .
>   :   Try the Create Program (CRTPGM) command again, supplying an object
> that
>   contains a definition for symbol READLINE.

 // stuff cut...

> Any help or advice would be appreciated.

If I may suggest, I have a series of ILE articles that I think would
help you a lot.  They deal specifically with the kinds of questions you
are asking.  Start at the top and read the first 5 or 6 in order.  You
can find them listed here:

http://www.rpgnext.com/tutorials.php

Best of Luck,

Joel Cochran
http://www.rpgnext.com