Re: Prototype in RPG/ILE

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

Question is: What is the purpose of Prototype and it's relationship to
the procedure.

I can give you one of 2 answers, one technical and one the reason.  First,
the reason.

Answer: The compiler requires it to compile.

Now the technical answer (behind the scenes)

Answer: When the compile goes through a program and it sees an external
call, it has to know how to put everything to be passed in order.  It also has
to ensure that all required parameters are there.  Since the Subprocedure
is usually not reached in the program before the call is found, or may not
even be found in that source at all, the compiler needs a way to have this
information.  Hence, the prototype (almost all languages use it in one way
or another).

The prototype tells the compiler that there is a certain called called such
and such that is expecting these types of parameters and returns this type
of parameter.  Now, when the compile finds the call, it has all the information
it needs to build the statements to call the routine.

Take a simple call to atol (Ascii to Long).

 D atol            PR            10I 0 ExtProc('atol')
 D                                 *   Value Options(*String)

The first line gives 4 pieces of information to the compiler.
"atol" - What I am calling this in my program, how I am going to call it.
      In this case atol
"10I" - The returning parameter, in this case 10 digit integer (Zoned)
"ExtProc" - This is an external procedure, not found in this source file.
"'atol'" - This is the name of the procedure in some other file I am going
     to link to.

The second line gives the compiler one piece of information, in this case
the one parameter I need to pass to the Subprocedure.
"*" - It is a pointer I am passing (location of a variable)
"Value" - I am passing the parameter by value, that is, a copy of it, not
     the real thing
"Options(*String)" - I'm going to be passing a pointer to a string.

Now the compiler has all the information it needs to compile without
having a copy of the subprocedure it is going to link to.

Regards,

Jim Langston

Bruce Collins wrote:

> I am trying to learn to use Subprocedures instead of Subroutines in RPG/ILE.
> I am having a problem understanding What the purpose of the Prototype and
> its relationship to the procudure. Could someone shed some light on this
> please.

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