Re: Subprocedure scoping?

[Joe Pluta <joepluta@xxxxxxxxxxxxxxxxx>]

Kim Spears wrote:
> To fellow ILEers
>
> I'm trying to use a pointer to a data structure to pass data back from a
> sub procedure.  I had it working at one point but have since made some
> changes and now it doesn't work.  What's happening is $obcGetType loads
> the data structure properly and the pointer is getting passed back to
> $obcSelect, however the data in the 'type' data structure in $obSelect
> is garbage.  It's almost like once $obcGetType goes out of scope the
> memory for type_ds gets corrupted. Any suggestions?
>   
You exactly nailed it. type_ds is "automatic" storage, meaning it is 
allocated when $obcGetType is called and released when $obcGetType 
returns.  Once it returns, you have no idea what is occupying that 
memory space.  You must never return a pointer to your local storage to 
an outside routine.

There are a few ways to get around this.

One is to allocate memory explicitly in your called routine.  The 
problem with that is that you have to remember to free the memory when 
you're done with it.  This is whee you get memory leaks.

Another option is to make the variable STATIC.  This means the variable 
will be placed in the general program storage when the program is 
called, as opposed to being allocated when the routine is called.  A 
cool thing about this is that you can use it to keep information between 
calls.  For example, say you have a method GetCustomer that takes a 
customer number as the input.  Well, with static storage you could 
compare the customer number to the number in the data structure and not 
do the CHAIN if the record is already there.  The downside to this 
method is that it doesn't re-initialize the variables on every call 
(although you can do that reasonably easily with the RESET opcode) and 
also you can run into some issues with multi-threading, although I'm not 
100% clear on the details in RPG, expecially in V6R1.

Yet another way is to define the data structure in the calling procedure 
and pass the address in to the called subprocedure.  I do this all the time.

Joe