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I am still interested on any thoughts about how the pointer based on a
pointer thing works (because it does), and if there is a better way.

Oh, now that I've looked it over again and read Tom's reply, I can tell you exactly how it works.

Basically what you were doing is equivalent to doing this:

     D                 ds
     D   InVar                 1     16*
     D   DTSField              1     17A


I don't know what the name of the field containing the DTS timestamp was (since you didn't post it) so I made one up. But hopefully you understand what I'm saying. They occupied the same spot in memory.

Granted, you used pointer logic rather than a data structure, but it had the exact same effect.

It didn't matter which one you passed to the API, since they contained the same value. As long as you made sure DTSField was set to a valid DTS timestamp (and therefore made InVar an illegal pointer value) the code would work.

It makes no sense to do that -- but that's what you did.

For example, if you wanted to dump all the messages in a data queue into a user space with the QMHRDQM API, and the API expects a character argument for the output variable, how would you do it?

Well, the method you're using wouldn't help with that. But you could do it this way if you wanted to:

     D mySpace         s               *
     D InVarPtr        s               *
     D InVar           s           1024A   based(InVarPtr)
     D x               s             10I 0

      ** Ask the OS for 1mb of memory:
     C                   eval      mySpace = %alloc(1024 * %Size(InVar))

      ** set the entire space to blanks:
     c                   for       x = 0 to 1023
     c                   eval      InVarPtr = mySpace + (x * %size(InVar))
     C                   eval      InVar = *blanks
     c                   endfor
      .
      .
      ** when you're done, let the OS have it's memory back
     C                   dealloc                 mySpace


Basically, InVar views 1k of memory at a time. So in the above example, I've reserved 1 mb of memory to the MySpace pointer. And then I've changed InVarPtr to tell it to view different sections of that 1mb of memory. (In this example, I used that functionality to blank it out.)

To pass the contents of the 1mb space to the API you'd do this:

      ** Ask the OS for 1mb of memory:
     C                   eval      InVarPtr = mySpace
     C                   CALL      'SOMEAPI'
     C                   PARM                    inVar

Since the current address of InVar is the same as the address stored in mySpace, it'd be the memory that's allocated to mySpace that'd be used by the API.

I needed a lot more than 32767 characters for a large data queue.

In this case, I've used the %alloc() BIF, which is capable of allocating up to (approx) 16mb. That's a lot more than 32k!

You could also use QUSCRTUS and QUSPTRUS to create and get a pointer to a user space instead of allocating RAM with the %alloc() BIF. It'd require more code, but you'd end up with the same results (more or less).

I don't know if the data queue APIs support it, but if 16mb wasn't enough, you COULD allocate the memory in teraspace. If you did that, you'd be able to allocate as much as 1 terabyte to MySpace. So... that should be enough, eh?


By basing a pointer variable on a pointer that is the address to the user space, and then passing that based pointer to the QMHRDQM API it works nicely, but I've sometimes wondered if there is a better way.

It makes no sense to base a pointer on a pointer for what you're trying to do. Base a character field or data struture, or whatever you need to work with on the pointer. Not another pointer!

Anyway, I'm glad you figured it out.

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