Re: Trigger program input buffer layout

Jim Langston <jlangston@conexfreight.com> wrote:
>
> Scott,
>
> I thought about doing it something like this, but how do
> you know the Parm1 size before hand?  Or does it matter?
>
> You are setting Parm1 up as 3072 bytes, how do you know
> it is this size?  And does it matter?

You don't need to know the size beforehand...  I picked 3072
because I felt it'd always be long enough to allow me to get
the address of the start of my record buffer.  Its possible that
it won't work if I'm using a really large record format, (but you
usually know at least the APPROXIMATE size of your record format
at compile time)

All the array is doing is proving me with a way of offsetting my
pointer.   IF you're using a newer version of the operating system,
and can do direct pointer arithmetic, you don't need the array at
all, and then you don't have to worry about sizes at all...

>
> I thought that if I specified a parameter larger than my actual
> parameter I would get problems.  For instance, say I had
> 2 parameters I was passing as length 5 strings.  If I declared
> them as length 10 in my program, wouldn't I actually get both
> passed values in my first parameter?

You might -- but you can't rely on that fact.  Parameters are passed
by "reference".  This means that for each parameter, the program
making the call passes the memory address that the paramter is
stored in, and the program thats being called puts its parameters
into the area of memory that the calling program passed.

Does that make sense?

Here's a (somewhat oversimplified) example:

The calling program has two strings declared, 5 bytes each.  Lets
call them String1 & String2.  At startup, it asks OS/400 for two
areas of memory, each 5-bytes long.  The operating system
tells the program that it can use (for example) area 1A amd area 27

So the memory might look like this (purely an example)
                     1               2               3
     0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABDEF
     ---already-used-memory----XXXXX--used--XXXXX--free-memory----

Where XXXXX denotes the areas of memory that were given to the
program to use.

Now, the program puts some data into those areas.
c                   eval      String1 = 'Data1'
c                   eval      String2 = 'Data2'

Memory now looks like this:
                     1               2               3
     0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABDEF
     ---already-used-memory----Data1--used--Data2--free-memory----

Now it calls another program:

c                   call      'PGM2'
c                   parm                    String1
c                   parm                    String2

Because String1 is area 1A, and String2 is area 27, what its actually
doing is telling the program that its to put its first parm in area
1A, and the 2nd parm in area 27.

So as Pgm2 starts up, it checks its parameter definitiions, and it
has this:

c     *entry        plist
c                   parm                    EntParm1         10
c                   parm                    EntParm2          5

Because these are parameters, rather than requesting an area of
memory from the operating system, it takes the area that was given
by the calling program.   EntParm1 is in area 1A, EntParm2 is in
area 27.

Memory still looks like this, so:
                     1               2               3
     0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABDEF
     ---already-used-memory----Data1--used--Data2--free-memory----
                               XXXXXXXXXX   XXXXX
This is where the parms would be...

So, because we made EntParm1 10 bytes long, it'll be pointed at
the "Data1" but then will go on to look at whatever is in the area
of memory beyond that...   (which is unknown to us)

If (by luck) the operating system had placed String1 in area 1A,
and String2 in area 1F, then we would've gotten both strings in one...
and this probably happens frequently, but its not something we can
rely upon.  If we did, we'd have "random crashes" (much like we
associate with Windows programs) when the area of memory allocated
didn't happen to be contiguous.


>
> Perhaps this is a good time to use the VARYING length
> definition?

If you're using a release of OS/400 new enough to use a VARYING
length field, you can probably do direct pointer manipulation,
as well.   If this is the case, you don't need to define a field
there at all, just offset the pointer "NewOffset" bytes from the
beginning of the data structure.

It doesn't matter that "NewOffset" takes you beyond the length of
"Parm1" because when you declare parameters, you're not allocating
the memory for them.   The CALLING program is.   In this case, the
operating system declares the memory, and you can trust it to declare
enough memory that the offset that its sending you won't exceed that
space.

Of course, if you're worried about it, the length of the data is also
passed to you in the 2nd parm.   You can double check it by doing
something like this:

c                   if        NewOffset+%SIZE(dsRecord) > Parm2
c***   .. handle situation where not space was allocated for the
c***      size of the externally defined data structure ..
c                   endif


Hope that helps...  (If it doesn't, I'm not sure how to explain it)

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