Re: Procedure PR/PI question

As I have said many times, VALUE means the compiler says I guarantee that
variable in the calling program will not be changed and it does that by
making a copy and pass a pointer to it. In the procedure you can do
anything you want with that parameter including changing it and it will
have no impact on the variable in the caller.

CONST means I will try to keep you from changing it but there are plenty of
ways to change the variable inside the procedure and have the calling
variable to be changed. These include passing as a parameter to a program
inside the procedure, getting a pointer to it, putting in a data structure.
The compiler may try to prevent you from changing but it is easy enough to
change which could result in some really nasty bugs.

That is why I always use a VALUE unless I passing something really big and
lately I have using Option(*String) to pass long variable strings. It only
passes the actual length of a variable instead of it's declared length. Now
if we could get IBM to put the actual length into the Operational
Descriptor we would be set. .


On Mon, Apr 29, 2013 at 11:05 AM, Rory Hewitt <rory.hewitt@xxxxxxxxx> wrote:

whatt,

If you have a variable passed as a Const parameter that is also defined as
a global variable, the yes, you can get problems. But that has nothing to
do with the Const parameter, and everything to do with the module design.
All you need to do is rename the parameter (and how it's defined in the
procedure) and the compiler will use a new variable.

Rory


On Mon, Apr 29, 2013 at 1:59 AM, whatt sson <whattssonn@xxxxxxxxx> wrote:

Hi Barbara,

I usually also default to const.
But i'm not sure if this is always a good idea because sometimes you get
aliasing problems.
And i would rather have a program that is correct instead of a program

that

performs a little better but where you have a chance that some nasty bug
gets introduced.

Now i default to value instead of const when i'm coding a local

procedure.

For example, when i call a local procedure with a const parameter and

pass

a variable that is globally defined within the same module chances are

that

the parameter value "suddenly" is changed while executing that procedure.
For example when this procedure calls another local procedure doing

things

with this global variable. This happens more often that you (i) think,

for

example if the variable is a (record) datastructure. I pass it to a local
procedure (by ref) which reads a file a fills the datastructure, etc.

This

pattern is quite common (passing record structures) but in this case you
really have to be careful.

However, if the procedure is in another module chances are much less that
calling this procedure has these kinds of side effects (and we don't want
side effects) happen because the other module is in another name space.

So i've chosen to always default to value for local procedures (i.e. they
are by defnition only called from within the same module), and const for
the other procedue (which can also be called locally of course).

But i'm still a bit in doubt. Always defaulting to value will make the

code

more robust and easier to reason about. And defaulting to const is a kind
of premature optimization. And in cases where you have to pass large

chunks

of data (>1k ?) it could be a good idea to use const for performance. But
in our environment which is mainly I/O bound this shouldn't be a problem
most of the time.


On Sat, Apr 27, 2013 at 6:14 AM, Barbara Morris <bmorris@xxxxxxxxxx>
wrote:

On 4/25/2013 8:19 PM, Alan Campin wrote:

If creating a procedure.

d pWriteScaleLog...
d pr
d PR_InScaleMessage...
d 128a Value

I would normally use CONST rather than VALUE for the string parameters
of a procedure. The difference in performance between CONST and VALUE
can be noticeable, especially for large varying length parameters where
the actual length is small. Maybe not particularly noticeable for 128,
but I still think it's a good practice to default to CONST rather than
VALUE for strings.

Unless the called procedure needs to change the parameter for its own
use, and even then, it might still perform much better to pass the
parameter by CONST, and make a local copy inside the procedure.

--
Barbara
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--
Rory Hewitt

http://www.linkedin.com/in/roryhewitt
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