RE: STDIN, STDOUT, STDERR in RPG

James, Scott, Jim.
Thanks. I'm aware that OS like Unix and the C (or C++) language provide an
operator that permits the output of one program to be easily piped to
another but the challenge is to prove that this can be achieved effectively
on OS/400 since the analagous facility to override stdout appears to be
ignored when the QtmhWrStout Api is used to generate stdout.

Populating stdout from within an RPGLE cgi program (or C) is a relatively
simple matter using the built-in Apis or functions that the language
provides since this is how all CGI programs are written. I suspect that
James's explanation that stdout does not actually exist as a file may be the
reason that the concept of subsequently opening it is not possible.

The piping technique would therefore appear be a mechanism the merely primes
the OS before making a program call so that it bypasses the usual low level
process that generates stdout and instead passes the output stream to a
redirected actual file that is available for read.

I'm inclined to agree that this cannot be done (unless emulating popen() and
pclose() is worth a look.)

Cheers, Peter



-----Original Message-----
From: James Rich [mailto:james@eaerich.com]
Sent: Monday, April 15, 2002 7:36 PM
To: 'rpg400-l@midrange.com'
Subject: RE: STDIN, STDOUT, STDERR in RPG


On Mon, 15 Apr 2002, Peter Connell wrote:

> Forgive me for mentioning this again. You may have spotted a recent
question
> I posed that asks if stdout (not stdin) can be read into an RPGLE pgm
after
> some previous process has already populated it via an api like
QtmhWrStout.
> Can this be done?

I know I'm not Scott but you get to hear from me anyway :)

I believe the straightforward answer to this is no.  For a more in depth
discussion on how stdin and friends work see the reply I just wrote to
David's question.

Using pipes one program's stdout can be read by another program's stdin.
I think you are trying to accomplish this reading without using a pipe,
which I believe cannot be done.  But there is a bigger problem with your
question.  stdout is not something that can be populated.  If there is
something to read it (using a pipe) then fine, or it can be printed on the
screen, or it can be redirected to a file.  But stdout is not saved or
stored anywhere.  It never fills up or gets empty.  So you can't have one
program write some stuff to its stdout and then fire up another program
and read the first's stdout.  stdin, stdout, and stderr aren't real files.
They only exist in the mind of the operating system.  To do what you want
you should use a pipe.

James Rich
james@eaerich.com

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