Re: STDIN, STDOUT, STDERR in RPG

["David Morris" <David.Morris@xxxxxxxxxxxxx>]

James,

Thanks for the help. I am still wondering
if you don't assign a specific file handle
to stdin/out/err, how do the utilities find
these files? For example, it looks like QSH
uses /dev/qsh-stdin-null. When I asked a
friend who is familiar with Linux how STDIN
is identified he said that /dev/stdin-null
is the norm, but he didn't know see know how
Linux finds this since redirection could pick
up STDIN from another source. How do nested
utilities know this unless there is some
standard file handle?

I suppose I could make this easy and just use
C, but I really want to find out how the
utilities make this association and don't end
up walking on each other.

Thanks,

David Morris

>>> james@eaerich.com 04/13/02 12:19 PM >>>
On Sat, 13 Apr 2002, David Morris wrote:

> STDIN should be assigned a handle of 0, STDOUT 1, and STDERR 2. At
least
> that is what the PASE manual says.

stdin, stdout, and stderr should not be assigned anything by your
program.
The OS assigns the values of the handles.  Your program just remembers
what was assigned.  I'm not sure of the RPG code but it should be
similar
to this:

C               eval    filehandle = open (stdin)

filehandle could be anything. Of course some values indicate an error
occurred.  Maybe some C code will help since stdin and friends come from
that world:

infile = fopen(stdin, "r");
if (infile == NULL)
{
        printf("Unable to open stdin\n");
}

However (in C anyway) stdin, stdout, and stderr are always open, even if
your program does not use them.  You don't need to do the above because
you would be trying to open an already open file.  Now I don't think the
same is true of RPG but that may help to explain your other questions
below.

> You should close these if you open them when your process is
completed,
> this just seems like a good practice.

In other languages you don't have to worry about this in the case of
stdin, etc. because they are always opened and closed for you.

> I found something strange. With QSH it opens a single file at 0 if
> possible, but it will use anything if 0 is not available. QSH closes
> this file when you press F3. Debug does not operate this way. Debug
> appears to 0,1,2 and never close them.
>
> How does a unix utility like grep locate STDIN, STDOUT, and STDERR?
Does
> it use the file handle 0,1,2 or some other means?

Since they are always opened and closed for you the utilities (and every
*nix program) work something like this (given in C since that what they
are written in on *nix):

int main()
{
char junk[10];

fprintf (stderr, "I am writing to stderr.\n");
fprintf (stdout, "I am writing to stdout.\n");
printf ("I am writing to stdout here, too.\n");
/* Read from stdin and store it in junk */
/* This is how you do a buffer overflow! */
fscanf (stdin, "%s", junk);
}

James Rich
james@eaerich.com