From:	CSBVAX::MRGATE!HOLLINGE%SASK.BITNET@CUNYVM.CUNY.EDU@SMTP  3-APR-1988 16:15
To:	ARISIA::EVERHART
Subj:	RE: Question on f$getdvi lexical function


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Date:     Tue, 29 Mar 88 00:11 CST
From:     <HOLLINGE%SASK.BITNET@CORNELLC.CCS.CORNELL.EDU> (Glenn Hollinger)
Subject:  RE: Question on f$getdvi lexical function
To:       info-vax@kl.sri.com
X-Original-To:  info-vax@kl.sri.com, HOLLINGER

>From: Jnet%"ACM_CSA@SWTEXAS"
>Date:         Mon, 28 Mar 88 01:38:00 CST
>Reply-To:     INFO-VAX@KL.SRI.COM
>Sender:       INFO-VAX Discussion <INFO-VAX@UBVM>
>From:         ACM_CSA@SWTEXAS
>Subject:      Question on f$getdvi lexical function
>
>[...]
>
>By executing this command file,
>
>$ On Control Then Exit
>$ TOP: Write sys$output F$Getdvi(F$GetJPI(F$Pid(CONTEXT),"TERMINAL"),"DEVTYPE")
>$ Goto TOP
>
>the error becomes illustrated. The value will be correct the first two loops,
>different the third, and then crash on the fourth.  (After executing this,
>you can stop your temporary sub-process by STOP TEST.)
>
> [example deleted]
>
>Although application of this syntax does not often require a loop, the
>F$GETDVI function is commonly called and I suspect it has something to
>do with its call.  We are presently running under V5.7.
>
>My question(s):
>
>  Why this error is occuring?

There might be your problem, this must be a beta-test of version 5.7! ;-)

The error message is being caused by handing an empty string, "", to
f$getdvi.  The F$pid wild-cards all the process IDs you have access to
in the system.  This includes your interactive job as well as any
subprocess, batch, network, and detached processes.  The non-interactive
jobs have no terminal associated with them, so getjpi returns an empty
string for the terminal name.  Getdvi tries to translate this empty
string as a device name and returns the error you report.  The error
will not be limited to just subprocesses.  The same error should occur
for batch and network jobs under your UIC/Username.

>  How it can be prevented?

The solution is not to hand the empty string to getdvi.  Two solutions
come to mind.  The first mearly discards invalid cases while the second
predicts which cases not to do based on the type of process.

1) quick and dirty

$ On Control Then Exit
$ TOP: x = F$GetJPI(F$Pid(CONTEXT),"TERMINAL")
$ if x .nes. "" then Write sys$output F$Getdvi(x,"DEVTYPE")
$ Goto TOP

2) check process first

$ On Control Then Exit
$ TOP: x = F$Pid(CONTEXT)
$ if f$getjpi(x,"MODE") .eqs. "INTERACTIVE" .and. -     ! only interactive
        f$getjpi(x,"MASTER_PID") .eqs. x then -         ! not subprocess
                Write sys$output F$Getdvi(F$GetJPI(x,"TERMINAL"),"DEVTYPE")
$ Goto TOP

>
>                        Ray Renteria
>                        ACM_CSA@SWTNYSSA

Glenn Hollinger,
Systems Programmer,
Computing Services,
University of Saskatchewan.