What are those <i>_nc</i> programs for?
<h3>What are those <i>_nc</i> programs for?</h3>
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Date: Fri, 20 Sep 1996 08:08:30 -0600
From: Peggy Bruehl 
To: Charles.Mcgill@noaa.gov
Subject: 960920:ASGQ, not!
Cc: peggy@comet.ucar.edu
Status: R

>Date: 19 Sep 1996 22:08:28 -0400
>From: "Charles McGill" >To: "peggy@comet.ucar.edu" (Return requested)
>Cc: "paul sisson" (Return requested)
>Subject: A.S.G.Q (Another Silly GEMPAK Question)

Chuck,

>I was wondering what the difference is between gdplot and gdplot_nc (and
>others like this) besides the nc versions being larger.  What (if any)
>advantage is there to using the nc programs for generating metafiles?

This is not a silly question at all.  The "_nc" versions of GEMPAK
programs are special executables that have the GPLT (graphics drawing)
process and the NC (metafile generation) driver pre-compiled and linked 
together with the GEMPAK application program.  

Ordinarily, those three processes (gdplot, gplt, nc) all run separately
and communicate with each other via message queues.  If something unusual
happens and one of processes crashes (or is killed with a control-C), then
the other two processes and the messages queues could become "orphaned".
Orphaned processes and queues can pile up on a workstation, slowing 
performance, and possibly even causing more crashes of other GEMPAK
processes.  Enough orphaned processes and queues could even hang the
workstation.  (Incidently, this "three process/two message queue"
system is the reason that the GPEND program is necessary.  After
the successful execution of a GEMPAK program (such as GDPLOT, or
GDCROSS), you must run GPEND in order to gently exit the gplt and
nc (or xw, ps, gf, etc) processes, and to close the message queues.
If you forget to run GPEND, you will be orphaning those processes and
queues on your workstation.)

When the three processes are linked together, there is no chance
for any orphaned processes.  If something happens to crash process,
there are no other processes or queues left behind.  This is why
the "_nc" programs are so popular in scripts, where crashes can
sometimes be common.  However, there is nothing about the "_nc"
programs that require they be run from scripts.  You can run
them interactively as well.  In the old days when I was at Unidata,
I also used to release "_gf" versions of the programs too, for
making GIF images.  There's no reason "_ps" versions can't be
made either, for making Postscript files.

The big drawback of the "_nc" programs is that you can not create
an overlay graphic from two different programs.  So, for example,
if you wanted to use GPMAP to drop a high resolution county
map, or a US roads & interstates map on top of your GDPLOT
graphic, you can't use GDPLOT_NC or GPMAP_NC because once either
of these programs is exited, the NC device driver is also 
exited.  Once the device driver is exited, all the navigation
information about the graphic is gone.

Peggy
__________________________________________________________________
Peggy Bruehl                     National STRC Coordinator - NWS
peggy@comet.ucar.edu             COMET/UCAR PO Box 3000
(303) 497-8356                   Boulder, CO 80307-3000
SOO/STRC Home Page: 
http://www.comet.ucar.edu/pub_html/STRC_html/index.html
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