Editing Graphics development with vplot (section)

==Handling errors==
Device routines SHOULD NOT simply exit when they get a fatal error!
All errors should be handled via the provided utility routine "ERR"!
<syntaxhighlight lang="c">
#include "err.h"
ERR (type, filter, fmt, a1, a2, a3)
int type;
char *filter;
char *fmt;
double a1, a2, a3;
</syntaxhighlight>
"Type" is one of the possibilities from "err.h". Currently there are three
possibilities: COMMENT, WARN, FATAL. Case COMMENT is just for making
remarks, not really error messages per se. Case WARN should be used when
something is wrong, but the filter can take reasonable corrective action.
Case FATAL is for errors that should cause abnormal termination of the
program. Calls to ERR with type FATAL will not return.

"Filter" should be the name of the filter generating the error or comment
as you want it to appear in the message. Normally this should just be
the variable "name" defined in <dev>conf.c.

From here on ERR has the same syntax as "printf", except that ERR will
automatically throw a carriage-return line-feed on for you (so you don't
want to end with "\n").

After ERR has been called with type FATAL, dovplot and frontend will do
necessary cleaning up, in the process calling dev.close. Device-dependent
cleaning up should be done by that routine at that time.

The define ERR is used so that conflicts with other subroutines named
"error", "err", etc, can be avoided. Currently "ERR" is defined to be
"filtererror", a name we haven't had any trouble with.