Editing Graphics development with vplot (section)

===Input routines===
<syntaxhighlight lang="c">
int dev.getpoint(termout, x, y)
FILE *termout;
int *x, *y;
{
int status;

return status;
}
</syntaxhighlight>
Turn on the cursor and let the user pick a point. Return the device
coordinate of the picked point in (*x,*y), and return 0. If the
user indicated that he doesn't want to pick any more points, return 1.
(The value in (*x,*y) is then considered to be junk by dovplot.)
Returned values should be in the normal range of device coordinates.
If (*x,*y) is returned unchanged, this will also indicate that no point
was picked and no more points should be picked.

Termout is a stream pointer reading from "/dev/tty". You can use it if
it is convenient and correct to do so, or ignore it.

Generic routines: nulldev
Linking in nulldev for this routine will simulate a user that always
declines to enter any points at all, since (*x,*y) is returned unchanged.

<syntaxhighlight lang="c">
int dev.interact(command, termout, string)
int command;
FILE *termout;
char string[];
</syntaxhighlight>
This routine handles string input from the user and pausing.
The various possible commands and return values are defined and
DOCUMENTED in ".../vplot/filters/include/intcom.h".

(Normally the return value will just always be (int) 0.)

Generic routines: nulldev, geninteract
Linking in nulldev will rudely zoom past all prompts to the user without
waiting. This is a reasonable thing to do for hardcopy devices. It is
a very rude thing to do for screen devices.

Geninteract will read the required input from termout, which is connected
to read from "/dev/tty". If you find it convenient to use termout, make
sure it isn't NULL before trying to use it (see geninteract for an example).
Termout == NULL means "/dev/tty" could not be opened for reading.