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Re: [groff] vertical resolution and page location traps

From: Dave Kemper
Subject: Re: [groff] vertical resolution and page location traps
Date: Fri, 15 Jun 2018 18:12:55 -0500

Thanks, Werner and Tadziu, for the prompt replies!  That clears a few things up.

Yes, I had read the Measurements section of the manual, but that's
part of what confused me about the .V register.  Where that section
said "This basic unit, represented by a 'u', is a device dependent
measurement," I took that to mean that the groff basic unit
corresponded to the output device's resolution, but this seems to not
(always) be the case, according to Tadziu's detailed explanation.
There is probably a clearer way to word all that in the manual.

Anyway, it turns out to be unrelated to the issue I was trying to
solve, that of page-location traps.

> Reason is that traps are not recursive, IIRC: Traps are not handled
> while another trap is active (this seems to be missing in the info
> file).

I'm not sure expected them to be, but the real problem seems to be:

> In other words, while your first trap is active, it moves down
> one line, which passes some traps without triggering them.

That bit of information is missing from the manual as well, but more
importantly, doesn't even seem like desired behavior for the traps as
written.  Nothing in them should cause vertical movement.  (Indeed,
vertical spacing in the output looks consistent whether or not a trap
was executed between two lines.)

> Below is an example that triggers all defined traps if called with the -Tps
> device.

Hmm.  When I run it, it overprints some output lines atop each other,
also not a desired result.  (And it additionally creates an infinite
loop when I change the .nop to .tm so the trap notices to go stderr
rather than the ps output.)

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