e-TeX \hbox to size of glue

Question

I have a piece of glue (\hskip 2.4in plus 2fil specifically) that's flexible, and I want to create an \hbox to the same length, but \hbox to 2.4in plus 2fil doesn't work. Is there a way to achieve a similar result?

The reason I want to use this is to make a 3-column format with boxed in columns more flexible (especially to differently sized columns). I'm using pdfTeX, so I can use e-TeX's extra features if that's helpful

Answer 1

You can use pdftex features to record the position before and after a glue node, and then (on the second run) use that information to construct a box of the desired size.

Here paragraph 1 just has glue, paragraph 2 has a box with Z...Z set to the same size.

\newwrite\zz % easier in latex where this is set up already
\openin\zz=\jobname.aux
\ifeof\zz\else
\input \jobname.aux
\closein\zz
\fi
\immediate\openout\zz=\jobname.aux

1) zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z
zzzz%
\hskip 2.4in plus 2fil
zzz zzzz zzz
zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z
zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z

2) zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z
zzzz%
\ifx\zzA\undefined\else
\leavevmode\rlap{\hbox to \dimexpr \zzB sp -\zzA sp{Z\dotfill Z}}%
\pdfsavepos\write\zz{\gdef\string\zzA{\the\pdflastxpos}}%
\hskip 2.4in plus 2fil
\pdfsavepos\write\zz{\gdef\string\zzB{\the\pdflastxpos}}%
zzz zzzz zzz
zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z
zzz zz zz zz zz zz zz zz zzzz zz zz zz zz zz zz zz z

\bye