[Dev-luatex] On \maxdimen in LuaTeX

Sjors Gielen sjors at sjorsgielen.nl
Mon Jun 10 13:25:50 CEST 2019

Hi all,

According to the TeX in Practice book, "the largest dimension value that
can be represented in a TeX program is 16383.99999pt. This value is
assigned to a dimension register \maxdimen [...]." This is a tiny bit
smaller than 2^14 pt. As a layman TeX user, my naive guess is that in the
compiler, floating point dimension registers probably use int32_t or some
other fixed-length type, and the compiler implements its own floating point
arithmetics. Either way, whenever an overflow is going to happen, the
compiler prints a "! Dimension too large" error.

This error also occurs when using LuaTeX. My gut feeling is that the
current limit of (almost) 16384 pt is perhaps mostly historic; could it
nowadays be loosened somehow? Perhaps the dimension calculations are
already in Lua and the error is there for legacy reasons, or perhaps the
calculations are still in C and if int32_t is indeed used, int64_t could
conceivably raise the max dimensions to hundreds of kilometers, more than
anyone would accidentally reach.

Looking forward to hearing your thoughts.

Sjors Gielen

P.S.: The discussion I am hoping to invoke is theoretical, but for
background, my motivation comes from the use of the 'mdframed' package to
split a large frame over many pages. Our automated testing system creates
mdframed environments containing test steps and their outcomes; we have
apparently crossed some new amount of steps where the virtual vbox size
goes over \maxdimen before the vbox is split over multiple pages. If the
error is ignored, the generated PDF looks fine. We can also end the
mdframed environment every N steps and immediately start a new one, which
is an acceptable workaround but I'm a bit unsatisfied that it's necessary.
Perhaps this is better considered a bug in mdframed than in the typesetting
engine, but I still find the theoretical question behind \maxdimen
interesting, hence this e-mail.
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