[texhax] \write17?

David Carlisle d.p.carlisle at gmail.com
Sat Oct 20 23:22:11 CEST 2018

On Sat, 20 Oct 2018 at 22:12, Hefferon, Jim S. <jhefferon at smcvt.edu> wrote:
> I'm trying to understand the \write<number> command, specifically \write16 and \write17.  I'd appreciate any help.
> I understand that the output files are ordinarily numbered  1 to 15,


> and that if <number> is positive and outside that range then TeX writes to both the log file and the terminal.  So I think I understand that \write16 was adopted  early on as a convention to write to both.  I see it in the TeXbook.  Is that understanding right?

yes although I don't think there is or was any particular convention to use 16.

> But \write17?  Google gives me a references to it, but what is it for?   How is it different than \write16?
It's the same as you say, the same as any value > 15 (in classic tex)

> If it is no different then why was it skipped over in getting to \write18?

I wasn't there but I think it's just a "margin of error" to make it
less likely to clash with existing use when the shell-escape system
was added,

> (I know that \write18 is an entirely different animal.)
> Thank you,
> Jim

In luatex there are 256 rather than 16 output streams, so it's different again.


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