[texhax] How can I copy all contents in \box255?

[Robin Fairbairns]

>         I use "lineno" package in my book and it works well. However,
> If I can get all contents from \box255 before it will be \unvbox at
> every line by the output routine, I will surely improve my book
> series.
>  
>         At first , I tried to copy the box's contents into a token
> register, but it didn't work.  
> 
>         So, could you please suggest me how to get that contents from \box255
> before \unvbox will blow all of my contents to vertical list by output
> routine in "lineno" package. 

since \box255 is worked on asynchronously in the output routine, the
only place you can sensibly approach it is through the output routine.

the code of the output routine is in the latex source, file
ltoutput.dtx; this is a documented source, though imo the documentation
is not terribly good.

there are no documented sources of the lineno package, so you read the
code and its source direct.

however, your belief that access to the vertical list will help you does
not tally with my understanding of the latex output routine.  i suggest
you offer an explanation of what you hope to do: with that, there is a
good chance that some one will offer alternative means of achieving your
aims.  (or will explain why your aim is not possible.)