groff: End-of-input Traps
5.24.6 End-of-input Traps
-------------------------
-- Request: .em macro
Set a trap at the end of input. MACRO is executed after the last
line of the input file has been processed.
For example, if the document had to have a section at the bottom of
the last page for someone to approve it, the 'em' request could be
used.
.de approval
\c
. ne 3v
. sp (\\n[.t]u - 3v)
. in +4i
. lc _
. br
Approved:\t\a
. sp
Date:\t\t\a
..
.
.em approval
The '\c' in the above example needs explanation. For historical
reasons (and for compatibility with AT&T 'troff'), the end macro
exits as soon as it causes a page break and no remaining data is in
the partially collected line.
Let us assume that there is no '\c' in the above 'approval' macro,
and that the page is full and has been ended with, say, a 'br'
request. The 'ne' request now causes the start of a new page,
which in turn makes 'troff' exit immediately for the reasons just
described. In most situations this is not intended.
To always force processing the whole end macro independently of
this behaviour it is thus advisable to insert something that starts
an empty partially filled line ('\c') whenever there is a chance
that a page break can happen. In the above example, the call of
the 'ne' request assures that the remaining code stays on the same
page, so we have to insert '\c' only once.
The next example shows how to append three lines, then starting a
new page unconditionally. Since '.ne 1' doesn't give the desired
effect - there is always one line available or we are already at
the beginning of the next page - we temporarily increase the page
length by one line so that we can use '.ne 2'.
.de EM
.pl +1v
\c
.ne 2
line one
.br
\c
.ne 2
line two
.br
\c
.ne 2
line three
.br
.pl -1v
\c
'bp
..
.em EM
Note that this specific feature affects only the first potential
page break caused by the end macro; further page breaks emitted by
the end macro are handled normally.
Another possible use of the 'em' request is to make 'gtroff' emit a
single large page instead of multiple pages. For example, one may
want to produce a long plain-text file for reading on-screen. The
idea is to set the page length at the beginning of the document to
a very large value to hold all the text, and automatically adjust
it to the exact height of the document after the text has been
output.
.de adjust-page-length
. br
. pl \\n[nl]u \" \n[nl] holds the current vert. position
..
.
.de single-page-mode
. pl 99999
. em adjust-page-length
..
.
.\" activate the above code
.single-page-mode
Since only one end-of-input trap does exist and other macro
packages may already use it, care must be taken not to break the
mechanism. A simple solution would be to append the above macro to
the macro package's end-of-input macro using the '.am' request.