5.28.1.1 Page Location Traps

A page location trap is a vertical position trap that applies to the page; that is, to undiverted output. Many can be present; manage them with the wh and ch requests.

Request: .wh dist [name]

Plant macro name as page location trap at dist. The default scaling unit is ‘v’. Non-negative values for dist set the trap relative to the top of the page; negative values set the trap relative to the bottom of the page. It is not possible to plant a trap less than one basic unit from the page bottom: a dist of -0 is interpreted as 0, the top of the page.¹⁰⁶ An existing visible trap (see below) at dist is removed; this is wh’s sole function if name is missing.

A trap is sprung only if it is visible, meaning that its location is reachable on the page¹⁰⁷ and it is not hidden by another trap at the same location already planted there.

A macro package might set headers and footers as follows; this example configures vertical margins of one inch to the body text, and one half-inch to the titles. Observe the use of the no-break control character with sp request to position our text baselines, and the page number character ‘%’ used with the tl request.

.\" hdfo.roff
.de hd                  \" page header
'  sp .5i
'  tl '\\*(Ti''\\*(Da'  \" title and date strings
'  sp .5i
..
.de fo                  \" page footer
'  sp .5i
.  tl ''%''
.  bp
..
.wh 0   hd             \" trap at top of the page
.wh -1i fo             \" trap 1 inch from bottom

To use these traps, copy the above (or load it from a file with the so or mso requests), then set up the strings it uses.

.so hdfo.roff
.ds Ti Final Report\"
.ds Da 21 May 2023\"
.ti
On 5 August of last year,
this committee tasked me with the investigation of the
CFIT (controlled flight into terrain) incident of
.\" ...and so on...

A trap above the top or at or below the bottom of the page can be made visible by either moving it into the page area or increasing the page length so that the trap is on the page. Negative trap values always use the current page length; they are not converted to an absolute vertical position. We can use the ptr request to dump our page location traps to the standard error stream (see Debugging). Their positions are reported in basic units; an nroff device example follows.

.pl 5i
.wh -1i xx
.ptr
    error→ xx      -240
.pl 100i
.ptr
    error→ xx      -240

It is possible to have more than one trap at the same location (although only one at a time can be visible); to achieve this, the traps must be defined at different locations, then moved to the same place with the ch request. In the following example, the many empty lines caused by the bp request are not shown in the output.

.de a
.  nop a
..
.de b
.  nop b
..
.de c
.  nop c
..
.
.wh 1i a
.wh 2i b
.wh 3i c
.bp
    ⇒ a b c

.ch b 1i
.ch c 1i
.bp
    ⇒ a

.ch a 0.5i
.bp
    ⇒ a b

Register: \n[.t]

The read-only register .t holds the distance to the next vertical position trap. If there are no traps between the current position and the bottom of the page, it contains the distance to the page bottom. Within a diversion, in the absence of a diversion trap, this distance is the largest representable integer in basic units—effectively infinite.

Request: .ch name [dist]

Change the location of a trap by moving macro name to new location dist, or by unplanting it altogether if dist is absent. The default scaling unit is ‘v’. Parameters to ch are specified in the opposite order from wh. If name is the earliest planted macro of multiple traps at the same location, (re)moving it from that location exposes the macro next least recently planted at the same place.¹⁰⁸

Changing a trap’s location is useful for building up footnotes in a diversion to allow more space at the bottom of the page for them.

The same macro can be installed simultaneously at multiple locations; however, only the earliest-planted instance—that has not yet been deleted with wh—will be moved by ch. The following example (using an nroff device) illustrates this behavior. Blank lines have been elided from the output.

.de T
Trap sprung at \\n(nlu.
.br
..
.wh 1i T
.wh 2i T
foo
.sp 11i
.bp
.ch T 4i
bar
.sp 11i
.bp
.ch T 5i
baz
.sp 11i
.bp
.wh 5i
.ch T 6i
qux
.sp 11i

    ⇒ foo
    ⇒ Trap sprung at 240u.
    ⇒ Trap sprung at 480u.
    ⇒ bar
    ⇒ Trap sprung at 480u.
    ⇒ Trap sprung at 960u.
    ⇒ baz
    ⇒ Trap sprung at 480u.
    ⇒ Trap sprung at 1200u.
    ⇒ qux
    ⇒ Trap sprung at 1440u.

Register: \n[.ne]

The read-only register .ne contains the amount of space that was needed in the last ne request that caused a trap to be sprung; it is useful in conjunction with the .trunc register. See Page Control. Since the .ne register is set only by traps, it doesn’t make sense to interpolate it outside of macros called by traps.

Register: \n[.trunc]

A read-only register containing the amount of vertical space truncated from an sp request by the most recently sprung vertical position trap, or, if the trap was sprung by an ne request, minus the amount of vertical motion produced by the ne request. In other words, at the point a trap is sprung, it represents the difference of what the vertical position would have been but for the trap, and what the vertical position actually is. Since the .trunc register is set only by traps, it doesn’t make sense to interpolate it outside of macros called by traps.

Register: \n[.pe]

This Boolean-valued, read-only register interpolates 1 while a page is being ejected, and 0 otherwise.

In the following example, we plant the same trap at the top and the bottom of the page. We also make the trap report its name and the vertical drawing position.

.de T
.tm \\$0: page \\n%, nl=\\n[nl] .pe=\\n[.pe]
..
.ll 46n
.wh 0 T
.wh -1v T
Those who can make you believe absurdities can make you
commit atrocities. \[em] Voltaire
    error→ T: page 1, nl=0 .pe=0
    error→ T: page 1, nl=2600 .pe=1
    ⇒ Those who can make you believe absurdities can
    ⇒ make you commit atrocities. -- Voltaire

When designing macros, keep in mind that diversions and traps do normally interact. For example, if a trap calls a header macro (while outputting a diversion) that tries to change the font on the current page, the effect is not visible before the diversion has completely been printed (except for input protected with \! or \?) since the data in the diversion is already formatted. In most cases, this is not the expected behaviour.