File: groff.info,  Node: Page Location Traps,  Next: The Implicit Page Trap,  Prev: Vertical_Position_Traps.php">Vertical Position Traps,  Up: Vertical_Position_Traps.php">Vertical Position Traps

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.(1)  (*note Page
     Location Traps-Footnote-1::) 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 page(2) (*note Page Location
     Traps-Footnote-2::) 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 (*note
     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.(3)  (*note Page Location Traps-Footnote-3::)

     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.  *Note 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.