This is groff, produced by makeinfo version 4.2 from ./groff.texinfo.

This manual documents GNU `troff' version 1.18.

   Copyright (C) 1994-2000, 2001, 2002 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.1 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being `A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     `GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: `You have freedom to copy and
     modify this GNU Manual, like GNU software.  Copies published by
     the Free Software Foundation raise funds for GNU development."
   
INFO-DIR-SECTION Miscellaneous
START-INFO-DIR-ENTRY
* Groff: (groff).               The GNU troff document formatting system.
END-INFO-DIR-ENTRY


File: groff,  Node: Strings,  Next: Conditionals and Loops,  Prev: Sizes,  Up: gtroff Reference

Strings
=======

   `gtroff' has string variables, which are entirely for user
convenience (i.e. there are no built-in strings exept `.T', but even
this is a read-write string variable).

 - Request: .ds name [string]
 - Request: .ds1 name [string]
 - Escape: \*N
 - Escape: \*(NM
 - Escape: \*[NAME ARG1 ARG2 ...]
     Define and access a string variable NAME (one-character name N,
     two-character name NM).  If NAME already exists, `ds' overwrites
     the previous definition.  Only the syntax form using brackets can
     take arguments which are handled identically to macro arguments;
     the single exception is that a closing bracket as an argument must
     be enclosed in double quotes.  *Note Request Arguments::, and
     *Note Parameters::.

     Example:


          .ds foo a \\$1 test
          .
          This is \*[foo nice].
              => This is a nice test.

     The `\*' escape "interpolates" (expands in-place) a
     previously-defined string variable.  To be more precise, the stored
     string is pushed onto the input stack which is then parsed by
     `gtroff'.  Similar to number registers, it is possible to nest
     strings, i.e. string variables can be called within string
     variables.

     If the string named by the `\*' escape does not exist, it is
     defined as empty, and a warning of type `mac' is emitted (see
     *Note Debugging::, for more details).

     *Caution:* Unlike other requests, the second argument to the `ds'
     request takes up the entire line including trailing spaces.  This
     means that comments on a line with such a request can introduce
     unwanted space into a string.


          .ds UX \s-1UNIX\s0\u\s-3tm\s0\d \" UNIX trademark

     Instead the comment should be put on another line or have the
     comment escape adjacent with the end of the string.


          .ds UX \s-1UNIX\s0\u\s-3tm\s0\d\"  UNIX trademark

     To produce leading space the string can be started with a double
     quote.  No trailing quote is needed; in fact, any trailing quote is
     included in your string.


          .ds sign "           Yours in a white wine sauce,

     Strings are not limited to a single line of text.  A string can
     span several lines by escaping the newlines with a backslash.  The
     resulting string is stored _without_ the newlines.


          .ds foo lots and lots \
          of text are on these \
          next several lines

     It is not possible to have real newlines in a string.  To put a
     single double quote character into a string, use two consecutive
     double quote characters.

     The `ds1' request turns off compatibility mode while interpreting
     a string.  To be more precise, a "compatibility save" input token
     is inserted at the beginning of  the string, and a "compatibility
     restore" input token at the end.


          .nr xxx 12345
          .ds aa The value of xxx is \\n[xxx].
          .ds1 bb The value of xxx ix \\n[xxx].
          .
          .cp 1
          .
          \*(aa
              => warning: number register `[' not defined
              => The value of xxx is 0xxx].
          \*(bb
              => The value of xxx ix 12345.

     Strings, macros, and diversions (and boxes) share the same name
     space.  Internally, even the same mechanism is used to store them.
     This has some interesting consequences.  For example, it is
     possible to call a macro with string syntax and vice versa.


          .de xxx
          a funny test.
          ..
          This is \*[xxx]
              => This is a funny test.
          
          .ds yyy a funny test
          This is
          .yyy
              => This is a funny test.

     Diversions and boxes can be also called with string syntax.

     Another consequence is that you can copy one-line diversions or
     boxes to a string.


          .di xxx
          a \fItest\fR
          .br
          .di
          .ds yyy This is \*[xxx]\c
          \*[yyy].
              => This is a test.

     As the previous example shows, it is possible to store formatted
     output in strings.  The `\c' escape prevents the insertion of an
     additional blank line in the output.

     Copying diversions longer than a single output line produces
     unexpected results.


          .di xxx
          a funny
          .br
          test
          .br
          .di
          .ds yyy This is \*[xxx]\c
          \*[yyy].
              => test This is a funny.

     Usually, it is not predictable whether a diversion contains one or
     more output lines, so this mechanism should be avoided.  With UNIX
     `troff', this was the only solution to strip off a final newline
     from a diversion.  Another disadvantage is that the spaces in the
     copied string are already formatted, making them unstretchable.
     This can cause ugly results.

     A clean solution to this problem is available in GNU `troff',
     using the requests `chop' to remove the final newline of a
     diversion, and `unformat' to make the horizontal spaces
     stretchable again.


          .box xxx
          a funny
          .br
          test
          .br
          .box
          .chop xxx
          .unformat xxx
          This is \*[xxx].
              => This is a funny test.

     *Note Gtroff Internals::, for more information.

 - Request: .as name [string]
 - Request: .as1 name [string]
     The `as' request is similar to `ds' but appends STRING to the
     string stored as NAME instead of redefining it.  If NAME doesn't
     exist yet, it is created.


          .as sign " with shallots, onions and garlic,

     The `as1' request is similar to `as', but compatibility mode is
     switched off while the appended string is interpreted.  To be more
     precise, a "compatibility save" input token is inserted at the
     beginning of the appended string, and a "compatibility restore"
     input token at the end.

   Rudimentary string manipulation routines are given with the next two
requests.

 - Request: .substring str n1 [n2]
     Replace the string named STR with the substring defined by the
     indices N1 and N2.  The first character in the string has index 0.
     If N2 is omitted, it is taken to be equal to the string's length.
     If the index value N1 or N2 is negative, it is counted from the
     end of the string, going backwards: The last character has
     index -1, the character before the last character has index -2,
     etc.


          .ds xxx abcdefgh
          .substring xxx 1 -4
          \*[xxx]
              => bcde


 - Request: .length reg str
     Compute the number of characters of STR and return it in the
     number register REG.  If REG doesn't exist, it is created.  `str'
     is read in copy mode.


          .ds xxx abcd\h'3i'efgh
          .length yyy \n[xxx]
          \n[yyy]
              => 14


 - Request: .rn xx yy
     Rename the request, macro, diversion, or string XX to YY.

 - Request: .rm xx
     Remove the request, macro, diversion, or string XX.  `gtroff'
     treats subsequent invocations as if the object had never been
     defined.

 - Request: .als new old
     Create an alias named NEW for the request, string, macro, or
     diversion object named OLD.  The new name and the old name are
     exactly equivalent (it is similar to a hard rather than a soft
     link). If OLD is undefined, `gtroff' generates a warning of type
     `mac' and ignores the request.

 - Request: .chop xx
     Remove (chop) the last character from the macro, string, or
     diversion named XX.  This is useful for removing the newline from
     the end of diversions that are to be interpolated as strings.
     This command can be used repeatedly; see *Note Gtroff Internals::,
     for details on nodes inserted additionally by `gtroff'.

   *Note Identifiers::, and *Note Comments::.


File: groff,  Node: Conditionals and Loops,  Next: Writing Macros,  Prev: Strings,  Up: gtroff Reference

Conditionals and Loops
======================

* Menu:

* Operators in Conditionals::
* if-else::
* while::


File: groff,  Node: Operators in Conditionals,  Next: if-else,  Prev: Conditionals and Loops,  Up: Conditionals and Loops

Operators in Conditionals
-------------------------

   In `if' and `while' requests, there are several more operators
available:

`e'
`o'
     True if the current page is even or odd numbered (respectively).

`n'
     True if the document is being processed in nroff mode (i.e., the
     `.nroff' command has been issued).

`t'
     True if the document is being processed in troff mode (i.e., the
     `.troff' command has been issued).

`v'
     Always false.  This condition is for compatibility with other
     `troff' versions only.

`'XXX'YYY''
     True if the string XXX is equal to the string YYY.  Other
     characters can be used in place of the single quotes; the same set
     of delimiters as for the `\D' escape is used (*note Escapes::).
     `gtroff' formats the strings before being compared:


          .ie "|"\fR|\fP" \
          true
          .el \
          false
              => true

     The resulting motions, glyph sizes, and fonts have to match,(1)
     (*note Operators in Conditionals-Footnote-1::) and not the
     individual motion, size, and font requests.  In the previous
     example, `|' and `\fR|\fP' both result in a roman `|' glyph with
     the same point size and at the same location on the page, so the
     strings are equal.  If `.ft I' had been added before the `.ie',
     the result would be "false" because (the first) `|' produces an
     italic `|' rather than a roman one.

`r XXX'
     True if there is a number register named XXX.

`d XXX'
     True if there is a string, macro, diversion, or request named XXX.

`m XXX'
     True if there is a color named XXX.

`c G'
     True if there is a glyph G available(2) (*note Operators in
     Conditionals-Footnote-2::); G is either an ASCII character or a
     special character (`\(GG' or `\[GGG]'); the condition is also true
     if G has been defined by the `char' request.

   Note that these operators can't be combined with other operators like
`:' or `&'; only a leading `!' (without whitespace between the
exclamation mark and the operator) can be used to negate the result.


     .nr xxx 1
     .ie !r xxx \
     true
     .el \
     false
         => false

   A whitespace after `!' always evaluates to zero (this bizarre
behaviour is due to compatibility with UNIX `troff').


     .nr xxx 1
     .ie ! r xxx \
     true
     .el \
     false
         => r xxx true

   It is possible to omit the whitespace before the argument to the
`r', `d', and `c' operators.

   *Note Expressions::.


File: groff,  Node: Operators in Conditionals-Footnotes,  Up: Operators in Conditionals

   (1) The created output nodes must be identical.  *Note Gtroff
Internals::.

   (2) The name of this conditional operator is a misnomer since it
tests names of output glyphs.


File: groff,  Node: if-else,  Next: while,  Prev: Operators in Conditionals,  Up: Conditionals and Loops

if-else
-------

   `gtroff' has if-then-else constructs like other languages, although
the formatting can be painful.

 - Request: .if expr anything
     Evaluate the expression EXPR, and executes ANYTHING (the remainder
     of the line) if EXPR evaluates to non-zero (true).  ANYTHING is
     interpreted as though it was on a line by itself (except that
     leading spaces are swallowed).  *Note Expressions::, for more info.


          .nr xxx 1
          .nr yyy 2
          .if ((\n[xxx] == 1) & (\n[yyy] == 2)) true
              => true


 - Request: .nop anything
     Executes ANYTHING.  This is similar to `.if 1'.

 - Request: .ie expr anything
 - Request: .el anything
     Use the `ie' and `el' requests to write an if-then-else.  The
     first request is the `if' part and the latter is the `else' part.


          .ie n .ls 2 \" double-spacing in nroff
          .el   .ls 1 \" single-spacing in troff


 - Escape: \{
 - Escape: \}
     In many cases, an if (or if-else) construct needs to execute more
     than one request.  This can be done using the `\{' and `\}'
     escapes.  The following example shows the possible ways to use
     these escapes (note the position of the opening and closing
     braces).


          .ie t \{\
          .    ds lq ``
          .    ds rq ''
          .\}
          .el \
          .\{\
          .    ds lq "
          .    ds rq "\}


   *Note Expressions::.


File: groff,  Node: while,  Prev: if-else,  Up: Conditionals and Loops

while
-----

   `gtroff' provides a looping construct using the `while' request,
which is used much like the `if' (and related) requests.

 - Request: .while expr anything
     Evaluate the expression EXPR, and repeatedly execute ANYTHING (the
     remainder of the line) until EXPR evaluates to 0.


          .nr a 0 1
          .while (\na < 9) \{\
          \n+a,
          .\}
          \n+a
              => 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

     Some remarks.

        * The body of a `while' request is treated like the body of a
          `de' request: `gtroff' temporarily stores it in a macro which
          is deleted after the loop has been exited.  It can
          considerably slow down a macro if the body of the `while'
          request (within the macro) is large.  Each time the macro is
          executed, the `while' body is parsed and stored again as a
          temporary macro.


               .de xxx
               .  nr num 10
               .  while (\\n[num] > 0) \{\
               .    \" many lines of code
               .    nr num -1
               .  \}
               ..

          The traditional and ofter better solution (UNIX `troff'
          doesn't have the `while' request) is to use a recursive macro
          instead which is parsed only once during its definition.


               .de yyy
               .  if (\\n[num] > 0) \{\
               .    \" many lines of code
               .    nr num -1
               .    yyy
               .  \}
               ..
               .
               .de xxx
               .  nr num 10
               .  yyy
               ..

          Note that the number of available recursion levels is set
          to 1000 (this is a compile-time constant value of `gtroff').

        * The closing brace of a `while' body must end a line.


               .if 1 \{\
               .  nr a 0 1
               .  while (\n[a] < 10) \{\
               .    nop \n+[a]
               .\}\}
                   => unbalanced \{ \}


 - Request: .break
     Break out of a `while' loop.  Be sure not to confuse this with the
     `br' request (causing a line break).

 - Request: .continue
     Finish the current iteration of a `while' loop, immediately
     restarting the next iteration.

   *Note Expressions::.


File: groff,  Node: Writing Macros,  Next: Page Motions,  Prev: Conditionals and Loops,  Up: gtroff Reference

Writing Macros
==============

   A "macro" is a collection of text and embedded commands which can be
invoked multiple times.  Use macros to define common operations.

 - Request: .de name [end]
 - Request: .de1 name [end]
 - Request: .dei name [end]
     Define a new macro named NAME.  `gtroff' copies subsequent lines
     (starting with the next one) into an internal buffer until it
     encounters the line `..' (two dots).  The optional second argument
     to `de' changes this to a macro to `.END'.

     There can be whitespace after the first dot in the line containing
     the ending token (either `.' or macro `END').

     Here a small example macro called `P' which causes a break and
     inserts some vertical space.  It could be used to separate
     paragraphs.


          .de P
          .  br
          .  sp .8v
          ..

     The following example defines a macro within another.  Remember
     that expansion must be protected twice; once for reading the macro
     and once for executing.


          \# a dummy macro to avoid a warning
          .de end
          ..
          .
          .de foo
          .  de bar end
          .    nop \f[B]Hallo \\\\$1!\f[]
          .  end
          ..
          .
          .foo
          .bar Joe
              => Hallo Joe!

     Since `\f' has no expansion, it isn't necessary to protect its
     backslash.  Had we defined another macro within `bar' which takes
     a parameter, eight backslashes would be necessary before `$1'.

     The `de1' request turns off compatibility mode while executing the
     macro.  On entry, the current compatibility mode is saved and
     restored at exit.


          .nr xxx 12345
          .
          .de aa
          The value of xxx is \\n[xxx].
          ..
          .de1 bb
          The value of xxx ix \\n[xxx].
          ..
          .
          .cp 1
          .
          .aa
              => warning: number register ' not defined
              => The value of xxx is 0xxx].
          .bb
              => The value of xxx ix 12345.

     The `dei' request defines a macro indirectly.  That is, it expands
     strings whose names are NAME or END before performing the append.

     This:


          .ds xx aa
          .ds yy bb
          .dei xx yy

     is equivalent to:


          .de aa bb

     Using `trace.tmac', you can trace calls to `de' and `de1'.

     Note that macro identifiers are shared with identifiers for
     strings and diversions.

 - Request: .am xx
 - Request: .am1 xx
 - Request: .ami xx yy
     Works similarly to `de' except it appends onto the macro named XX.
     So, to make the previously defined `P' macro actually do indented
     instead of block paragraphs, add the necessary code to the
     existing macro like this:


          .am P
          .ti +5n
          ..

     The `am1' request turns off compatibility mode while executing the
     appended macro piece.  To be more precise, a "compatibility save"
     input token is inserted at the beginning of the appended code, and
     a "compatibility restore" input token at the end.

     The `ami' request appends indirectly, meaning that `gtroff'
     expands strings whose names are XX or YY before performing the
     append.

     Using `trace.tmac', you can trace calls to `am' and `am1'.

   *Note Strings::, for the `als' request to rename a macro.

   The `de', `am', `di', `da', `ds', and `as' requests (together with
its variants) only create a new object if the name of the macro,
diversion or string diversion is currently undefined or if it is
defined to be a request; normally they modify the value of an existing
object.

 - Request: .return
     Exit a macro, immediately returning to the caller.

* Menu:

* Copy-in Mode::
* Parameters::


File: groff,  Node: Copy-in Mode,  Next: Parameters,  Prev: Writing Macros,  Up: Writing Macros

Copy-in Mode
------------

   When `gtroff' reads in the text for a macro, string, or diversion,
it copies the text (including request lines, but excluding escapes) into
an internal buffer.  Escapes are converted into an internal form,
except for `\n', `\$', `\*', `\\' and `\<RET>' which are evaluated and
inserted into the text where the escape was located.  This is known as
"copy-in" mode or "copy" mode.

   What this means is that you can specify when these escapes are to be
evaluated (either at copy-in time or at the time of use) by insulating
the escapes with an extra backslash.  Compare this to the `\def' and
`\edef' commands in TeX.

   The following example prints the numbers 20 and 10:


     .nr x 20
     .de y
     .nr x 10
     \&\nx
     \&\\nx
     ..
     .y


File: groff,  Node: Parameters,  Prev: Copy-in Mode,  Up: Writing Macros

Parameters
----------

   The arguments to a macro or string can be examined using a variety of
escapes.

 - Register: \n[.$]
     The number of arguments passed to a macro or string.  This is a
     read-only number register.

   Any individual argument can be retrieved with one of the following
escapes:

 - Escape: \$N
 - Escape: \$(NN
 - Escape: \$[NNN]
     Retrieve the Nth, NNth or NNNth argument.  As usual, the first
     form only accepts a single number (larger than zero), the second a
     two-digit number (larger or equal to 10), and the third any
     positive integer value (larger than zero).  Macros and strings can
     have an unlimited number of arguments.  Note that due to copy-in
     mode, use two backslashes on these in actual use to prevent
     interpolation until the macro is actually invoked.

 - Request: .shift [n]
     Shift the arguments 1 position, or as many positions as specified
     by its argument.  After executing this request, argument I becomes
     argument I-N; arguments 1 to N are no longer available.  Shifting
     by negative amounts is currently undefined.

 - Escape: \$*
 - Escape: \$@
     In some cases it is convenient to use all of the arguments at once
     (for example, to pass the arguments along to another macro).  The
     `\$*' escape concatenates all the arguments separated by spaces.  A
     similar escape is `\$@', which concatenates all the arguments with
     each surrounded by double quotes, and separated by spaces.  If not
     in compatibility mode, the input level of double quotes is
     preserved (see *Note Request Arguments::).

 - Escape: \$0
     The name used to invoke the current macro.  The `als' request can
     make a macro have more than one name.


          .de generic-macro
          .  ...
          .  if \\n[error] \{\
          .    tm \\$0: Houston, we have a problem.
          .    return
          .  \}
          ..
          .
          .als foo generic-macro
          .als bar generic-macro


   *Note Request Arguments::.


File: groff,  Node: Page Motions,  Next: Drawing Requests,  Prev: Writing Macros,  Up: gtroff Reference

Page Motions
============

   *Note Manipulating Spacing::, for a discussion of the main request
for vertical motion, `sp'.

 - Request: .mk [reg]
 - Request: .rt [dist]
     The request `mk' can be used to mark a location on a page, for
     movement to later.  This request takes a register name as an
     argument in which to store the current page location.  With no
     argument it stores the location in an internal register.  The
     results of this can be used later by the `rt' or the `sp' request
     (or the `\v' escape).

     The `rt' request returns _upwards_ to the location marked with the
     last `mk' request.  If used with an argument, return to a position
     which distance from the top of the page is DIST (no previous call
     to `mk' is necessary in this case).  Default scaling indicator is
     `v'.

     Here a primitive solution for a two-column macro.


          .nr column-length 1.5i
          .nr column-gap 4m
          .nr bottom-margin 1m
          .


          .de 2c
          .  br
          .  mk
          .  ll \\n[column-length]u
          .  wh -\\n[bottom-margin]u 2c-trap
          .  nr right-side 0
          ..
          .


          .de 2c-trap
          .  ie \\n[right-side] \{\
          .    nr right-side 0
          .    po -(\\n[column-length]u + \\n[column-gap]u)
          .    \" remove trap
          .    wh -\\n[bottom-margin]u
          .  \}
          .  el \{\
          .    \" switch to right side
          .    nr right-side 1
          .    po +(\\n[column-length]u + \\n[column-gap]u)
          .    rt
          .  \}
          ..
          .


          .pl 1.5i
          .ll 4i
          This is a small test which shows how the
          rt request works in combination with mk.
          
          .2c
          Starting here, text is typeset in two columns.
          Note that this implementation isn't robust
          and thus not suited for a real two-column
          macro.

     Result:


          This is a small test which shows how the
          rt request works in combination with mk.
          
          Starting  here,    isn't    robust
          text is typeset    and   thus  not
          in two columns.    suited  for   a
          Note that  this    real two-column
          implementation     macro.


   The following escapes give fine control of movements about the page.

 - Escape: \v'E'
     Move vertically, usually from the current location on the page (if
     no absolute position operator `|' is used).  The argument E
     specifies the distance to move; positive is downwards and negative
     upwards.  The default scaling indicator for this escape is `v'.
     Beware, however, that `gtroff' continues text processing at the
     point where the motion ends, so you should always balance motions
     to avoid interference with text processing.

     `\v' doesn't trigger a trap.  This can be quite useful; for
     example, consider a page bottom trap macro which prints a marker
     in the margin to indicate continuation of a footnote or something
     similar.

   There are some special-case escapes for vertical motion.

 - Escape: \r
     Move upwards 1v.

 - Escape: \u
     Move upwards .5v.

 - Escape: \d
     Move down .5v.

 - Escape: \h'E'
     Move horizontally, usually from the current location (if no
     absolute position operator `|' is used).  The expression E
     indicates how far to move: positive is rightwards and negative
     leftwards.  The default scaling indicator for this escape is `m'.

   There are a number of special-case escapes for horizontal motion.

 - Escape: \<SP>
     An unbreakable and unpaddable (i.e. not expanded during filling)
     space.  (Note: This is a backslash followed by a space.)

 - Escape: \~
     An unbreakable space that stretches like a normal inter-word space
     when a line is adjusted.

 - Escape: \|
     A 1/6th em space.  Ignored for TTY output devices (rounded to
     zero).

 - Escape: \^
     A 1/12th em space.  Ignored for TTY output devices (rounded to
     zero).

 - Escape: \0
     A space the size of a digit.

   The following string sets the TeX logo:


     .ds TeX T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X

 - Escape: \w'TEXT'
 - Register: \n[st]
 - Register: \n[sb]
 - Register: \n[rst]
 - Register: \n[rsb]
 - Register: \n[ct]
 - Register: \n[ssc]
 - Register: \n[skw]
     Return the width of the specified TEXT in basic units.  This
     allows horizontal movement based on the width of some arbitrary
     text (e.g. given as an argument to a macro).


          The length of the string `abc' is \w'abc'u.
              => The length of the string `abc' is 72u.

     Font changes may occur in TEXT which don't affect current settings.

     After use, `\w' sets several registers:

    `st'
    `sb'
          The highest and lowest point of the baseline, respectively,
          in TEXT.

    `rst'
    `rsb'
          Like the `st' and `sb' registers, but takes account of the
          heights and depths of glyphs.  With other words, this gives
          the highest and lowest point of TEXT.

    `ct'
          Defines the kinds of glyphs occurring in TEXT:

         0
               only short glyphs, no descenders or tall glyphs.

         1
               at least one descender.

         2
               at least one tall glyph.

         3
               at least one each of a descender and a tall glyph.

    `ssc'
          The amount of horizontal space (possibly negative) that
          should be added to the last glyph before a subscript.

    `skw'
          How far to right of the center of the last glyph in the `\w'
          argument, the center of an accent from a roman font should be
          placed over that glyph.

 - Escape: \kP
 - Escape: \k(PS
 - Escape: \k[POSITION]
     Store the current horizontal position in the _input_ line in
     number register with name POSITION (one-character name P,
     two-character name PS).  Use this, for example, to return to the
     beginning of a string for highlighting or other decoration.

 - Register: \n[hp]
     The current horizontal position at the input line.

 - Register: \n[.k]
     A read-only number register containing the current horizontal
     output position.

 - Escape: \o'ABC'
     Overstrike glyphs A, B, C, ...; the glyphs are centered, and the
     resulting spacing is the largest width of the affected glyphs.

 - Escape: \zG
     Print glyph G with zero width, i.e., without spacing.  Use this to
     overstrike glyphs left-aligned.

 - Escape: \Z'ANYTHING'
     Print ANYTHING, then restore the horizontal and vertical position.
     The argument may not contain tabs or leaders.

     The following is an example of a strike-through macro:


          .de ST
          .nr ww \w'\\$1'
          \Z@\v'-.25m'\l'\\n[ww]u'@\\$1
          ..
          .
          This is
          .ST "a test"
          an actual emergency!



File: groff,  Node: Drawing Requests,  Next: Traps,  Prev: Page Motions,  Up: gtroff Reference

Drawing Requests
================

   `gtroff' provides a number of ways to draw lines and other figures
on the page.  Used in combination with the page motion commands (see
*Note Page Motions::, for more info), a wide variety of figures can be
drawn.  However, for complex drawings these operations can be quite
cumbersome, and it may be wise to use graphic preprocessors like `gpic'
or `ggrn'.  *Note gpic::, and *Note ggrn::, for more information.

   All drawing is done via escapes.

 - Escape: \l'L'
 - Escape: \l'LG'
     Draw a line horizontally.  L is the length of the line to be
     drawn.  If it is positive, start the line at the current location
     and draw to the right; its end point is the new current location.
     Negative values are handled differently: The line starts at the
     current location and draws to the left, but the current location
     doesn't move.

     L can also be specified absolutely (i.e. with a leading `|') which
     draws back to the beginning of the input line.  Default scaling
     indicator is `m'.

     The optional second parameter G is a glyph to draw the line with.
     If this second argument is not specified, `gtroff' uses the
     underscore glyph, `\[ru]'.

     To separate the two arguments (to prevent `gtroff' from
     interpreting a drawing glyph as a scaling indicator if the glyph is
     represented by a single character) use `\&'.

     Here a small useful example:


          .de box
          \[br]\\$*\[br]\l'|0\[rn]'\l'|0\[ul]'
          ..

     Note that this works by outputting a box rule (a vertical line),
     then the text given as an argument and then another box rule.
     Finally, the line drawing escapes both draw from the current
     location to the beginning of the _input_ line - this works because
     the line length is negative, not moving the current point.

 - Escape: \L'L'
 - Escape: \L'LG'
     Draw vertical lines.  Its parameters are similar to the `\l'
     escape, except that the default scaling indicator is `v'.  The
     movement is downwards for positive values, and upwards for
     negative values.  The default glyph is the box rule glyph,
     `\[br]'.  As with the vertical motion escapes, text processing
     blindly continues where the line ends.


          This is a \L'3v'test.

     Here the result, produced with `grotty'.


          This is a
                    |
                    |
                    |test.


 - Escape: \D'COMMAND ARG ...'
     The `\D' escape provides a variety of drawing functions.  Note
     that on character devices, only vertical and horizontal lines are
     supported within `grotty'; other devices may only support a subset
     of the available drawing functions.

     The default scaling indicator for all subcommands of `\D' is `m'
     for horizontal distances and `v' for vertical ones.  Exceptions
     are `\D'f ...'' and `\D't ...'' which use `u' as the default.

    `\D'l DX DY''
          Draw a line from the current location to the relative point
          specified by (DX,DY).

          The following example is a macro for creating a box around a
          text string; for simplicity, the box margin is taken as a
          fixed value, 0.2m.


               .de BOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \D'l 0 -(\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l (\\n[@wd]u + .4m) 0'\
               \D'l 0 (\\n[rst]u - \\n[rsb]u + .4m)'\
               \D'l -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \\$1\
               \h'.2m'
               ..

          First, the width of the string is stored in register `@wd'.
          Then, four lines are drawn to form a box, properly offset by
          the box margin.  The registers `rst' and `rsb' are set by the
          `\w' escape, containing the largest height and depth of the
          whole string.

    `\D'c D''
          Draw a circle with a diameter of D with the leftmost point at
          the current position.

    `\D'C D''
          Draw a solid circle with the same parameters as an outlined
          circle.  No outline is drawn.

    `\D'e X Y''
          Draw an ellipse with a horizontal diameter of X and a vertical
          diameter of Y with the leftmost point at the current position.

    `\D'E X Y''
          Draw a solid ellipse with the same parameters as an outlined
          ellipse.  No outline is drawn.

    `\D'a DX1 DY1 DX2 DY2''
          Draw an arc clockwise from the current location through the
          two specified relative locations (DX1,DY1) and (DX2,DY2).
          The coordinates of the first point are relative to the
          current position, and the coordinates of the second point are
          relative to the first point.

    `\D'~ DX1 DY1 DX2 DY2 ...''
          Draw a spline from the current location to the relative point
          (DX1,DY1) and then to (DX2,DY2), and so on.

    `\D'f N''
          Set the shade of gray to be used for filling solid objects
          to N; N must be an integer between 0 and 1000, where 0
          corresponds solid white and 1000 to solid black, and values
          in between correspond to intermediate shades of gray.  This
          applies only to solid circles, solid ellipses, and solid
          polygons.  By default, a level of 1000 is used.

    `\D'p DX1 DY1 DX2 DY2 ...''
          Draw a polygon from the current location to the relative
          position (DX1,DY1) and then to (DX2,DY2) and so on.  When the
          specified data points are exhausted, a line is drawn back to
          the starting point.

    `\D'P DX1 DY1 DX2 DY2 ...''
          Draw a solid polygon with the same parameters as an outlined
          polygon.  No outline is drawn.

          Here a better variant of the box macro to fill the box with
          some color.  Note that the box must be drawn before the text
          since colors in `gtroff' are not transparent; the filled
          polygon would hide the text completely.


               .de BOX
               .  nr @wd \w'\\$1'
               \h'.2m'\
               \h'-.2m'\v'(.2m - \\n[rsb]u)'\
               \M[lightcyan]\
               \D'P 0 -(\\n[rst]u - \\n[rsb]u + .4m) \
                    (\\n[@wd]u + .4m) 0 \
                    0 (\\n[rst]u - \\n[rsb]u + .4m) \
                    -(\\n[@wd]u + .4m) 0'\
               \h'.2m'\v'-(.2m - \\n[rsb]u)'\
               \M[]\
               \\$1\
               \h'.2m'
               ..

    `\D't N''
          Set the current line thickness to N machine units.  A value of
          zero selects the smallest available line thickness.  A
          negative value makes the line thickness proportional to the
          current point size (this is the default behaviour of AT&T
          `troff').

   *Note Graphics Commands::.

 - Escape: \b'STRING'
     "Pile" a sequence of glyphs vertically, and center it vertically
     on the current line.  Use it to build large brackets and braces.

     Here an example how to create a large opening brace:


          \b'\[lt]\[bv]\[lk]\[bv]\[lb]'

     The first glyph is on the top, the last glyph in STRING is at the
     bottom.  Note that `gtroff' separates the glyphs vertically by 1m,
     and the whole object is centered 0.5m above the current baseline;
     the largest glyph width is used as the width for the whole object.
     This rather unflexible positioning algorithm doesn't work with
     `-Tdvi' since the bracket pieces vary in height for this device.
     Instead, use the `eqn' preprocessor.

     *Note Manipulating Spacing::, how to adjust the vertical spacing
     with the `\x' escape.


File: groff,  Node: Traps,  Next: Diversions,  Prev: Drawing Requests,  Up: gtroff Reference

Traps
=====

   "Traps" are locations, which, when reached, call a specified macro.
These traps can occur at a given location on the page, at a given
location in the current diversion, at a blank line, after a certain
number of input lines, or at the end of input.

   Setting a trap is also called "planting".  It is also said that a
trap is "sprung" if the associated macro is executed.

* Menu:

* Page Location Traps::
* Diversion Traps::
* Input Line Traps::
* Blank Line Traps::
* End-of-input Traps::


File: groff,  Node: Page Location Traps,  Next: Diversion Traps,  Prev: Traps,  Up: Traps

Page Location Traps
-------------------

   "Page location traps" perform an action when `gtroff' reaches or
passes a certain vertical location on the page.  Page location traps
have a variety of purposes, including:

   * setting headers and footers

   * setting body text in multiple columns

   * setting footnotes

 - Request: .vpt flag
 - Register: \n[.vpt]
     Enable vertical position traps if FLAG is non-zero, or disables
     them otherwise.  Vertical position traps are traps set by the `wh'
     or `dt' requests.  Traps set by the `it' request are not vertical
     position traps.  The parameter that controls whether vertical
     position traps are enabled is global.  Initially vertical position
     traps are enabled.  The current setting of this is available in the
     `.vpt' read-only number register.

 - Request: .wh dist [macro]
     Set a page location trap.  Positive 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.  Default scaling indicator is
     `v'.

     MACRO is the name of the macro to execute when the trap is sprung.
     If MACRO is missing, remove the first trap (if any) at DIST.

     The following is a simple example of how many macro packages set
     headers and footers.


          .de hd                \" Page header
          '  sp .5i
          .  tl 'Title''date'
          '  sp .3i
          ..
          .
          .de fo                \" Page footer
          '  sp 1v
          .  tl ''%''
          '  bp
          ..
          .
          .wh 0   hd            \" trap at top of the page
          .wh -1i fo            \" trap one inch from bottom

     A trap at or below the bottom of the page is ignored; it can be
     made active by either moving it up or increasing the page length
     so that the trap is on the page.

     It is possible to have more than one trap at the same location; to
     do so, the traps must be defined at different locations, then
     moved together with the `ch' request; otherwise the second trap
     would replace the first one.  Earlier defined traps hide later
     defined traps if moved to the same position (the many empty lines
     caused by the `bp' request are omitted):


          .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]
     A read-only number register holding the distance to the next trap.

     If there are no traps between the current position and the bottom
     of the page, it contains the distance to the page bottom.  In a
     diversion, the distance to the page bottom is infinite (the
     returned value is the biggest integer which can be represented in
     `groff') if there are no diversion traps.

 - Request: .ch macro dist
     Change the location of a trap.  The first argument is the name of
     the macro to be invoked at the trap, and the second argument is
     the new location for the trap (note that the parameters are
     specified the opposite of the `wh' request).  This is useful for
     building up footnotes in a diversion to allow more space at the
     bottom of the page for them.

     Default scaling indicator for DIST is `v'.  If DIST is missing,
     the trap is removed.


 - Register: \n[.ne]
     The read-only number register `.ne' contains the amount of space
     that was needed in the last `ne' request that caused a trap to be
     sprung.  Useful in conjunction with the `.trunc' register.  *Note
     Page Control::, for more information.

 - Register: \n[.trunc]
     A read-only register containing the amount of vertical space
     truncated 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.


File: groff,  Node: Diversion Traps,  Next: Input Line Traps,  Prev: Page Location Traps,  Up: Traps

Diversion Traps
---------------

 - Request: .dt dist macro
     Set a trap _within_ a diversion.  DIST is the location of the trap
     (identical to the `.wh' request; default scaling indicator is `v')
     and MACRO is the name of the macro to be invoked.  The number
     register `.t' still works within diversions.  *Note Diversions::,
     for more information.


File: groff,  Node: Input Line Traps,  Next: Blank Line Traps,  Prev: Diversion Traps,  Up: Traps

Input Line Traps
----------------

 - Request: .it n macro
 - Request: .itc n macro
     Set an input line trap.  N is the number of lines of input which
     may be read before springing the trap, MACRO is the macro to be
     invoked.  Request lines are not counted as input lines.

     For example, one possible use is to have a macro which prints the
     next N lines in a bold font.


          .de B
          .  it \\$1 B-end
          .  ft B
          ..
          .
          .de B-end
          .  ft R
          ..

     The `itc' request is identical, except that a line interrupted
     with `\c' counts as one input line.

     Both requests are associated with the current environment (*note
     Environments::); switching to another environment disables the
     current input trap, and going back reactivates it, restoring the
     number of already processed lines.


File: groff,  Node: Blank Line Traps,  Next: End-of-input Traps,  Prev: Input Line Traps,  Up: Traps

Blank Line Traps
----------------

 - Request: .blm macro
     Set a blank line trap.  `gtroff' executes MACRO when it encounters
     a blank line in the input file.


File: groff,  Node: End-of-input Traps,  Prev: Blank Line Traps,  Up: Traps

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
          .  ne 5v
          .  sp |(\\n[.t] - 6v)
          .  in +4i
          .  lc _
          .  br
          Approved:\t\a
          .  sp
          Date:\t\t\a
          ..
          .
          .em approval


