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diff --git a/doc/groff.html.node/Page-Location-Traps.html b/doc/groff.html.node/Page-Location-Traps.html new file mode 100644 index 0000000..3d388bf --- /dev/null +++ b/doc/groff.html.node/Page-Location-Traps.html @@ -0,0 +1,326 @@ +<!DOCTYPE html> +<html> +<!-- Created by GNU Texinfo 7.0.3, https://www.gnu.org/software/texinfo/ --> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> +<!-- This manual documents GNU troff version 1.23.0. + +Copyright © 1994-2023 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 1.3 or +any later version published by the Free Software Foundation; with no +Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A +copy of the license is included in the section entitled "GNU Free +Documentation License". --> +<title>Page Location Traps (The GNU Troff Manual)</title> + +<meta name="description" content="Page Location Traps (The GNU Troff Manual)"> +<meta name="keywords" content="Page Location Traps (The GNU Troff Manual)"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta name="viewport" content="width=device-width,initial-scale=1"> + +<link href="index.html" rel="start" title="Top"> +<link href="Request-Index.html" rel="index" title="Request Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Vertical-Position-Traps.html" rel="up" title="Vertical Position Traps"> +<link href="The-Implicit-Page-Trap.html" rel="next" title="The Implicit Page Trap"> +<link href="Vertical-Position-Traps.html" rel="prev" title="Vertical Position Traps"> +<style type="text/css"> +<!-- +a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em} +div.example {margin-left: 3.2em} +span.r {font-family: initial; font-weight: normal; font-style: normal} +span:hover a.copiable-link {visibility: visible} +strong.def-name {font-family: monospace; font-weight: bold; font-size: larger} +--> +</style> + + +</head> + +<body lang="en"> +<div class="subsubsection-level-extent" id="Page-Location-Traps"> +<div class="nav-panel"> +<p> +Next: <a href="The-Implicit-Page-Trap.html" accesskey="n" rel="next">The Implicit Page Trap</a>, Previous: <a href="Vertical-Position-Traps.html" accesskey="p" rel="prev">Vertical Position Traps</a>, Up: <a href="Vertical-Position-Traps.html" accesskey="u" rel="up">Vertical Position Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Request-Index.html" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<h4 class="subsubsection" id="Page-Location-Traps-1">5.28.1.1 Page Location Traps</h4> +<a class="index-entry-id" id="index-page-location-traps"></a> +<a class="index-entry-id" id="index-traps_002c-page-location"></a> + +<p>A <em class="dfn">page location trap</em> is a vertical position trap that applies to +the page; that is, to undiverted output. Many can be present; manage +them with the <code class="code">wh</code> and <code class="code">ch</code> requests. +</p> +<dl class="first-deffn"> +<dt class="deffn" id="index-_002ewh"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.wh</code></strong> <var class="def-var-arguments">dist [<span class="r"><i class="slanted">name</i></span>]</var><a class="copiable-link" href='#index-_002ewh'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-wh"></a> +<p>Plant macro <var class="var">name</var> as page location trap at <var class="var">dist</var>. The default +scaling unit is ‘<samp class="samp">v</samp>’. Non-negative values for <var class="var">dist</var> 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 <var class="var">dist</var> of <code class="code">-0</code> +is interpreted as <code class="code">0</code>, the top of the page.<a class="footnote" id="DOCF106" href="groff.html_fot.html#FOOT106"><sup>106</sup></a> An existing <em class="emph">visible</em> trap (see below) at +<var class="var">dist</var> is removed; this is <code class="code">wh</code>’s sole function if <var class="var">name</var> +is missing. +</p> +<p>A trap is sprung only if it is <em class="dfn">visible</em>, meaning that its location +is reachable on the page<a class="footnote" id="DOCF107" href="groff.html_fot.html#FOOT107"><sup>107</sup></a> and it +is not hidden by another trap at the same location already planted +there. +</p> +<a class="index-entry-id" id="index-page-headers"></a> +<a class="index-entry-id" id="index-page-footers"></a> +<a class="index-entry-id" id="index-headers-1"></a> +<a class="index-entry-id" id="index-footers-1"></a> +<a class="index-entry-id" id="index-top-margin"></a> +<a class="index-entry-id" id="index-margin_002c-top"></a> +<a class="index-entry-id" id="index-bottom-margin"></a> +<a class="index-entry-id" id="index-margin_002c-bottom"></a> +<p>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 <code class="code">sp</code> request to position our text baselines, +and the page number character ‘<samp class="samp">%</samp>’ used with the <code class="code">tl</code> request. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.\" 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 +</pre></div></div> + +<p>To use these traps, copy the above (or load it from a file with the +<code class="code">so</code> or <code class="code">mso</code> requests), then set up the strings it uses. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.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 +.\" <i class="i">...and so on...</i> +</pre></div></div> + +<p>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 <em class="emph">current</em> page length; they are not converted to an absolute +vertical position. +<a class="index-entry-id" id="index-page-location-traps_002c-debugging"></a> +<a class="index-entry-id" id="index-debugging-page-location-traps"></a> +We can use the <code class="code">ptr</code> request to dump our page location traps to the +standard error stream (see <a class="pxref" href="Debugging.html">Debugging</a>). Their positions are reported +in basic units; an <code class="code">nroff</code> device example follows. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.pl 5i +.wh -1i xx +.ptr + error→ xx -240 +.pl 100i +.ptr + error→ xx -240 +</pre></div></div> + +<p>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 +<code class="code">ch</code> request. In the following example, the many empty lines +caused by the <code class="code">bp</code> request are not shown in the output. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.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 +</pre></div></div> +<div class="example"> +<div class="group"><pre class="example-preformatted">.ch b 1i +.ch c 1i +.bp + ⇒ a +</pre></div></div> +<div class="example"> +<div class="group"><pre class="example-preformatted">.ch a 0.5i +.bp + ⇒ a b +</pre></div></div> +</dd></dl> + +<dl class="first-deffn"> +<dt class="deffn" id="index-_005cn_005b_002et_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.t]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002et_005d'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-_002et"></a> +<a class="index-entry-id" id="index-distance-to-next-vertical-position-trap-register-_0028_002et_0029"></a> +<a class="index-entry-id" id="index-trap_002c-distance-to-next-vertical-position_002c-register-_0028_002et_0029"></a> +<p>The read-only register <code class="code">.t</code> 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. +</p></dd></dl> + +<dl class="first-deffn"> +<dt class="deffn" id="index-_002ech"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ch</code></strong> <var class="def-var-arguments">name [<span class="r"><i class="slanted">dist</i></span>]</var><a class="copiable-link" href='#index-_002ech'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-ch"></a> +<a class="index-entry-id" id="index-changing-trap-location-_0028ch_0029"></a> +<a class="index-entry-id" id="index-trap_002c-changing-location-_0028ch_0029"></a> +<p>Change the location of a trap by moving macro <var class="var">name</var> to new location +<var class="var">dist</var>, or by unplanting it altogether if <var class="var">dist</var> is absent. The +default scaling unit is ‘<samp class="samp">v</samp>’. Parameters to <code class="code">ch</code> are specified +in the opposite order from <code class="code">wh</code>. If <var class="var">name</var> 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.<a class="footnote" id="DOCF108" href="groff.html_fot.html#FOOT108"><sup>108</sup></a> +</p> +<p>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. +</p> + +</dd></dl> + +<p>The same macro can be installed simultaneously at multiple locations; +however, only the earliest-planted instance—that has not yet been +deleted with <code class="code">wh</code>—will be moved by <code class="code">ch</code>. The following +example (using an <code class="code">nroff</code> device) illustrates this behavior. Blank +lines have been elided from the output. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.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 +</pre></div></div> +<div class="example"> +<div class="group"><pre class="example-preformatted"> ⇒ 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. +</pre></div></div> + +<dl class="first-deffn"> +<dt class="deffn" id="index-_005cn_005b_002ene_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.ne]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002ene_005d'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-_002ene-1"></a> +<p>The read-only register <code class="code">.ne</code> contains the amount of space that was +needed in the last <code class="code">ne</code> request that caused a trap to be sprung; +it is useful in conjunction with the <code class="code">.trunc</code> register. See <a class="xref" href="Page-Control.html">Page Control</a>. Since the <code class="code">.ne</code> register is set only by traps, it +doesn’t make sense to interpolate it outside of macros called by traps. +</p></dd></dl> + +<dl class="first-deffn"> +<dt class="deffn" id="index-_005cn_005b_002etrunc_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.trunc]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002etrunc_005d'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-_002etrunc"></a> +<a class="index-entry-id" id="index-ne-request_002c-and-the-_002etrunc-register"></a> +<a class="index-entry-id" id="index-truncated-vertical-space-register-_0028_002etrunc_0029"></a> +<p>A read-only register containing the amount of vertical space truncated +from an <code class="code">sp</code> request by the most recently sprung vertical +position trap, or, if the trap was sprung by an <code class="code">ne</code> request, +minus the amount of vertical motion produced by the <code class="code">ne</code> +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 <code class="code">.trunc</code> register is set only by traps, it doesn’t make +sense to interpolate it outside of macros called by traps. +</p></dd></dl> + +<dl class="first-deffn"> +<dt class="deffn" id="index-_005cn_005b_002epe_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.pe]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002epe_005d'> ¶</a></span></dt> +<dd><a class="index-entry-id" id="index-_002epe"></a> +<a class="index-entry-id" id="index-bp-request_002c-and-traps-_0028_002epe_0029"></a> +<a class="index-entry-id" id="index-traps_002c-sprung-by-bp-request-_0028_002epe_0029"></a> +<a class="index-entry-id" id="index-page-ejection-status-register-_0028_002epe_0029"></a> +<p>This Boolean-valued, read-only register interpolates 1 while a page +is being ejected, and 0 otherwise. +</p> +<p>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. +</p> +<div class="example"> +<div class="group"><pre class="example-preformatted">.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 +</pre></div></div> +</dd></dl> + +<a class="index-entry-id" id="index-diversions_002c-and-traps"></a> +<a class="index-entry-id" id="index-traps_002c-and-diversions"></a> +<p>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 <code class="code">\!</code> or <code class="code">\?</code>) since +the data in the diversion is already formatted. In most cases, this is +not the expected behaviour. +</p> + +</div> +<hr> +<div class="nav-panel"> +<p> +Next: <a href="The-Implicit-Page-Trap.html">The Implicit Page Trap</a>, Previous: <a href="Vertical-Position-Traps.html">Vertical Position Traps</a>, Up: <a href="Vertical-Position-Traps.html">Vertical Position Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Request-Index.html" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |