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<title>Page Control (The GNU Troff Manual)</title>
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<h3 class="section" id="Page-Control-1">5.18 Page Control</h3>
<a class="index-entry-id" id="index-page-control"></a>
<a class="index-entry-id" id="index-control_002c-page"></a>
<a class="index-entry-id" id="index-page-break-1"></a>
<a class="index-entry-id" id="index-break_002c-page-1"></a>
<a class="index-entry-id" id="index-page-ejection-1"></a>
<a class="index-entry-id" id="index-ejection_002c-page-1"></a>
<p>Discretionary page breaks can prevent the unwanted separation of
content. A new page number takes effect during page ejection; see
<a class="ref" href="The-Implicit-Page-Trap.html">The Implicit Page Trap</a>.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002ebp"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.bp</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">page-number</i></span>]</var><a class="copiable-link" href='#index-_002ebp'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-bp"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_002ebp-1"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.bp</code></strong> <var class="def-var-arguments"><code class="t">+</code><span class="r"><i class="slanted">page-number</i></span></var><a class="copiable-link" href='#index-_002ebp-1'> ¶</a></span></dt>
<dt class="deffnx def-cmd-deffn" id="index-_002ebp-2"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.bp</code></strong> <var class="def-var-arguments"><code class="t">-</code><span class="r"><i class="slanted">page-number</i></span></var><a class="copiable-link" href='#index-_002ebp-2'> ¶</a></span></dt>
<dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_0025_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[%]</code></strong><a class="copiable-link" href='#index-_005cn_005b_0025_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_0025-2"></a>
<a class="index-entry-id" id="index-new-page-_0028bp_0029"></a>
<a class="index-entry-id" id="index-page_002c-new-_0028bp_0029"></a>
<p>Break the page and change (increase or decrease) the next page number
per the numeric expression <var class="var">page-number</var>. If <var class="var">page-number</var> is
invalid, GNU <code class="code">troff</code> emits a warning in category ‘<samp class="samp">number</samp>’ and
ignores the argument. This request causes a break. A page break
advances the vertical drawing position to the bottom of the page,
springing traps. See <a class="xref" href="Page-Location-Traps.html">Page Location Traps</a>.
<a class="index-entry-id" id="index-bp-request_002c-and-top_002dlevel-diversion"></a>
<a class="index-entry-id" id="index-top_002dlevel-diversion_002c-and-bp"></a>
<a class="index-entry-id" id="index-diversion_002c-top_002dlevel_002c-and-bp"></a>
<code class="code">bp</code> has effect only if invoked within the top-level
diversion.<a class="footnote" id="DOCF71" href="groff.html_fot.html#FOOT71"><sup>71</sup></a>
<a class="index-entry-id" id="index-CSTR-_002354-errata-2"></a>
<a class="index-entry-id" id="index-CSTR-_002354-erratum_002c-bp-request"></a>
This request is incorrectly documented in the <abbr class="acronym">AT&T</abbr>
<code class="code">troff</code> manual as having a default scaling unit of ‘<samp class="samp">v</samp>’.
</p>
<a class="index-entry-id" id="index-page-number-register-_0028_0025_0029"></a>
<a class="index-entry-id" id="index-current-page-number-_0028_0025_0029"></a>
<p>The register <code class="code">%</code> interpolates the current page number.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de BP
' bp \" schedule page break once current line is output
..
</pre></div></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002ene"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.ne</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">space</i></span>]</var><a class="copiable-link" href='#index-_002ene'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-ne"></a>
<a class="index-entry-id" id="index-orphan-lines_002c-preventing-with-ne"></a>
<a class="index-entry-id" id="index-conditional-page-break-_0028ne_0029"></a>
<a class="index-entry-id" id="index-page-break_002c-conditional-_0028ne_0029"></a>
<p>Force a page break if insufficient vertical space is available (assert
“needed” space). <code class="code">ne</code> tests the distance to the next page
location trap; see <a class="ref" href="Page-Location-Traps.html">Page Location Traps</a>, and breaks the page if
that amount is less than <var class="var">space</var>. The default scaling unit is
‘<samp class="samp">v</samp>’. If <var class="var">space</var> is invalid, GNU <code class="code">troff</code> emits a warning
in category ‘<samp class="samp">number</samp>’ and ignores the argument. If <var class="var">space</var> is
not specified, ‘<samp class="samp">1v</samp>’ is assumed.
</p>
<a class="index-entry-id" id="index-widow"></a>
<p>We can require space for at least the first two output lines of a
paragraph, preventing its first line from being <i class="slanted">widowed</i> at the
page bottom.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.ne 2v
Considering how common illness is,
how tremendous the spiritual change that it brings,
how astonishing,
when the lights of health go down,
the undiscovered countries that are then disclosed,
what wastes and deserts of the soul a slight attack
of influenza brings to view,
</pre></div></div>
<p>This method is reliable only if no output line is pending when <code class="code">ne</code>
is invoked. When macro packages are used, this is often not the case:
their paragraphing macros perform the break. You may need to experiment
with placing the <code class="code">ne</code> after the paragraphing macro, or <code class="code">br</code>
and <code class="code">ne</code> before it.
</p>
<a class="index-entry-id" id="index-orphan"></a>
<a class="index-entry-id" id="index-widow-1"></a>
<p><code class="code">ne</code> is also useful to force grouping of section headings with
their subsequent paragraphs, or tables with their captions and/or
explanations. Macro packages often use <code class="code">ne</code> with diversions to
implement keeps and displays; see <a class="ref" href="Diversions.html">Diversions</a>. They may also offer
parameters for widow and orphan management.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_002esv"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.sv</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">space</i></span>]</var><a class="copiable-link" href='#index-_002esv'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-sv"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_002eos"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.os</code></strong><a class="copiable-link" href='#index-_002eos'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-os"></a>
<a class="index-entry-id" id="index-ne-request_002c-comparison-with-sv"></a>
<p>Require vertical space as <code class="code">ne</code> does, but also <i class="slanted">save</i> it for
later output by the <code class="code">os</code> request. If <var class="var">space</var> is available
before the next page location trap, it is output immediately. Both
requests ignore a partially collected line, taking effect at the next
break.
<a class="index-entry-id" id="index-sv-request_002c-and-no_002dspace-mode"></a>
<a class="index-entry-id" id="index-os-request_002c-and-no_002dspace-mode"></a>
<code class="code">sv</code> and <code class="code">os</code> ignore no-space mode (recall <a class="ref" href="Manipulating-Spacing.html">Manipulating Spacing</a>). While the <code class="code">sv</code> request allows negative values for
<var class="var">space</var>, <code class="code">os</code> ignores them. The default scaling unit is
‘<samp class="samp">v</samp>’. If <var class="var">space</var> is not specified, ‘<samp class="samp">1v</samp>’ is assumed.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-_005cn_005bnl_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[nl]</code></strong><a class="copiable-link" href='#index-_005cn_005bnl_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-nl"></a>
<a class="index-entry-id" id="index-vertical-drawing-position-_0028nl_0029"></a>
<a class="index-entry-id" id="index-vertical-position_002c-drawing-_0028nl_0029"></a>
<a class="index-entry-id" id="index-drawing-position_002c-vertical-_0028nl_0029"></a>
<p><code class="code">nl</code> interpolates or sets the vertical drawing position. When the
formatter starts, the first page transition hasn’t happened yet, and
<code class="code">nl</code> is negative. If a header trap has been planted on the page
(typically at vertical position <code class="code">0</code>), you can assign a negative
value to <code class="code">nl</code> to spring it if that page has already started
(see <a class="pxref" href="Page-Location-Traps.html">Page Location Traps</a>).
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">.de HD
. sp
. tl ''Goldbach Solution''
. sp
..
.
First page.
.bp
.wh 0 HD \" plant header trap at top of page
.nr nl (-1)
Second page.
⇒ First page.
⇒
⇒ <span class="r"><i class="i">(blank lines elided)</i></span>
⇒
⇒ Goldbach Solution
⇒
⇒ <span class="r"><i class="i">(blank lines elided)</i></span>
⇒
⇒ Second page.
</pre></div></div>
<p>Without resetting <code class="code">nl</code> to a negative value, the trap just planted
would be active beginning with the <em class="emph">next</em> page, not the current
one.
</p>
<p>See <a class="xref" href="Diversions.html">Diversions</a>, for a comparison of <code class="code">nl</code> with the <code class="code">.h</code> and
<code class="code">.d</code> registers.
</p></dd></dl>
</div>
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