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-<head>
-<title>UK TeX FAQ -- question label whatengine</title>
-</head><body>
-<h3>Am I using PDFTeX, XeTeX or LuaTeX?</h3>
-<p/><p/>You often need to know what &#8220;engine&#8221; your macros are running on (the
-engine is the TeX-derivative or TeX-alike processor that
-typesets your document). The reason that you need to know is that the
-set of functions available in each engine is different. Thus, for
-TeX macros to run on any engine, they need to &#8220;know&#8221; what they
-can and cannot do, which depends on the engine in use. Getting the
-right answer is surprisingly tricky (see below for an elaboration of
-one apparently simple test).
-<p/>There is now a comprehensive set of packages that answer the question
-for you. They all create a TeX conditional command:
-<ul>
-<li> <i>ifpdf</i> creates a command <code>\</code><code>ifpdf</code>,
-<li> <i>ifxetex</i> creates a command <code>\</code><code>ifxetex</code> and
-<li> <i>ifluatex</i> creates a command <code>\</code><code>ifluatex</code>.
-</ul>
-These TeX commands may be used within the LaTeX conditional
-framework, as (for example):
-<blockquote>
- <code>\</code><code>ifthenelse</code><code>\char&#8216;{</code><code>\</code><code>boolean{pdf}</code><code>\char&#8216;}\char&#8216;&lbrace;&lt;<i>if pdf</i>&gt;\char&#8216;&rbrace;\char&#8216;&lbrace;&lt;<i>not pdf</i>&gt;\char&#8216;&rbrace;</code>
-</blockquote><p>
-<p/>The <i>ifxetex</i> package also provides a command
-<code>\</code><code>RequireXeTeX</code> which creates an error if the code is not running
-under XeTeX; while the other packages don&#8217;t provide such a command,
-it&#8217;s not really difficult to write one for yourself.
-<p/>
-
-<p/>Now for those who want to do the job for themselves: here&#8217;s a
-discussion of doing the job for PDFTeX and <code>\</code><code>ifpdf</code> &#8212; the
-eager programmer can regenerate <code>\</code><code>ifxetex</code> or <code>\</code><code>ifluatex</code> in the
-same fashion. It&#8217;s not recommended\dots
-<p/>Suppose you need to test whether your output will be PDF or
-DVI. The natural thing is to check whether you have access to
-some PDFTeX-only primitive; a good one to try (not least because it
-was present in the very first releases of PDFTeX) is
-<code>\</code><code>pdfoutput</code>. So you try
-<blockquote>
-<pre>
-\ifx\pdfoutput\undefined
- ... % not running PDFTeX
-\else
- ... % running PDFTeX
-\fi
-</pre>
-</blockquote><p>
-Except that neither branch of this conditional is rock-solid. The
-first branch can be misleading, since the &#8220;awkward&#8221; user could have
-written:
-<blockquote>
-<pre>
-\let\pdfoutput\undefined
-</pre>
-</blockquote><p>
-so that your test will falsely choose the first alternative. While
-this is a theoretical problem, it is unlikely to be a major one.
-<p/>More important is the user who loads a package that uses
-LaTeX-style testing for the command name&#8217;s existence (for example,
-the LaTeX <i>graphics</i> package, which is useful even to the
-Plain TeX user). Such a package may have gone ahead of you, so the
-test may need to be elaborated:
-<blockquote>
-<pre>
-\ifx\pdfoutput\undefined
- ... % not running PDFTeX
-\else
- \ifx\pdfoutput\relax
- ... % not running PDFTeX
- \else
- ... % running PDFTeX
- \fi
-\fi
-</pre>
-</blockquote><p>
-If you only want to know whether some PDFTeX extension (such as
-marginal kerning) is present, you can stop at this point: you know as
-much as you need.
-<p/>However, if you need to know whether you&#8217;re creating PDF
-output, you also need to know about the value of <code>\</code><code>pdfoutput</code>:
-<blockquote>
-<pre>
-\ifx\pdfoutput\undefined
- ... % not running PDFTeX
-\else
- \ifx\pdfoutput\relax
- ... % not running PDFTeX
- \else
- % running PDFTeX, with...
- \ifnum\pdfoutput&#62;0
- ... % PDF output
- \else
- ... % DVI output
- \fi
- \fi
-\fi
-</pre>
-</blockquote><p>
-<dl>
-<dt><tt><i>ifpdf.sty</i></tt><dd>Distributed as part Heiko Oberdiek&#8217;s bundle
- <a href="http://mirror.ctan.org/macros/latex/contrib/oberdiek.zip">macros/latex/contrib/oberdiek</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/oberdiek/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/ifpdf.html">catalogue entry</a>
-<dt><tt><i>ifluatex.sty</i></tt><dd>Distributed as part of Heiko Oberdiek&#8217;s bundle
- <a href="http://mirror.ctan.org/macros/latex/contrib/oberdiek.zip">macros/latex/contrib/oberdiek</a> (or <a href="http://mirror.ctan.org/macros/latex/contrib/oberdiek/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/ifluatex.html">catalogue entry</a>
-<dt><tt><i>ifxetex.sty</i></tt><dd><a href="http://mirror.ctan.org/macros/generic/ifxetex.zip">macros/generic/ifxetex</a> (or <a href="http://mirror.ctan.org/macros/generic/ifxetex/">browse the directory</a>); <a href="http://mirror.ctan.org/help/Catalogue/entries/ifxetex.html">catalogue entry</a>
-</dl>
-<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=whatengine">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=whatengine</a>
-</body>