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-<head>
-<title>UK TeX FAQ -- question label ifpdf</title>
-</head><body>
-<h3>Am I using PDFTeX?</h3>
-<p/>It&rsquo;s often useful to know whether your macros are operating within
-PDFTeX or within (&ldquo;normal&rdquo;) TeX; getting the right answer is
-surprisingly tricky.
-<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
- &lt;not running PDFTeX&gt;
-\else
- &lt;running PDFTeX&gt;
-\fi
-</pre>
-</blockquote><p>
-Except that neither branch of this conditional is rock-solid. The
-first branch can be misleading, since the &ldquo;awkward&rdquo; 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&rsquo;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
- &lt;not running PDFTeX&gt;
-\else
- \ifx\pdfoutput\relax
- &lt;not running PDFTeX&gt;
- \else
- &lt;running PDFTeX&gt;
- \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&rsquo;re creating PDF
-output, you also need to know about the value of <code>\</code><code>pdfoutput</code>:
-<blockquote>
-<pre>
-\ifx\pdfoutput\undefined
- &lt;not running PDFTeX&gt;
-\else
- \ifx\pdfoutput\relax
- &lt;not running PDFTeX&gt;
- \else
- &lt;running PDFTeX, with...&gt;
- \ifnum\pdfoutput&gt;0
- &lt;...PDF output&gt;
- \else
- &lt;...DVI output&gt;
- \fi
- \fi
-\fi
-</pre>
-</blockquote><p>
-The above is, in essence, what Heiko Oberdiek&rsquo;s <i>ifpdf</i>
-package does; the reasoning is the FAQ&rsquo;s interpretation of
-Heiko&rsquo;s explanation.
-<dl>
-<dt><tt><i>ifpdf.sty</i></tt><dd>Distributed with Heiko Oberdiek&rsquo;s packages <a href="ftp://cam.ctan.org/tex-archive/macros/latex/contrib/oberdiek.zip">macros/latex/contrib/oberdiek</a> (<a href="ftp://cam.ctan.org/tex-archive/macros/latex/contrib/oberdiek.tar.gz">gzipped tar</a>, <a href="http://www.tex.ac.uk/tex-archive/macros/latex/contrib/oberdiek/">browse</a>)
-</dl>
-<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ifpdf">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ifpdf</a>
-</body>