# Am I using PDFTeX, XeTeX or LuaTeX?

You often need to know what “engine” 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 “know” 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).

There is now a comprehensive set of packages that answer the question for you. They all create a TeX conditional command:

• ifpdf creates a command `\ifpdf`,
• ifxetex creates a command `\ifxetex` and
• ifluatex creates a command `\ifluatex`.
These TeX commands may be used within the LaTeX conditional framework, as (for example):
`\ifthenelse`{`\boolean``{pdf}`}{if pdf}{if not pdf}

The ifxetex package also provides a command `\RequireXeTeX` which creates an error if the code is not running under XeTeX; while the other packages don’t provide such a command, it’s not really difficult to write one for yourself.

Now for those who want to do the job for themselves: here’s a discussion of doing the job for PDFTeX and `\ifpdf` — the eager programmer can regenerate `\ifxetex` or `\ifluatex` in the same fashion. It’s not recommended…

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 `\pdfoutput`. So you try

```\ifx\pdfoutput\undefined
... % not running PDFTeX
\else
... % running PDFTeX
\fi
```
Except that neither branch of this conditional is rock-solid. The first branch can be misleading, since the “awkward” user could have written:
```\let\pdfoutput\undefined
```
so that your test will falsely choose the first alternative. While this is a theoretical problem, it is unlikely to be a major one.

More important is the user who loads a package that uses LaTeX-style testing for the command name’s existence (for example, the LaTeX graphics 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:

```\ifx\pdfoutput\undefined
... % not running PDFTeX
\else
\ifx\pdfoutput\relax
... % not running PDFTeX
\else
... % running PDFTeX
\fi
\fi
```
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.

However, if you need to know whether you’re creating PDF output, you also need to know about the value of `\pdfoutput`:

```\ifx\pdfoutput\undefined
... % not running PDFTeX
\else
\ifx\pdfoutput\relax
... % not running PDFTeX
\else
% running PDFTeX, with...
\ifnum\pdfoutput>0
... % PDF output
\else
... % DVI output
\fi
\fi
\fi
```
ifpdf.sty
Distributed as part Heiko Oberdiek’s bundle oberdiek
ifluatex.sty
Distributed as part of Heiko Oberdiek’s bundle oberdiek
ifxetex.sty
ifxetex