[Jeff McCune]

As an example, when using a microcontroller, using this:

#define PI (22 / 7)

is *considerably* faster than using 3.14159... and it gives a fairly decent approximation.

Also, to prevent re-inclusion of header files, people usually do something like this:

myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H

.. Entire header file ...

#endif
/* ifndef MYHEADER_H */

This prevents weird recursion problems from having a header file included twice at the top of a C file.

It's sloppy, and I don't recommend doing this, because if you *need* to do this, you're already down the dangerous road they've been speaking of and you ultimately need to fix that or you'll just have a *monstrous* headache at competition when you're rapidly changing code.