A computer-nerd friend was writing something or other in PHP and was disappointed that the <sys/errno.h> constants weren't available. This was my suggestion:

#!/bin/sh

cpp2sh()
{
	cpp -dM | sed -e 's/^#define //' -e 's/ /=/'
}

( echo '#include &lt;sys/errno.h&gt;' | cpp2sh; echo | cpp2sh ) | sort | uniq -u

I just think I'm sooo clever.