The original usage for this macro was for converting integers to a string, which is better served by sprintf, but it could be used for other applications. 

Code:

#include <math.h>

#define MASKDIGIT(digitnum, n)  (n/pow(10, digitnum) - (n/pow(10,digitnum+1)*10))

Compile with:

cc -lm digitmask.c

Usage:

This macro could be used if you're expecting to return multiple single digit numbers from a function.

If you change the +1 to a +3 and *10 to *1000, you can use this as part of a function to display numbers nicely, like "1,000,000" instead of "1000000"

Extensions:

The width of the mask (one digit in this case) could be made variable realatively easily.

Sources:

http://codingexperiments.com/archives/324