C :: Create Function That Is Then Called To Calculate Powers

As I am taking my first steps in C, I study K&R (I guess most of you did the same, right?)

In the introductory chapter about functions (1.7) there is an example showing how to create a function that is then called to calculate powers (b**n). I simplified it to calculate only one given power, 2**5:

Code:

#include <stdio.h>
int power(int m, int n);
main() {
printf("%d", power(2,5));

[Code]....

It will then be called to make the calculation in the above string.

First things first: we already know how to use while(getchar!=EOF) to count characters (K&R chapter 1.5.2) but what if -instead- the input is a specific string? How to "read" it and how to tell my program that the string is finished? And most important: "reading" will be done in the function or in the rest of the body?