Converting a string to an unsigned integer value in C

atoi.c

// Implements a standard library function `atoi` which returns an
// int for an input string. Also contains functions to calculate
// the length of a string and the value of an exponent expression.

#include <stdio.h>

unsigned int
str_length(const char str[])
{
	int len;

	for (len = 0; str[len]; len++) continue;

	return len;
}

unsigned int
power(unsigned int base, unsigned int exp)
{
	if (exp == 0) return 1;

	for (; exp > 1; exp--) base = base * base;

	return base;
}

unsigned long int
atoi(const char str[])
{
	int len, n, i;

	len = str_length(str) - 1;

	n = 0;

	for (i = 0; str[i]; i++, len--) {
		switch (str[i]) {
			case '1': n +=     power(10, len); break;
			case '2': n += 2 * power(10, len); break;
			case '3': n += 3 * power(10, len); break;
			case '4': n += 4 * power(10, len); break;
			case '5': n += 5 * power(10, len); break;
			case '6': n += 6 * power(10, len); break;
			case '7': n += 7 * power(10, len); break;
			case '8': n += 8 * power(10, len); break;
			case '9': n += 9 * power(10, len); break;
			default: break;
		}
	}

	return n;
}

int
main(int argc, char **argv)
{
	if (argc != 3) return 1;

	printf("%li\n", atoi(argv[1]) + atoi(argv[2]));

	return 0;
}