base26.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* toBase26(int num){
	char* str;
	str = malloc(sizeof(char)*10);
	char* temp = str;

	while(num > 0){
		temp[0] = (num%26) + 0x40;
		temp++;
		num /= 26;
	}
	
	temp = '\0';
	
	return str;
}

int toBase10(char* str){
	int len = strlen(str);
	int num = 0;
	int i;
	
	for(i = len-1; i >= 0; i--){
		num = num*26 + ((int)str[i] - 0x40);
	}

	return num;
}

int main(){
	printf("%s\n", toBase26(27)); // prints AA
	printf("%d\n", toBase10("AA")); // prints 27

	return 0;
}

Not really base 26 - but I am converting numbers <-> letters in some form. Based on base 26 to/from base 10 conversion.