Edit: After prompting the user to choose between “login (1)” and “create an account (2)”, I am trying to have the program check that:

• the input is an integer
• that the input, in case of an integer, is only valid if one (1) or two (2)
• (optionally) that the data size of the user input is not greater than the integer number one (1) or two (2), as a security measure, to hinder overflow exploits

If I allocate [] or [1] to intCheck the program goes into an endless loop from the very start. No user input required. If, however, I allocate [2] or more, the program works as intended, as long as the total size of bytes of the user input is less than the number of bytes previously allocated. If, however, the user input results in a byte size greater than what was preallocated, the program repeats the else condition of the while loop - here, printf - that number of times. Is this an overflow of choice or what?

//Bank of Haruhi  
 
//TODO  
// - Create user account: require user first name, last name, user name, password, age (deny if < 20 yrs)  
// - Prompt login (deny if != password && username)  
// - Display menu ("About the Bank of Haru", "Account Settings", "Check your balance", "Deposit", "Withdraw", "Close account", "Logout")  
 
#include <stdio.h>  
#include <string.h>  
#include <stdbool.h>  
 
int main(void) {  
 
//Base 
char intCheck[3] = ""; //Will "if (sscanf(char,%d,&int) == true)" check for ANY "char" or a predetermined amount of "char"?  
 
//Registration and login  
char firstName[] = "";  
char lastName[] = "";  
char password[] = "";  
int age = 0;  
 
//Menues and selections  
int choice = 0;  
int arithmeticChoice = 0;  
 
//Balance, deposit and withdrawal  
float balance = 0.0;  
float deposit = 0.0;  
float withdraw = 0.0;  
 
printf("Welcome to the Bank of Haruhi! Please login (1) or create an account (2): ");  
 
while (fgets(intCheck, sizeof(intCheck),stdin)) {  
intCheck[strlen(intCheck) - 1] = '\0';  
if (sscanf(intCheck, "%d", &choice) == true && sizeof(intCheck) == 4 && choice == 1 || choice == 2) break; //I'm keeping "sizeof(intCheck)" in order to excercise byte size input validation, but I could just remove it and let "if choice == 1 || 2" restrict the valid input.  
else printf("Please enter ""1"" to login or ""2"" to create a new account: ");  
}  
 
return 0;  
 
}  

The program is, of course, not nearly complete. I just stopped doing anything else, as soon as I stumbled upon this phenomenon.