Run-Time Check Failure - Stack around variable was corrupted

Question

I am getting the error

Run-Time Check Failure #2 - Stack around the variable 'IDU' was corrupted.

I have googled and tried everything and I have read that I need to increase the array size. However, for this specific task I am not allowed to do that. I also tried changing my condition and it seemed to work for the one variable 'newsal' and now is showing this error for 'IDU'.
Please help me out here.

#define _CRT_SECURE_NO_WARNINGS
#define SIZE 4
#include <stdio.h>

// Define Number of Employees "SIZE" to be 2


// Declare Struct Employee 
struct employee {
    int ID;
    int age;
    double salary;

};

/* main program */
int main(void) {

    int option = 0;
    int count = 0, count2 = 0, valid = 0;
    int IDU;
    double newsal = 0;

    struct employee emp[SIZE] = { { 0 } };

    // Declare a struct Employee array "emp" with SIZE elements 
    // and initialize all elements to zero


    printf("---=== EMPLOYEE DATA ===---\n\n");

    do {
        // Print the option list
        printf("1. Display Employee Information\n");
        printf("2. Add Employee\n");
        printf("3. Update Employee Salary\n");
        printf("4. Remove Employee\n");
        printf("0. Exit\n\n");
        printf("Please select from the above options: ");

        // Capture input to option variable
        scanf("%d", &option);
        printf("\n");

        switch (option) {
        case 0: // Exit the program
            printf("Exiting Employee Data Program. Good Bye!!!\n");
            break;
        case 1: // Display Employee Data
                // @IN-LAB
            printf("EMP ID  EMP AGE EMP SALARY\n");
            printf("======  ======= ==========\n");

            for (count = 0; count < count2; count++) {
                if (emp[count].ID > 0) {
                    printf("%6d%9d%11.2lf", emp[count].ID, emp[count].age, emp[count].salary);
                    printf("\n");
                }
            }
            printf("\n");

            // Use "%6d%9d%11.2lf" formatting in a   
            // printf statement to display
            // employee id, age and salary of 
            // all  employees using a loop construct 

            // The loop construct will be run for SIZE times 
            // and will only display Employee data 
            // where the EmployeeID is > 0

            break;
        case 2: // Adding Employee
                // @IN-LAB
            printf("Adding Employee\n");
            printf("===============\n");
            if (valid == SIZE) {
                printf("ERROR!!! Maximum Number of Employees Reached\n");
                printf("\n");
                break;
            }

            printf("Enter Employee ID: ");
            scanf("%d", &emp[count2].ID);

            printf("Enter Employee Age: ");
            scanf("%d", &emp[count2].age);

            printf("Enter Employee Salary: ");
            scanf("%lf", &emp[count2].salary);
            valid++;
            count2++;
            printf("\n");


            // Check for limits on the array and add employee 
            // data accordingly. 



            break;
        case 3:
            printf("Update Employee Salary\n");
            printf("===============\n");

            do {
                printf("Enter Employee ID: ");
                scanf("%d", &IDU);
                for (count2 = 0; count2 <= SIZE; count2++) {
                    if (IDU == emp[count2].ID) {
                        printf("The current salary is %.2lf\n", emp[count2].salary);
                        printf("Enter Employee New Salary: ");
                        scanf("%lf", &newsal);
                        emp[count2].salary = newsal;
                    }
                }
            } while (IDU == emp[count2].ID);


                break;
        case 4:
            printf("Remove Employee\n");
            printf("===============\n");
            do {
                printf("Enter Employee ID: ");
                scanf("%d", &IDU);
                for (count2 = 0; count2 <= SIZE; count2++) {
                    if (IDU == emp[count2].ID) {
                        printf("Employee %d will be removed\n", emp[count2].ID);
                        emp[count2].ID = 0;
                        emp[count2].age = 0;
                        emp[count2].salary = 0;
                        printf("\n");
                        valid--;
                    }
                }
            } while (IDU == emp[count2].ID);
                break;
        default:
            printf("ERROR: Incorrect Option: Try Again\n\n");
        }

    } while (option != 0);


    return 0;
}




//PROGRAM OUTPUT IS SHOW BELOW

/*
---=== EMPLOYEE DATA ===---

1. Display Employee Information
2. Add Employee
0. Exit

Please select from the above options: 2

Adding Employee
===============
Enter Employee ID: 111
Enter Employee Age: 34
Enter Employee Salary: 78980.88

1. Display Employee Information
2. Add Employee
0. Exit

Please select from the above options: 2

Adding Employee
===============
Enter Employee ID: 112
Enter Employee Age: 41
Enter Employee Salary: 65000

1. Display Employee Information
2. Add Employee
0. Exit

Please select from the above options: 2

Adding Employee
===============
ERROR!!! Maximum Number of Employees Reached

1. Display Employee Information
2. Add Employee
0. Exit

Please select from the above options: 1

EMP ID  EMP AGE EMP SALARY
======  ======= ==========
111       34   78980.88
112       41   65000.00

1. Display Employee Information
2. Add Employee
0. Exit

Please select from the above options: 0

Exiting Employee Data Program. Good Bye!!!

*/

Answer 1

You loop like this

for (count2 = 0; count2 <= SIZE; count2++)

so count2 is up to SIZE inclusive.
Then you access like this

emp[count2].ID

i.e.

emp[SIZE].ID

And emp is

struct employee emp[SIZE];

So you have access beyond the array, nothing more is needed to corrupt the stack.

My favorite attempt to solve this would be to loop like this instead:

    for (count2 = 0; count2 < SIZE; count2++)