Why am I not getting proper output in the C program?

Question

I wrote a C program to register a user and login using the same username and password but I am getting Username invalid/doesn't exist message when trying to login. Any idea what I have been doing wrong in the following code? Also when I re-run the program I can't get the Login Successfully message. Even if I use the commented fscanf() function in the code I don't get the proper output.

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

struct database {
    char name[20];
    char email[30];
    char user[10];
    char pass[20];
} store;

int main() {
    int count, entries, choice;
    char username[10];
    char password[20];
    FILE *fptr;
    fptr = fopen("E:\\login.bin", "ab+");
    printf("Welcome to the user authentication program v1.2 .\n");
 Again:
    printf("\n1. Register\n");
    printf("2. Login\n");
    printf("3. Exit\n");
    printf("\nEnter your choice: ");
    scanf("%d", &choice);
    switch(choice) {
      case 1:
        printf("\nEnter the number of users.\n");
        printf("Users = ");
        scanf("%d", &entries);
        for (count = 1; count <= entries; count++) {
            /*printf("\nEnter your email: ");
            scanf("%s", &store.email);
            fprintf(fptr, "%s\n", store.email);*/

            printf("\nEnter a username: ");
            scanf("%s", &store.user);
            fprintf(fptr, "%s\n", store.user);

            printf("\nEnter a password: ");
            scanf("%s", &store.pass);
            fprintf(fptr, "%s\n", store.pass);

            printf("\nRegistration successful.\n");

        }
        goto Again;
        break;

      case 2:
        printf("Enter your username: ");
        scanf("%s", &username);
        ///fscanf(fptr, "%s", &store.user);
        printf("Enter your password: ");
        scanf("%s", &password);
        ///fscanf(fptr, "%s", &store.pass);
        if (strcmp(username, store.user) == 0) {
            if (strcmp(password, store.pass) == 0) {
                printf("\nLogin Successful.\n");
            } else {
                printf("\nIncorrect password!\n");
            }
        } else {
            printf("\nUsername invalid/doesn't exist.\n");
        }
        break;

      case 3:
        exit(1);
        break;
    }
    fclose(fptr);
}

Answer 1

Your program is incomplete. Your writing auth data to a file you never read. You should consider building an array of database records and iterating over each one when you are trying to find a match.

Learn to use your debugger. You would have noticed that only the last auth record created in case 1 is lingering in store when you get to case 2 . You also would have noticed that the fscanf(fptr, "%s", &store.user) was reading only one record.

Consider this program outline:

if exist authfile LoadAuth()
if user enter new auth data, update internal database, then write to file.
if user attempts login, create a temp record with their auth data and then 
iterate through the database, comparing that record to ones in the database.
  If not found, fail the login.

Answer 2

You read the login and password into a local structure and append the credentials to the file, but you never read the file to check against all the login/passwords in the database.

The program only works if you register a single user and log in immediately after. Furthermore you have no error handling, so invalid input can cause undefined behaviour in many places.