Creating Username & Password

[cGuru]

NOTE: If you could help me out with just one of these, then that would be greatly appreciated!
NOTE: Code is at bottom!

Goal:
Create a user account and password for game

Problems:
(1) File shows the username/password I created, as well as a bunch of weird characters.
(2) Each record (username and password) is appended to the end of line 1, versus to a new line each time.
(3) File search algorithm(s) that I have tried do not work correctly

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(1) I figure that my file shows the following:

abc IIIIIIIIIIIIIIIIdef IIIIIIIIIIIIIIII

Because I specified the array as a certain amount of characters, and the user inputted less than that max character limit. I want to either (A) Delete these alien character from my file, or (B) Know that they will not affect an algorithm that searches the file for the username string.

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(2) Each new username simply just goes to the end of the first line. I want each new username created to be appended to a new line. Can anyone give me insight into this?

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(3) I tried a couple of search algorithms to search my file for a username, and tell the user that they entered a username is already in use, but it hasn't worked out so far. Can anyone suggest an algorithm that can search and/or replace a string in a file that might work with my code?

Pseudo code:

Get input (username)
If that username is found Then
Keep prompting for input until it does not match

Note: A problem that can occur though is that each time the user enters a new username, then I want to search the whole file to see if it is found ANYWHERE, not just where the current read position is and on forward...
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Here is the relevant code:

const int USERNAME_SIZE = 20, PASSWORD_SIZE = 20;

struct UserAccount
{
	char username[USERNAME_SIZE];
	char password[PASSWORD_SIZE];
};

void create_account(UserAccount &u)
{
	string s;
	fstream accounts("user_accounts.txt",ios::app|ios::binary);
	cout << "Create your username:\n";
	cin >> u.username;

	cout << "\n\nCreate your password:\n";
	cin >> u.password;
	accounts.write(reinterpret_cast<char *>(&u),sizeof(u));
	accounts.close();
}