C++ :: Read And Rewrite Lines In File

My task is to write a function for borrowing books form library called borrow(name,surname, student_id, book_id), which will connect information of borrowed book's ID and student's information in the file.

File looks like this:

John Jackson 45 0
Michael Gregory 34 56
Ann Cawitch 23 0
Chris Lamb 34 50
...

First two words are name and surname of students. First number represents student's ID and second one represents ID of book. If ID of book is 0, it means that student hasn't borrowed book yet, so he can borrow some book. If ID of book is not 0, it means that student can't borrow a book until he returns the old one. So, when student borrows book, ID of book in file should change from zero to some number

This is my code:

void borrow(string name, string surname, int student_id,int book_id ) {
string a;
string b;
int c=0;
int d;
fstream f2;
f2.open("students.txt", ios::out |ios::in);

[Code] ....

Whenever I call function (for example borrow(John, Jackson,45,15)), I get answer "Student isn't sign up library."

Am I on a right path? How to make this code to work?