Как исправить ошибку при компиляции?

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
struct DBTable
{
string title, line;
int n;
};

void readTable();
void main()
{
    system("chcp 1251 > null");
    FILE *sf, *suf, *bf, *af, *mt;
    DBTable line;
    sf = fopen("Students.txt", "a+b");
    suf = fopen("Subjects.txt", "a+b"); 
    bf = fopen("Books.txt", "a+b");
    af = fopen("Abonements.txt", "a+b");
    readTable();
    system("pause");
}
void readTable()
{
    ifstream sf("Students");
    ifstream suf("Subjects.txt");
    ifstream bf("Books.txt");
    ifstream af("Abonements.txt");
    char buf[20], real[20];
    sf.getline(buf,20);
    char *leftS = strtok(buf, "|");
    cout << leftS;

    //printf("%s \n",real);
}

Сама ошибка:

Exception thrown: read access violation. _First was nullptr. If there is a handler for this exception, the program may be safely continued.