Sudoku solver

Hello, I'm asked to make a program that will solved a 9 X 9 sudoku using backtracking.
I am having a problem in backtracking. It backtracks but it do not know if it already tested a number so it creates an infinite loop. I do not know what to do.
Please help me.

Code:

---

//===============================================================================//
void backtrack(int a, int b, int value)
{
    bool check = true;
    if(sudoku2[a][b] == 0 ) // This will check the contents of the array "sudoku2" and change the ones with the value of zero
    {
        check = true;
        while(check == true && value < 9)
        {
            value++;
            sudoku2[a][b] = value; // assigns a value to sudoku2[a][b]
            if((check_row(a, b)) && (check_col(a, b)) && (check_box(a, b))) // if there is no conflict with the value. the previous while and if statements will terminate and moves to the next one.
            {
                cout << "lol3";
                check = false;
            }
            else // If the value is not compatible, it will try the next value
            {
                if(value > 8)// If there is no value from 1 to 9 that is compatible, backtrack
                {
                    sudoku2[a][b] = 0;
                    do
                    {
                        if(b > 0)
                        {
                            b = b - 1;
                        }
                        else
                        {
                            a = a - 1;
                            b = 0;
                        }
                    }while(sudoku[a][b] != 0);
                    value = sudoku2[a][b];
                    cout << value;
                    sudoku2[a][b] = 0;
                    backtrack(a, b, value);
                }
            }
        }
    }
    if(b < 8)
    {
        backtrack(a, b + 1, 0);
    }
    else
    {
        backtrack(a + 1, 0, 0);
    }
}


//===============================================================================//

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Can you outline the algorithm you're trying to use? Also what are the starting conditions with which you call backtrack()?