Present message and prompt re-enter into array if too many characters "out of bounds" entered into an array

Question

I just can't figure it out. I'm not allowed to use the string library, first of all. But, I want the user to enter their major into a char array, but if they enter too many (runs into input buffer), then present a message and prompt to re-enter their major into the same array. Do I need to somehow erase the array and then have them re-enter? Or, do I check input buffer with cin.peek() != '\n'? I just don't know what to do...

I've been trying a bunch of different things but I'm just stuck going back to my code below for a new solution.

const int MAJORSIZE = 21;

int main()
{

char major[MAJORSIZE];

//Get user's major
cout << "What is your major?: ";
cin.get(major, MAJORSIZE, '\n');
cin.ignore(100, '\n');

return 0;
}