rand() giving same number each time [duplicate]

This question already has an answer here:

• rand() returns the same number each time the program is run 6 answers

I'm using the rand() function to set up my game board, however every time I run my program it gives me the exact same values. I want the row/col positions to be random each time the program is run. Here's my code:

   for (int x = 0; x < 30; x++)
    {
        int rowPos = (rand()%(21))+1;
        int colPos = (rand()%(21))+1;
        if(GetCellState(b, rowPos, colPos) != 7)
           SetCellState(b, rowPos, colPos, 7);
        else
            --x;
    }