Here are a few typical mistakes:

a) you forgot to fill the array with ' '
b) you're doing gets( buff ), and the size of buff is less than 10000
c) you're doing printf( "%s", out ), and you forgot to do out[80] = '\0'
d) if the position becomes 80, you go to 79 instead of 0
e) you don't print trailing spaces

Good luck!

faint! I didn't see the word 'mistakes' ......

d) seems to be wrong,
"When the cursor reaches left or right edge of the screen it is automatically placed at the first position to the left."
so if the position becomes 80, you should go to 0.