Print input lines longer than 80 characters in C
That's too complicated a solution.

It also not only imposes an arbitrary limit on the maximum possible input line length, it fails to check that the input doesn't exceed this limit.

80 is the only significant number in this program.
There's no need to worry about anything more than that.

Simply get characters into an 80 character buffer.

If you get '\n' before it fills, empty the buffer.

Otherwise, print the full buffer and get and print characters until the next '\n'.