What happened when don't write any things between two parentheses?

Answer 1

Your program enters to a infinite loop but syntax should be like that
for ( ; ; )
{
//codes
}

If you don't put semicolons there an error occurs. (I guess syntax error.)

// Edit:
In Other loops, like while, do while
you have to put statement otherwise an error occurs.
while ( //statement )
{ }

or

do{
//codes
while( //statement );

Answer 2

Nothing happens except the useless loop.