how is this while-loop iterating done?

In C, = is an operator just like every other operator. a = b sets the variable a to the value b, then returns the value b. So, y = (x = 1) + 2 sets x to 1, then adds 2 to that 1, then sets y to 3. readdir takes a DIR* as an argument, and that DIR* has an internal state that determines which file to read. Every time you call readdir on it, it’ll return the next file. See the docs here.