Y4M header C

Hello /prog,
I need help parsing the header of a y4m file.
I took a look at the y4m.c file from the link below and i cannot understand what are all these 4 inputs for in the open_file func. The task is just to read the header.
Thanks in advance.

http://www.v.8mv.net/2010/mar/x264/input/y4m.c

But from which one do i get the whole header?