when will HTXParse_put_* be used?

Now I am planning to use HTXParse module.
But I found it seems only HTXParse_write() is called
when I composed a small program to test, and
my question is :
when will HTXparse_put_* be called?

Tao Jiang     Visiting Scientist, CECI, MIT
email:   jiangt@ceci.mit.edu    URL:  http://ceci.mit.edu/staff/jiangt/