Apache API notes:
"Special considerations for response handlers
Handlers for most phases do their work by simply setting a few fields in the request_rec structure (or, in the case of access checkers, simply by returning the correct error code). However, response handlers have to actually send a request back to the client.
They should begin by sending an HTTP response header, using the function send_http_header. (You don't have to do anything special to skip sending the header for HTTP/0.9 requests; the function figures out on its own that it shouldn't do anything). If the request is marked header_only, that's all they should do; they should return after that, without attempting any further output.
Otherwise, they should produce a request body which responds to the client as appropriate. The primitives for this are rputc and rprintf, for internally generated output, and send_fd, to copy the contents of some FILE * straight to the client."
