difference between two call backs in libevent

Question

In libevent, are both of the following APIs going to invoke the call back function when the HTTP transaction completes?

//from event2/http.h
/**
 * Creates a new request object that needs to be filled in with the request
 * parameters.  The callback is executed when the request completed or an
 * error occurred.
 */
struct evhttp_request *evhttp_request_new(
    void (*cb)(struct evhttp_request *, void *), void *arg);

/*The callback function will be called on the completion of the request after
 * the output data has been written and before the evhttp_request object
 * is destroyed ....*/
void evhttp_request_set_on_complete_cb(struct evhttp_request *req,
    void (*cb)(struct evhttp_request *, void *), void *cb_arg);

The wording for evhttp_request_new(..) is different from that of evhttp_request_set_on_complete_cb(..), but in my test, the call back in evhttp_request_new(..) is indeed invoked at the end of HTTP transaction.

Answer 1

I've inspected those functions in http.c (libevent 2.1.8) and I found that their inner usage differ:

1) evhttp_request_new() CREATES evhttp_request object AND SETS req->cb that is USED BY:

• evhttp_connection_incoming_fail() ---> calls req->cb as reply-function about this failing that comes from anywhere - corrupted error, too long, firstline function, etc...
• evhttp_connection_done() ---> calls as (*req->cb)(req, req->cb_arg); just after processing request/response.

It is positioned by evhttp_connection and error

2) evhttp_request_set_on_complete_cb() SETS req->on_complete_cb FOR EXISTING evhttp_request object in additional to req->cb. USED ONLY BY: evhttp_send_done (this function called from evhttp_send()->evhttp_write_buffer(): evcon->cb=evhttp_send_done() after whole requested page data is sent / chunked request finish).

It is positioned after data writing over, and only if some error did not occure.