Libevent 2 - write callback / infinite loop - libevent

Question

I'm playing with Libevent as a server and I'm having a problem
Basically I'm using the example at the bottom of this page, http://www.wangafu.net/~nickm/libevent-book/Ref6_bufferevent.html:
I have the main/accept/readcb working well - however trying to understand the writecb.
Before I go further I want to make sure I have this correct.
When a client send (curl in this instance) sends some JSON, this is called in the read_callback, when I want to write back to the curl I use the write_callback.
the problem I have in short, when I do a curl with json. It requests that the server sends a http/1.1 100 continue before sending the JSON. I could ignore that but would rather not.
So this is what I have for my write callback:
void writecb(struct bufferevent *bev, void *ctx){
struct evbuffer *tmp = evbuffer_new();
evbuffer_add_printf(tmp, "HTTP/1.1 100 Continue\r\n\r\n");
bufferevent_write_buffer(bev, tmp);
evbuffer_free(tmp);
printf("finish sending data back to client!\n");
}
What happens is when I do a curl statement:
curl -vv -H "Content-Type: application/json" --data #test.json http://localhost:40714
On the console there is an infinite loop.
However if I put this code within the readcb, it works like a charm. So my question, what is the point of writecb? Is it when you finalize a write and what to close a connection you send 'stuff'?

Answer 1

The point of writecb is opposite to readcb: writecb is invoked by libevent automatically when there is enough room in the OUTPUT side of bufferevent for some new data to be written into.
Here you described a good example of infinite loop:
writecb was invoked by libevent (because there is enough room on OUTPUT side)
inside writecb you generating new data and writing it into the OUTPUT by bufferevent_write_buffer. Then writecb returns context back to libevent main loop.
libevent is passing all the data from OUTPUT to the kernel. And when done, libevent sees that a free place became available in OUTPUT buffer. This generates next writecb event and the callback is invoked again... And here is the infinite loop.