Re: [Libevent-users] Dynamic changes of read water mark

[Mark Ellzey <mthomas@xxxxxxxxxx>]

On Sun, Aug 29, 2010 at 08:08:12AM -0700, Gilad Benjamini wrote:
> My eventloop needs to go through this cycle
>    - Read 16 bytes off the network
>    - The 16 bytes contain a length parameter
>    - Read additional "length" bytes from the network.
> 
> It seems that this can be done with a buffered event which keeps changing
> the read low watermark (twice per message).
> Is this a "proper" way to use buffered events ?
> 

Yes, bufferevents are fine for this, this abstracts some of the hard
work. I do the same thing in one of my applications. Something along
the lines of:

struct frame {
	uint16_t something;
  uint16_t length;
};

size_t frame_length = 4;

buf = bufferevent_get_input(bev);

while (evbuffer_get_length(buf) > 0) {
  unsigned char *data;
	unsigned char *payload;
	struct frame   frame;

	data = evbuffer_pullup(buf, frame_length);

	if (evbuffer_get_length(data) < frame_length) {
	  /* we don't have enough data yet to fill in our frame, return and wait for more */
		break;
	}

	memcpy((char *)&frame.something, data, sizeof(uint16_t));
  memcpy((char *)&frame.length, &data[sizeof(uint16_t)], sizeof(uint16_t))/

	if ((evbuffer_get_length(buf) - frame_length) < frame.length) {
		/* whole payload hasn't been seen */
		break;
	}

	/* drain the header portion from the evbuffer */
	evbuffer_drain(buf, frame_length);

	if (!(payload = malloc(frame.length))) {
		perror("malloc");
		return;
	}

	evbuffer_remove_buffer(buf, payload, frame.length); 
}
	
(not tested, just real quick example I could come up with)
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