How libevent triggers callback

Question

I am wondering how libevent triggers the callback. For example, let's say a client repeatedly sends a 4-byte message. In server's side, it seems not each 4-byte send will trigger a read callback. For example, I found the callback is sometimes triggered at a point of receiving 8-byte message (or other number). Is it possible to configure the libevent such that the read callback on server's side is triggered for each 4-byte message sent from the client?

Answer 1

I believe your problem is about sending, not about receiving. Libevent is probably not sending your data immediately because of output buffering, thus your read callback is only invoked when the client really receives the data you've sent.

See Nagle's algorithm (aka TCP_NODELAY). Then try disabling it to see if that's really the case:

int flag = 1;
int ret = setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &flag, sizeof(flag));

Is it possible to configure the libevent such that the read callback on server's side is triggered for each 4-byte message sent from the client?

Not really. You may receive multiple messages "simultaneously" as TCP is a stream-oriented protocol. Your application's only concern is to know where each message starts and ends.

Disregarding libevent for the purpose of explanation, you'd normally:

1. Keep calling recv in a loop;
2. Append the received data/payload to a buffer;
3. Invoke a method to process that buffer;
4. The process method checks the buffer size to see if it contains at least 1 full message. In your case, you want it to be at least 4 bytes, so you know you have at least 1 full message to process. If your buffer has a size that's not a multiple of 4 bytes (which should never happen given the very small size of your payload), say 10 bytes, you process the initial 4 bytes, remove them from the buffer, repeat once again, and then keep the remaining 2 bytes in the buffer.