What value of backlog should I use?

Question

I read the man 2 listen.

I don't understand what is the backlog value, it says

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow

Right, how can I define what is the best value?

Thanks

Cladophyll

possible duplicate of socket listen backlog parameter, how to determine this value? – Chronometry

Answer 1

Basically, what the listen() backlog affects is how many incoming connections can queue up if your application isn't accept()ing connections as soon as they come in. It's not particularly important to most applications. The maximum value used by most systems is 128, and passing that is generally safe.

Reindeer

Okay, if I listen(sockfd, 5), should I test in my accept() (in my infinite loop) if current_nb_client < 5 in order to send a error message to my client or can I trust backlog and handle this in client-side? – Cladophyll

No, that's not necessary -- as long as you're accepting connections as soon as they come in, the length of your listen backlog is irrelevant. You can have as many active connections as you need; the listen backlog only affects connections which haven't been fully established. – Reindeer

Oh, I've just understand! I thought they do not disappear from the queue. But no! It's a pending queue, and I just have to accept() each and they disappear from the queue. – Cladophyll

Answer 2

It's a fight between clients trying to connect. pushing accept requests onto the queue, and the accept thread/s sucking them off. Usually, the threads win. I usually set at 32, but it's not usually an important parameter.

Aldoaldol

Okay, if I listen(sockfd, 5), should I test in my accept() (in my infinite loop) if current_nb_client < 5 in order to send a error message to my client or can I trust backlog and handle this in client-side? – Cladophyll

The connection attempt will be refused if the queue is full, but it's just so unlikely to happen that you should forget about it. There has to be a bit of a queue to cover those times when it just happens that a lot of connection rquests come in in a burst but, overall, todays processors/memory/OS/whatever are easily able to keep up with busy networks. – Aldoaldol