listen() queue length in socket-programing in c?

Question

I have written two pair of codes(server.c and client.c) in Linux. One for UNIX-domain AF_UNIX other for INTERNET-domain AF_INET. Both are working fine!

listen() is called for backlog queue length = 3 in both servers

listen(sockfd, 3);  

In UNIX domain (AF_UNIX): While one client is connected with server, If I try to connect more clients to server. Three are kept in queue, and request of fourth is declined. (as I desired - 3 in waiting queue).

In INTERNET domain (AF_INET): Request of more than three are kept in a pending queue.

Why isn't a request from a fourth client rejected, even when the backlog queue length is three? And why is the behavior of listen() (and others) protocol dependent?

Answer 1

Operating systems actually use larger queues for incoming TCP connections than the one specified to listen(). How much larger depends on the operating system.

 listen(int socket_fd, int backlog)  

For a given listening socket kernal maintains two queue.

1. An incomplete connection queue - for which SYN has been come but three-way handshaking (TCP) is not done completely. (SYN_RCV state)
2. A complete connection queue - Three-way handshaking done. (ESTABLISHED state)

backlog argument historically specify sum of both queues. But there is no formal definition of what backlog means.

Berkeley-derived implementation add a fudge factor to the backlog. So total queue length = factor * backlog.

A very detailed and deep explanation given in a book by W. Richard Stevens. Also a table showing the values for seven operating systems can be found in Stevens, Fenner, Rudoff, "Unix Network Programming: The Sockets Network API", Volume 1, Third Edition, Page 108.

Answer 2

The platform is entitled to adjust the specified backlog up or down, according to its minimum and its default. These days the default is more like 500 than five, which is where it started in about 1983. You can't rely on it being what you specified, and there is no API for finding out what it really is, and there is no apparent valid application reason for wanting it to be shorter than the default.