Unix-socket-faq for network programming

Vic Metcalfe, Andrew Gierth and other contributers

1. General Information and Concepts

• 1.1 About this FAQ
• 1.2 Who is this FAQ for?
• 1.3 What are Sockets?
• 1.4 How do Sockets Work?
• 1.5 Where can I get source code for the book book title]?

• 1.6 Where can I get more information?

2. Questions regarding both Clients and Servers (TCP/SOCK_STREAM)

• 2.1 How can I tell when a socket is closed on the other end?
• 2.2 What's with the second parameter in bind()?
• 2.3 How do I get the port number for a given service?
• 2.4 If bind() fails, what should I do with the socket descriptor?
• 2.5 How do I properly close a socket?
• 2.6 When should I use shutdown()?
• 2.7 Please explain the TIME_WAIT state.
• 2.8 Why does it take so long to detect that the peer died?
• 2.9 What are the pros/cons of select(), non-blocking I/O and SIGIO?
• 2.10 Why do I get EPROTO from read()?
• 2.11 How can I force a socket to send the data in its buffer?
• 2.12 Where can a get a library for programming sockets?
• 2.13 How come select says there is data, but read returns zero?
• 2.14 Whats the difference between select() and poll()?
• 2.15 How do I send this] over a socket?

• 2.16 How do I use TCP_NODELAY?
• 2.17 What exactly does the Nagle algorithm do?
• 2.18 What is the difference between read() and recv()?
• 2.19 I see that send()/write() can generate SIGPIPE. Is there any
• 2.20 After the chroot(), calls to socket() are failing. Why?
• 2.21 Why do I keep getting EINTR from the socket calls?
• 2.22 When will my application receive SIGPIPE?
• 2.23 What are socket exceptions? What is out-of-band data?
• 2.24 running on? How can I find the full hostname (FQDN) of the

3. Writing Client Applications (TCP/SOCK_STREAM)

• 3.1 How do I convert a string into an internet address?
• 3.2 How can my client work through a firewall/proxy server?
• 3.3 Why does connect() succeed even before my server did an
• 3.4 Why do I sometimes lose a server's address when using more than
• 3.5 How can I set the timeout for the connect() system call?
• 3.6 system choose one for me on the connect() call? Should I bind()
• 3.7 Why do I get "connection refused" when the server isn't running?
• 3.8 over the socket ? Is there a way to have a dynamic buffer ?

4. Writing Server Applications (TCP/SOCK_STREAM)

• 4.1 How come I get "address already in use" from bind()?
• 4.2 Why don't my sockets close?
• 4.3 How can I make my server a daemon?
• 4.4 How can I listen on more than one port at a time?
• 4.5 What exactly does SO_REUSEADDR do?
• 4.6 What exactly does SO_LINGER do?
• 4.7 What exactly does SO_KEEPALIVE do?
• 4.8 How can I bind() to a port number < 1024?
• 4.9 How do I get my server to find out the client's address / host-
• 4.10 How should I choose a port number for my server?

5. Choose one that isn't already registered, and isn't in

• 5.1 What is the difference between SO_REUSEADDR and SO_REUSEPORT?
• 5.2 How can I write a multi-homed server?
• 5.3 How can I read only one character at a time?
• 5.4 I'm trying to exec() a program from my server, and attach my

6. Writing UDP/SOCK_DGRAM applications

• 6.1 When should I use UDP instead of TCP?
• 6.2 What is the difference between "connected" and "unconnected"
• 6.3 of the socket? Does doing a connect() call affect the receive
• 6.4 How can I read ICMP errors from "connected" UDP sockets?
• 6.5 How can I be sure that a UDP message is received?
• 6.6 How can I be sure that UDP messages are received in order?
• 6.7 How often should I re-transmit un-acknowleged messages?
• 6.8 How come only the first part of my datagram is getting through?
• 6.9 Why does the socket's buffer fill up sooner than expected?

7. Advanced Socket Programming

• 7.1 How would I put my socket in non-blocking mode?
• 7.2 How can I put a timeout on connect()?

8. Sample Source Code