Sockets

Sockets are a type of inter-process communication that allows processes in different physical machines to communicate with each other, typically over a network.

There are four main steps to use sockets as a server:

• socket — creates the socket.
• bind — attaches the socket to some location (i.e., a file, IP with port, etc).
• listen — indicate that we’re accepting connections, and also set the queue limit (before connections start dropping).
• accept — return the next incoming connection for us to handle.

As a client, we only need two steps:

• socket — creates the socket.
• connect — connects to some location, and the socket can now send/receive data.

Basics

Stream sockets use a protocol called TCP, where all data sent by a client appears in the same order on the server. It forms a persistent connection between the client and server. It’s reliable, but may be slow.

Datagram sockets use UDP. It sends messages between the client and server without a persistent connection. It’s fast but messages may be reordered or dropped altogether.

In UNIX systems

We have a few relevant syscalls:

• int socket(int domain, int type, int protocol)
  • domain — general protocol, further specified with the protocol input (which is mostly unused).
    • AF_UNIX — for local communication on the same physical machine.
    • AF_INET — for IPv4 protocol using your network interface.
    • AF_INET6 — for IPv6, similarly with network interface.
  • type — either stream or datagram sockets, SOCK_STREAM.
• int bind(int socket, const struct sockaddr *address, socklen_t address_len)
  • socket — file descriptor returned from the socket syscall.
  • address — a sockaddr structure¹
    • struct sockaddr_un — for local communication, i.e., just a path.
    • struct sockaddr_in — for IPv4.
    • struct sockaddr_in6 — for IPv6
• int listen(int socket, int backlog)
  • backlog — limit of outstanding connections. This queue is managed by the kernel.
• int accept(int socket, struct sockaddr *restrict address, socklen_t *restrict address_len) — a blocking syscall until a new connection.
  • address and address_len — locations to write the connecting address. Acts as an optional return value. We can set to NULL to ignore.
  • Returns — a new file descriptor, which we can read or write to.
• int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen)
  • sockfd — file descriptor returned by the socket syscall. Client needs to be using the same protocol and type as the server.

Footnotes

1. ”Computers have less manners than people” - Prof Eyolfson ↩