22.1. Low-Level Socket Programming

The original Berkeley socket API was a fairly simple set of extensions to the original UNIX file handling mechanisms. It extended the UNIX “everything is a file” approach by allowing you to create new file handles corresponding to network connections.

The original idea of the socket API was to abstract away connection details of the underlying connection mechanism. In UNIX, everything is a file. A simple UNIX program starts with three files open, one for reading and two for writing. These are the standard input, output, and error streams. They are typically connected to the terminal, but can be redirected to point to files or anything that behaves like a file. On a traditional UNIX system, this even includes devices. OS X exports a number of devices in this way. The /dev/ directory lists a lot of devices that are exposed with file-like interfaces. You can read from these or write to them just as you would any other file, although typically access to them is restricted to the superuser.