Linux – Do all devices under Linux have a device node

Parse the root= parameter from /proc/cmdline.

On many devices, the main operations are to send bytes from the computer to a peripheral, or to receive bytes from a peripheral on the computer. Such devices are similar to pipes and work well as character devices. For operations that aren't reading and writing (such as flow control on a serial line), the device provides ad-hoc commands called ioctl.

Some devices are very much like regular files: they're made of a finite number of bytes, and what you write at a given position can later be read from the same position. These devices are called block devices.

Network interfaces are more complex: what they read and write is not bytes but packets. While it would still be possible to use the usual interface with read and write, it would be awkward: presumably each call to write would send a packet, and each call to read would receive a packet (and if the buffer is too small for the packet to fit, the packet would be lost).

Network interfaces could exist as devices providing only ioctl. In fact, this is what some unix variants do, but not Linux. There is some advantage to this approach; for example, on Linux, network interfaces could leverage udev. But the advantages are limited, which is why it hasn't been done.

Most network-related applications don't care about individual network interfaces, they work at a higher level. For example, a web browser wants to make TCP connections, and a web server wants to listen for TCP connections. For this purpose, what would be useful is devices for high-level network protocols, e.g.

{ echo $'GET http://www.google.com/ HTTP/1.0\r';
  echo $'Host: www.google.com\r';
  echo $'\r' >&0; cat; } <>/dev/tcp/www.google.com/80

In fact ksh and bash provide such an interface for TCP and UDP clients. In general, however, network applications are more complex than file-accessing applications. While most data exchanges are conducted with calls analogous to read and write, establishing the connection requires more information than just a file name. For example, listening for TCP connections takes two steps: one to be performed when the server starts listening, and one to be performed each time a client connects. Such extra steps don't fit well into the file API, which is the main reason why networking has its own API.

Another class of devices that typically doesn't have entries in /dev on Linux (but does on some other unix variants) is video adapters. In principle, simple video adapters could be exposed as framebuffer devices, which could be block devices made of blocks representing the color of each pixel. Accelerated video adapters could be represented as character devices onto which applications send commands. Here, the drawback of the device interface is that it's slow: the displaying application (in practice, an X server) would need to make kernel calls whenever displaying anything. What happens instead is that the X server mostly writes directly to the memory of the video adapter, because it's faster.