3.1. The Design of scull

3.1. The Design of scull
The first step of driver writing is defining the capabilities (the mechanism) the driver will offer to user programs. Since our "device" is part of the computer's memory, we're free to do what we want with it. It can be a sequential or random-access device, one device or many, and so on.
To make scull useful as a template for writing real drivers for real devices, we'll show you how to implement several device abstractions on top of the computer memory, each with a different personality.
The scull source implements the following devices. Each kind of device implemented by the module is referred to as a type .
scull0 to scull3
Four devices, each consisting of a memory area that is both global and persistent. Global means that if the device is opened multiple times, the data contained within the device is shared by all the file descriptors that opened it. Persistent means that if the device is closed and reopened, data isn't lost. This device can be fun to work with, because it can be accessed and tested using conventional commands, such as cp, cat, and shell I/O redirection.
scullpipe0 to scullpipe3
Four FIFO (first-in-first-out) devices, which act like pipes. One process reads what another process writes. If multiple processes read the same device, they contend for data. The internals of scullpipe will show how blocking and nonblocking read and write can be implemented without having to resort to interrupts. Although real drivers synchronize with their devices using hardware interrupts, the topic of blocking and nonblocking operations is an important one and is separate from interrupt handling (covered in Chapter 10).
scullsingle
scullpriv
sculluid
scullwuid
These devices are similar to scull0 but with some limitations on when an open is permitted. The first (scullsingle) allows only one process at a time to use the driver, whereas scullpriv is private to each virtual console (or X terminal session), because processes on each console/terminal get different memory areas. sculluid and scullwuid can be opened multiple times, but only by one user at a time; the former returns an error of "Device Busy" if another user is locking the device, whereas the latter implements blocking open. These variations of scull would appear to be confusing policy and mechanism, but they are worth looking at, because some real-life devices require this sort of management.
Each of the scull devices demonstrates different features of a driver and presents different difficulties. This chapter covers the internals of scull0 to scull3; the more advanced devices are covered in Chapter 6. scullpipe is described in the section Section 3.4 and the others are described in Section 6.6