Douglas Clarke

Mac OS X and iOS

Mac OS X is a modern Unix-based operating system developed by Apple Inc for their Macintosh computer series. OS X is the tenth incarnation of Mac OS.

Advanced Kernel Programming

This chapter covers miscellaneous topics that are of interest to more advanced kernel programmers. We’ll discuss how Streaming SIMD Extensions (SSE) and floating point can be used in the kernel. (SIMD is short for Single Input Multiple Data.) We will also examine strategies for dealing with drivers for multifunction devices, and discuss the implementation of I/O Kit families. We’ll cover the kernel control KPI that can be used for user space communication with KEXTs such as Network Kernel Extensions (NKE) that does not use the I/O Kit. We also show how to work with and manipulate processes from the kernel, such as getting the process identifier (PID) of a process and sending signals to the process. Some drivers may need additional resources loaded from the file system, such as firmware images. This chapter provides a discussion of how these resources can be loaded using the OSKextRequestResource() function. The chapter concludes with a discussion of how a driver can send messages to a user space daemon using notifications.

Operating System Fundamentals

The role of an operating system is to provide an environment in which the user is able to run application software. The applications that users run rely on services provided by the operating system to perform tasks while they execute, in many cases without the user—or even the programmer—giving much thought to them. For an application to read a file from disk, for example, the programmer simply needs to call a function that the operating system provides. The operating system handles the specific steps required to perform that read. This frees the application programmer from having to worry about the differences between reading a file that resides on the computer’s internal hard disk or a file on an external USB flash drive; the operating system takes care of such matters.

Universal Serial Bus

The Universal Serial Bus (USB) is a ubiquitous technology found in a wide variety of products, notably computer peripherals, including mice, keyboards, hard drives, and printers, as well as almost any other type of device or equipment that can be connected to a computer. The USB is a specification that defines the communication between a device, such as a printer or mobile phone, and a host controlled by a computer device, such as your Mac or iPad. The USB specification was developed in 1996 by a consortium of companies, including Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel. The motivation was to replace a series of connectors with a universal connector, making it easier to connect external devices to personal computers. The USB specification is currently at version 3.0. Support for 3.0 is still emerging, and support for version 2.0 is by far the most ubiquitous at this time. Apple has yet to release hardware capable of supporting the latest USB 3.0 specification, but Apple computers have shipped with USB support since before OS X. The iOS series of devices are themselves USB devices, but they can also act as USB hosts. An example of this is the iPad, which can act as a host for USB devices, such as digital cameras.

PCI Express and Thunderbolt

PCI (Peripheral Component Interconnect) is a high-speed bus developed by Intel, in the early nineties, to replace various older and slower bus technologies such as EISA, ISA, MCA, and VESA. The term PCI is often used to describe the family of technologies based on the original PCI specification. Throughout this chapter, when we refer to PCI, we refer to commonalities found in the PCI-based technologies; namely, PCI Express, Thunderbolt, and to a lesser extent ExpressCard. Most people associate PCI with expansion boards plugged into a computer, but it is worth noting that PCI is fundamental to many computer systems—even those without PCI slots, such as iMacs— that have internal PCI buses that connect the CPU to USB, Firewire, and SATA controllers. Recent PCI-based advancements (like Thunderbolt) allow the PCI bus to be extended outside of the computer, much in the same way as USB and Firewire.

Interacting with Drivers from Applications

In the previous chapter, we learnt about I/O Kit drivers, which live in the kernel. On the other hand, the applications that users interact with live in user space. So, if the user is going to use the services provided by your driver, the kernel/user space boundary needs to be crossed.

User-Space USB Drivers

From a user’s perspective, an application that requires a kernel driver detracts from the user experience. To begin with, driver installation involves writing to the “Extensions” directory, which requires administrative privileges. Therefore, the user needs to run an installer and enter the password of an administrative account, and then possibly restart before they can begin using the application. If, on the other hand, the application doesn’t require a kernel driver, the installation procedure can be as simple as downloading an application from the Mac App Store.

The I/O Kit Framework

Device drivers for Mac OS X are written using a framework known as the I/O Kit. The I/O Kit consists of header files and libraries that provide the services required by drivers, as well as header files and libraries that are used by user space code to locate a kernel driver and interact with it. There are two main parts of the I/O Kit: Kernel.framework IOKit.framework

Synchronization and Threading

As we have seen throughout this book, the role of a driver is to make the functionality that is provided by a hardware device available to the operating system and to user applications. This means that the code inside a driver may be called from any number of running applications at any time, depending on when an application wishes to request the services of the hardware device. In handling these requests, the driver runs in the thread context of the application that made the control call. In addition to these requests, the hardware itself can require servicing and may generate interrupts at arbitrary times that the driver must respond to. The end result for the driver developer is that driver code runs in a complex multithreaded environment, even without the driver creating any additional threads of its own.

Xcode and the Kernel Development Environment

Apple has a good track record of taking care of its developers and providing them with intuitive, user-friendly tools and APIs to develop for the Mac and iOS platforms. Anyone who has written application software for the Mac or iPhone will be familiar with the object-oriented Cocoa framework, which provides a rich set of interfaces to support graphical user interfaces and other services required by user applications. Likewise, kernel developers are provided with APIs that are designed to help with the tasks performed by a kernel extension. For driver development, Apple provides the I/O Kit, which is an object-oriented framework for interfacing with hardware. The following chapter discusses the tools and frameworks you will need to get started with kernel development and includes a tutorial for building and installing a simple kernel extension.