Introduction to Accessing Hardware From Applications

There are many reasons your application might need to access hardware: Receiving mouse and keyboard events, accessing devices, such as a FireWire DV camcorder, and driving a device from an application are just a few. Although only code that resides in the kernel can access hardware directly, OS X provides many services that allow you to communicate with hardware from plug-ins, applications, shared libraries, and other code running outside the kernel.

Who Should Read This Document?

This document describes how software running in OS X can access hardware by communicating with the kernel, focusing on services the I/O Kit provides to develop an application-based driver. You should read this document if you need to access a device from an application. Note that many applications will be able to handle all their hardware-access needs using high-level APIs, such as Open Transport and QuickTime, that are available through Carbon and Cocoa. To help you determine which approach is right for you, and for a summary of other services OS X provides for hardware access from applications, see Hardware-Access Options.

This document includes many code fragments illustrating the various tasks involved in developing an application that accesses hardware, but it is not intended to be a step-by-step cookbook for accessing a particular type of device. To determine how to access a particular device, see I/O Kit Family Device-Access Support; For each device family it specifies how to access a device in that family and where to find more detailed documentation.

This document does not describe how to write kernel-resident code to access hardware. Kernel programmers should refer to Kernel Programming Guide and in-kernel device-driver developers should read IOKit Device Driver Design Guidelines. In particular, if you are developing your own device interface and user client to create a custom solution to access your device, you should read the Making Hardware Accessible to Applications chapter in that document. For other documents that cover how to access particular devices, visit Reference Library > Hardware & Drivers.

Organization of This Document

Accessing Hardware From Applications includes the following chapters:

• Hardware-Access Options

  Describes many other methods you can use to access hardware from an application, such as Core Audio, QuickTime, and the Carbon Event Manager. Read this chapter to determine if such high-level APIs can meet your needs.

• Device Access and the I/O Kit

  Summarizes I/O Kit architecture, providing a list of terms used throughout this document and describing how the I/O Kit models I/O connections. It then describes the two fundamental hardware-access methods the I/O Kit supports: device interfaces and device files.

• Finding and Accessing Devices

  Describes the steps you take to access a device using an I/O Kit device interface and, for appropriate devices, using a device file.

• The IOKitLib API

  Categorizes and describes the functions of the main API that supports user-space device access through the I/O Kit.

• Handling Errors

  Describes how to interpret I/O Kit return values and provides information on the exclusive-access error.

• I/O Kit Family Device-Access Support

  Lists the current I/O Kit families and describes what support they provide for hardware access from applications.

• Document Revision History

  Lists changes to this document.

• Glossary

  Defines key terms used in this document.