Red Hat Enterprise Linux 5 Administration Unleashed

HAL 165HALIf the kernel knows about a piece of hardware, how does an application gain access to it?As a user or administrator, you want it to “just work.” This is now possible with HAL(Hardware Abstraction Layer). HAL was introduced in Fedora Core 3 and Red HatEnterprise Linux 4.HAL works by broadcasting a signal to the system message bus when a new device isadded. Then, an application can connect to the message bus instead of the kernel to learnabout the hardware. Just like Kudzu runs at boot time to detect new hardware, the HALdaemon runs while the system is running to detect new hardware. The HAL daemoncollects information about the device from the kernel as well as other resources. Thisallows the system bus to send as much information as possible to the application, and theapplication only needs to gather information from one place.For developers who need to use HAL, the hal-gnome package includes an exampleprogram and development tool for HAL. It can be started by executing the hal-devicemanagercommand. As shown in Figure 6.1, it provides a tree view of all the devices HALknows about. Because it is a Python program, the hal-gnome package installs the sourcefiles for the program in /usr/share/hal/device-manager/. They can be used to understandhow to interact with devices via HAL.6FIGURE 6.1HAL Device Manager