12.3.3 Associating Devices with Device Drivers
As discussed in the section on standard I/O functions, the create function is used to create a virtual instance of a device. The I/O subsystem tracks these virtual instances using the device table. A newly created virtual instance is given a unique name and is inserted into the device table, as shown in Figure 12.8. Figure 12.8 also illustrates the device table's relationship to the driver table.
Figure 12.8: Associating devices with drivers.
Each entry in the device table holds generic information, as well as instance-specific information. The generic part of the device entry can include the unique name of the device instance and a reference to the device driver. In Figure 12.8, a device instance name is constructed using the generic device name and the instance number. The device named tty0 implies that this I/O device is a serial terminal device and is the first instance created in the system. The driver-dependent part of the device entry is a block of memory allocated by the driver for each instance to hold instance-specific data. The driver initializes and maintains it. The content of this information is dependent on the driver implementation. The driver is the only entity that accesses and interprets this data.
A reference to the newly created device entry is returned to the caller of the create function. Subsequent calls to the open and destroy functions use this reference.