Entry Points for Block Device Drivers

Devices that support a file system are known as block devices. Drivers written for these devices are known as block device drivers. Block device drivers take a file system request, in the form of a buf(9S) structure, and issue the I/O operations to the disk to transfer the specified block. The main interface to the file system is the strategy(9E) routine. See Chapter 16, Drivers for Block Devices for more information.

A block device driver can also provide a character driver interface to enable utility programs to bypass the file system and to access the device directly. This device access is commonly referred to as the raw interface to a block device.

The following table lists additional entry points that can be used by block device drivers. See also Entry Points Common to All Drivers.

Table 1-2 Additional Entry Points for Block Drivers

Entry Point	Usage	Description
`aread`(9E)	Optional	Performs an asynchronous read. Drivers that do not support an `aread()` entry point should use the `nodev`(9F) error return function. Additional information: Differences Between Synchronous and Asynchronous I/O DMA Transfers (Asynchronous)
`awrite`(9E)	Optional	Performs an asynchronous write. Drivers that do not support an `awrite()` entry point should use the `nodev`(9F) error return function. Additional information: Differences Between Synchronous and Asynchronous I/O DMA Transfers (Asynchronous)
`print`(9E)	Required	Displays a driver message on the system console. Additional information: print() Entry Point (Block Drivers)
`strategy`(9E)	Required	Perform block I/O. Additional information: Canceling DMA Callbacks DMA Transfers (Synchronous) strategy() Entry Point DMA Transfers (Asynchronous) General Flow of Control x86 Target Driver Configuration Properties