Linux Device Drivers, Second Edition

The ioctl Method

Like char devices, block devices can be acted on by using the ioctl system call. The only relevant difference between block and char ioctl implementations is that block drivers share a number of common ioctl commands that most drivers are expected to support.

The commands that block drivers usually handle are the following, declared in <linux/fs.h>.

BLKGETSIZE

Retrieve the size of the current device, expressed as the number of sectors. The value of arg passed in by the system call is a pointer to a long value and should be used to copy the size to a user-space variable. This ioctl command is used, for instance, by mkfs to know the size of the filesystem being created.

BLKFLSBUF

Literally, “flush buffers.” The implementation of this command is the same for every device and is shown later with the sample code for the whole ioctl method.

BLKRRPART

Reread the partition table. This command is meaningful only for partitionable devices, introduced later in this chapter.

BLKRAGET , BLKRASET

Used to get and change the current block-level read-ahead value (the one stored in the read_ahead array) for the device. For GET, the current value should be written to user space as a long item using the pointer passed to ioctl in arg; for SET, the new value is passed as an argument.

BLKFRAGET , BLKFRASET

Get and set the filesystem-level read-ahead value (the one stored in max_readahead) for this device.

BLKROSET , BLKROGET

These commands are used to change ...