Device Addressing Using loctls

Even though character and block devices generally fit snugly in the structures of the filesystem and therefore conform to the Unix philosophy that ''everything is a file,'' some tasks are very difficult to perform using only input and output commands. These involve checking device-specific functions and properties that are not within the general file framework. A prime example is setting the configuration of a device.

If is, of course, possible to carry out such tasks by defining ''magic'' strings with special meanings and using generic read and write functions. For example, an approach of this kind can be used with a floppy disk drive to support software ejection. The device driver could monitor the data stream to the device and eject the disk from the drive when it encounters the string floppy: eject. Special codes can likewise be defined for other tasks.

This method has one obvious disadvantage. What happens if a text file containing the above string is written to floppy disk (the text may be part of an operating guide for the disk drive)? The driver would eject the disk — to the annoyance of the user because this is not what is wanted. Naturally, it is possible to prevent this situation by having a userspace application check that the string does not appear in the text or by masking it if it does (an appropriate method would also have to be defined to do this too). This whole process is not only a waste of time and resources but lacks elegance and sophistication.4

The kernel must therefore provide a way of supporting special device properties without having recourse to normal Read and Write commands. One way of doing this is to introduce special system calls. However, this a deprecated practice among kernel developers and is therefore used only for a few very popular devices. A more appropriate solution goes by the name of IOCTL, which stands for input output control interface and is a general interface for configuring and modifying specific device characteristics. There's also a third alternative available: Sysfs, a file system that hierarchically represents all devices of the system, and also provides means to set parameters for these devices. More information about this mechanism is contained in Section 10.3. For now, I will stick to the slightly more old-fashioned, but still valid IOCTL method.

Ioctls are implemented by means of a special method that can be used to process files. This method produces the desired results when applied to device files but has no effect when used with regular files. Chapter 8 discusses how the implementation fits into the virtual filesystem schema. All we need to know at this point is that each device driver is free to define an ioctl routine that enables control data to be transferred separately from the actual input/output channel.

How are ioctls employed from a user and programming point of view? The standard libraries provide the ioctl function to direct an ioctl command to an opened file by means of a specific code. The

4 For historical reasons, some drivers do employ this method. It is used widely with terminals to transfer control characters to modify device properties such as text color, cursor position, and so on.

implementation of this function is based on the ioctl system call that is handled by sys_ioctl in the kernel (see Chapter 13 for information on the implementation of system calls).

Continue reading here: Fsioctlc

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