IOCTL on Device Drivers

ioctl();
Input/Output Control (ioctl, in short) is a common operation, or system call, available in most driver categories.basically, anything to do with device input/output, or device-specific operations, yet versatile enough for any kind of operation

The question is: how can all this be achieved by a single function prototype? The trick lies in using its two key parameters: command and argument. The command is a number representing an operation. The argument command is the corresponding parameter for the operation. The function ioctl(); implementation does a switch … case over the commmand to implement the corresponding functionality. The following has been its prototype in the Linux kernel for quite some time:

int ioctl(struct inode *i, struct file *f, unsigned int cmd, unsigned long arg);

user-space using the ioctl()system call, prototyped in <sys/ioctl.h> as:

#include <sys/ioctl.h>

int ioctl(int d, int request, ...);  

DESCRIPTION

The ioctl function manipulates the underlying device parameters of special files. In particular, many operating characteristics of character special files (e.g. terminals) may be controlled with ioctl requests. The argument d must be an open file descriptor.

The second argument is a device-dependent request code. The third argument is an untyped pointer to memory. It's traditionally char *argp (from the days before void * was valid C), and will be so named for this discussion.

An ioctl request has encoded in it whether the argument is an in parameter or out parameter, and the size of the argument argpin bytes. Macros and defines used in specifying an ioctl request are located in the file <sys/ioctl.h>.  

RETURN VALUE

Usually, on success zero is returned. A few ioctls use the return value as an output parameter and return a nonnegative value on success. On error, -1 is returned, and errno is set appropriately.  

ERRORS

EBADF

d is not a valid descriptor.

EFAULT

argp references an inaccessible memory area.

ENOTTY

d is not associated with a character special device.

ENOTTY

The specified request does not apply to the kind of object that the descriptor d references.

EINVAL

Request or argp is not valid.