What are ioctl commands?

In computing, ioctl (an abbreviation of input/output control) is a system call for device-specific input/output operations and other operations which cannot be expressed by regular system calls. It takes a parameter specifying a request code; the effect of a call depends completely on the request code.

What is ioctl in Linux drivers?

IOCTL is referred to as Input and Output Control, which is used to talking to device drivers. This system call, available in most driver categories. The major use of this is in case of handling some specific operations of a device for which the kernel does not have a system call by default.

What is the purpose of ioctl?

The ioctl function is useful for implementing a device driver to set the configuration on the device. e.g. a printer that has configuration options to check and set the font family, font size etc. ioctl could be used to get the current font as well as set the font to a new one.

What is _iowr?

The second argument to _IO, _IOW, _IOR, or _IOWR is a sequence number to distinguish ioctls from each other. The third argument to _IOW, _IOR, or _IOWR is the type of the data going into the kernel or coming out of the kernel (e.g. ‘int’ or ‘struct foo’).

What is __ user in C?

2. __user is used to mark a pointer as userspace, as in, to indicate that the pointer exists in userspace and that it should not be dereferenced. 3. __force is used to, well, force a conversion, like casting from or to a bitwise type, else the Sparse checker will throw a warning.

What is ioctl used for in Linux?

The ioctl function is useful for implementing a device driver to set the configuration on the device. e.g. a printer that has configuration options to check and set the font family, font size etc. ioctl could be used to get the current font as well as set the font to a new one.

What is the difference between _IOR and _IOW?

_IOR(int type, int number, data_type), used for an ioctl that reads data from the device driver. The driver will be allowed to return sizeof(data_type) bytes to the user. _IOW(int type, int number, data_type), similar to _IOR, but used to write data to the driver.

Why should I care about ioctl numbers?

If each ioctl number is unique, the application gets an EINVAL error rather than succeeding in doing something unintended. Furthermore, an ioctl can require to write data to and/or read data from kernel space. When one creates it’s own driver that performs ioctls, he will need to describe all this in the ioctl command.

What is the difference between __IOW(magic) and TYPE(SEQ_no)?

__IOW(MAGIC, SEQ_NO, TYPE)MAGICand SEQ_NOare the same as above, and TYPEgives the type of the next argument, recall the third argument of ioctlis void *. W in __IOWindicates that the data flow is from user application to driver module. As an example, suppose we want to set the printer font to “Arial”.