Find IOCTL number in linux

Question

I see into header file

#define IOCTL_MAGIC 'A'
#define IOCTL_NAME _IOWR(IOCTL_MAGIC, 2, ioctl_param)

How can I know that is the ioctl number of IOCTL_NAME ?

Answer 1

The way of constructing of ioctl number is described at the top of the header include/uapi/asm-generic/ioctl.h:

ioctl command encoding: 32 bits total, command in lower 16 bits, size of the parameter structure in the lower 14 bits of the upper 16 bits.

...

De facto, however, the top 8 bits of the lower 16 bits are indeed used as a type field

That is, an ioctl number is constructed from 4 fields, from upper to lower:

1. dir - direction, 2 bits.
   • Upper bit denotes that a user writes an argument,
   • Lower bit denotes that a user reads an argument.
2. size - size of the argument, 14 bits.
3. type - a number uniquely representing a driver, 8 bit.
4. nr - a number which is unique for the type (for the driver), 8 bit.

For decode

#define IOCTL_MAGIC 'A'
#define IOCTL_NAME _IOWR(IOCTL_MAGIC, 2, ioctl_param)

you need to know the size of ioctl_param structure (sizeof(ioctl_param)).

E.g., if the size of the structure is 16 bytes, then ioctl fields are:

1. dir - 0x3 (both read and write).
2. size - 0x10 (size of the structure, 16).
3. type - 0x41 (ASCII code of the character A).
4. nr - 0x2 (the second argument).

and the ioctl number itself is 0xc0104102.