Using macros to generate ioctl command numbers

The Linux header file /usr/include/asm/ioctl.h defines macros that must be used to create the ioctl command number. These macros take various combinations of three arguments:

• type--an 8-bit integer selected to be specific to the device driver. type should be chosen so as not to conflict with other drivers that might be ``listening'' to the same file descriptor. (Inside the kernel, for example, the TCP and IP stacks use distinct numbers since an ioctl sent to a socket file descriptor might be examined by both stacks.)

• number--an 8-bit integer ``command number.'' Within a driver, distinct numbers should be chosen for each different kind of ioctl command that the driver services.

• data_type--The name of a type used to compute how many bytes are exchanged between the client and the driver. This argument is, for example, the name of a structure.

The macros used to generate command numbers are:

• _IO(int type, int number) - used for a simple ioctl that sends nothing but the type and number, and receives back nothing but an (integer) retval.

• _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.

• _IORW(int type, int number, data_type) - a combination of _IOR and _IOW. That is, data is both written to the driver and then read back from the driver by the client.

Program 6 is an example header file showing the use of these macros. In real programs, the client executing an ioctl and the driver that services it must share the same header file.