Linux Device Drivers, Second Edition

Quick Reference

This chapter introduced the following symbols and header files.

#include <linux/ioctl.h>

This header declares all the macros used to define ioctl commands. It is currently included by <linux/fs.h>.

_IOC_NRBITS , _IOC_TYPEBITS , _IOC_SIZEBITS , _IOC_DIRBITS

The number of bits available for the different bitfields of ioctl commands. There are also four macros that specify the MASKs and four that specify the SHIFTs, but they’re mainly for internal use. _IOC_SIZEBITS is an important value to check, because it changes across architectures.

_IOC_NONE , _IOC_READ , _IOC_WRITE

The possible values for the “direction” bitfield. “Read” and “write” are different bits and can be OR’d to specify read/write. The values are 0 based.

_IOC(dir,type,nr,size) , _IO(type,nr) , _IOR(type,nr,size) , _IOW(type,nr,size) , _IOWR(type,nr,size)

Macros used to create an ioctl command.

_IOC_DIR(nr) , _IOC_TYPE(nr) , _IOC_NR(nr) , _IOC_SIZE(nr)

Macros used to decode a command. In particular, _IOC_TYPE(nr) is an OR combination of _IOC_READ and _IOC_WRITE.

#include <asm/uaccess.h> , int access_ok(int type, const void *addr, unsigned long size);

This function checks that a pointer to user space is actually usable. access_ok returns a nonzero value if the access should be allowed.

VERIFY_READ , VERIFY_WRITE

The possible values for the type argument in access_ok. VERIFY_WRITE is a superset of VERIFY_READ.

#include <asm/uaccess.h> , int put_user(datum,ptr); ...