Ubuntu Manpage: ioctl - control device

NAME

       ioctl - control device

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/ioctl.h>

       int ioctl(int fd, unsigned long request, ...);

DESCRIPTION

       The ioctl() system call 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 fd 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  argp  in  bytes.   Macros  and  defines  used  in
       specifying an ioctl() request are located in the file <sys/ioctl.h>.  See NOTES.

RETURN VALUE

       Usually,  on  success zero is returned.  A few ioctl() requests use the return value as an
       output parameter and return a nonnegative value on success.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS

       EBADF  fd is not a valid file descriptor.

       EFAULT argp references an inaccessible memory area.

       EINVAL request or argp is not valid.

       ENOTTY fd is not associated with a character special device.

       ENOTTY The specified request does not apply to the kind of object that the file descriptor
              fd references.

STANDARDS

       No single standard.  Arguments, returns, and semantics of ioctl() vary  according  to  the
       device  driver  in  question  (the  call  is used as a catch-all for operations that don't
       cleanly fit the UNIX stream I/O model).

       The ioctl() system call appeared in Version 7 AT&T UNIX.

NOTES

In order to use this call, one needs an open file descriptor. Often the open(2) call has
unwanted side effects, that can be avoided under Linux by giving it the O_NONBLOCK flag.

ioctl structure
Ioctl command values are 32-bit constants. In principle these constants are completely
arbitrary, but people have tried to build some structure into them.

The old Linux situation was that of mostly 16-bit constants, where the last byte is a
serial number, and the preceding byte(s) give a type indicating the driver. Sometimes the
major number was used: 0x03 for the HDIO_* ioctls, 0x06 for the LP* ioctls. And sometimes
one or more ASCII letters were used. For example, TCGETS has value 0x00005401, with 0x54
= 'T' indicating the terminal driver, and CYGETTIMEOUT has value 0x00435906, with 0x43
0x59 = 'C' 'Y' indicating the cyclades driver.

Later (0.98p5) some more information was built into the number. One has 2 direction bits
(00: none, 01: write, 10: read, 11: read/write) followed by 14 size bits (giving the size
of the argument), followed by an 8-bit type (collecting the ioctls in groups for a common
purpose or a common driver), and an 8-bit serial number.

The macros describing this structure live in <asm/ioctl.h> and are _IO(type,nr) and
{_IOR,_IOW,_IOWR}(type,nr,size). They use sizeof(size) so that size is a misnomer here:
this third argument is a data type.

Note that the size bits are very unreliable: in lots of cases they are wrong, either
because of buggy macros using sizeof(sizeof(struct)), or because of legacy values.

Thus, it seems that the new structure only gave disadvantages: it does not help in
checking, but it causes varying values for the various architectures.