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 op, ...);  /* glibc, BSD */
       int ioctl(int fd, int op, ...);            /* musl, other UNIX */

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()  operations.   The  argument  fd  must  be an open file
       descriptor.

       The second argument is a device-dependent  operation  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() op 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() op are located in the file <sys/ioctl.h>.  See NOTES.

RETURN VALUE

       Usually, on success zero is returned.  A few ioctl() operations 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 op or argp is not valid.

       ENOTTY fd is not associated with a character special device.

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

VERSIONS

       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).

STANDARDS

       None.

HISTORY

       Version 7 AT&T UNIX has
           ioctl(int fildes, int op, struct sgttyb *argp);
       (where struct sgttyb has historically been used by stty(2) and gtty(2), and is polymorphic
       by operation type (like a void * would be, if it had been available)).

       SysIII documents arg without a type at all.

       4.3BSD has
           ioctl(int d, unsigned long op, char *argp);
       (with char * similarly in for void *).

       SysVr4 has
           int ioctl(int fildes, int op, ... /* arg */);

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 op 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.