ioctl(2) System Calls Manual ioctl(2)
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 charac-
ter special files (e.g., terminals) may be controlled with ioctl() oper-
ations. The argument fd must be an open file descriptor.
The second argument is a device-dependent operation code. The third ar-
gument 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 er-
ror.
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 de-
vice 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 in-
dicating 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) fol-
lowed 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 ar-
chitectures.
SEE ALSO
execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2), ioctl_fs(2),
ioctl_fsmap(2), ioctl_nsfs(2), ioctl_tty(2), ioctl_userfaultfd(2),
ioctl_eventpoll(2), open(2), sd(4), tty(4)
Linux man-pages 6.9.1 2024-06-13 ioctl(2)
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