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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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ioctl(2) System Calls Manual ioctl(2)
ioctl - control device
Standard C library (libc, -lc)
#include <sys/ioctl.h>
int ioctl(int fd, unsigned long op, ...); /* glibc, BSD */
int ioctl(int fd, int op, ...); /* musl, other UNIX */
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.
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.
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.
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).
None.
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 */);
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.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 ioctl(2)
Pages that refer to this page: apropos(1), man(1), pipesz(1), setterm(1), whatis(1), FAT_IOCTL_GET_VOLUME_ID(2const), FAT_IOCTL_SET_ATTRIBUTES(2const), FICLONE(2const), FIDEDUPERANGE(2const), FIONREAD(2const), FS_IOC_SETFLAGS(2const), FS_IOC_SETFSLABEL(2const), getsockopt(2), ioctl_console(2), ioctl_eventpoll(2), ioctl_fat(2), ioctl_fs(2), ioctl_fsmap(2), ioctl_kd(2), ioctl_nsfs(2), ioctl_pipe(2), ioctl_tty(2), ioctl_userfaultfd(2), ioctl_vt(2), ioctl_xfs_ag_geometry(2), ioctl_xfs_bulkstat(2), ioctl_xfs_commit_range(2), ioctl_xfs_exchange_range(2), ioctl_xfs_fsbulkstat(2), ioctl_xfs_fscounts(2), ioctl_xfs_fsgeometry(2), ioctl_xfs_fsgetxattr(2), ioctl_xfs_fsinumbers(2), ioctl_xfs_getbmapx(2), ioctl_xfs_getparents(2), ioctl_xfs_getresblks(2), ioctl_xfs_goingdown(2), ioctl_xfs_inumbers(2), ioctl_xfs_rtgroup_geometry(2), ioctl_xfs_scrub_metadata(2), ioctl_xfs_scrubv_metadata(2), io_uring_enter2(2), io_uring_enter(2), NS_GET_NSTYPE(2const), NS_GET_OWNER_UID(2const), NS_GET_USERNS(2const), open(2), PAGEMAP_SCAN(2const), perf_event_open(2), PR_SET_TAGGED_ADDR_CTRL(2const), read(2), seccomp_unotify(2), socket(2), syscalls(2), TCSBRK(2const), TCSETS(2const), TCXONC(2const), timerfd_create(2), TIOCCONS(2const), TIOCEXCL(2const), TIOCLINUX(2const), TIOCMSET(2const), TIOCPKT(2const), TIOCSCTTY(2const), TIOCSETD(2const), TIOCSLCKTRMIOS(2const), TIOCSPGRP(2const), TIOCSSOFTCAR(2const), TIOCSTI(2const), TIOCSWINSZ(2const), TIOCTTYGSTRUCT(2const), UFFDIO_API(2const), UFFDIO_CONTINUE(2const), UFFDIO_COPY(2const), UFFDIO_MOVE(2const), UFFDIO_POISON(2const), UFFDIO_REGISTER(2const), UFFDIO_UNREGISTER(2const), UFFDIO_WAKE(2const), UFFDIO_WRITEPROTECT(2const), UFFDIO_ZEROPAGE(2const), userfaultfd(2), VFAT_IOCTL_READDIR_BOTH(2const), write(2), errno(3), grantpt(3), if_nameindex(3), if_nametoindex(3), openpty(3), dsp56k(4), fd(4), lirc(4), loop(4), lp(4), random(4), rtc(4), sd(4), smartpqi(4), st(4), tty(4), vcs(4), proc_pid_io(5), arp(7), capabilities(7), inotify(7), landlock(7), namespaces(7), pipe(7), pty(7), signal(7), socket(7), tcp(7), termio(7), udp(7), unix(7), systemd-makefs@.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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