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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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IOCTL(2) Linux Programmer's Manual IOCTL(2)
ioctl - control device
#include <sys/ioctl.h>
int ioctl(int fd, unsigned long request, ...);
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.
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 appropriately.
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.
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.
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.
execve(2), fcntl(2), ioctl_console(2), ioctl_fat(2),
ioctl_ficlonerange(2), ioctl_fideduperange(2), ioctl_fslabel(2),
ioctl_getfsmap(2), ioctl_iflags(2), ioctl_ns(2), ioctl_tty(2),
ioctl_userfaultfd(2), open(2), sd(4), tty(4)
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-04-11 IOCTL(2)
Pages that refer to this page: apropos(1), man(1), whatis(1), creat(2), getsockopt(2), ioctl_console(2), ioctl_fat(2), ioctl_ficlone(2), ioctl_ficlonerange(2), ioctl_fideduperange(2), ioctl_fslabel(2), ioctl_getfsmap(2), ioctl_iflags(2), ioctl_ns(2), ioctl_tty(2), ioctl_userfaultfd(2), ioctl_xfs_ag_geometry(2), ioctl_xfs_bulkstat(2), ioctl_xfs_fsbulkstat(2), ioctl_xfs_fscounts(2), ioctl_xfs_fsgetxattr(2), ioctl_xfs_fsgetxattra(2), ioctl_xfs_fsinumbers(2), ioctl_xfs_fsop_geometry(2), ioctl_xfs_fssetxattr(2), ioctl_xfs_getbmap(2), ioctl_xfs_getbmapa(2), ioctl_xfs_getbmapx(2), ioctl_xfs_getresblks(2), ioctl_xfs_goingdown(2), ioctl_xfs_inumbers(2), ioctl_xfs_scrub_metadata(2), ioctl_xfs_setresblks(2), open(2), openat(2), perf_event_open(2), read(2), setsockopt(2), socket(2), syscalls(2), timerfd_create(2), timerfd_gettime(2), timerfd_settime(2), userfaultfd(2), write(2), errno(3), forkpty(3), if_freenameindex(3), if_indextoname(3), if_nameindex(3), if_nametoindex(3), login_tty(3), openpty(3), pcap_set_immediate_mode(3pcap), console_ioctl(4), dsp56k(4), fd(4), loop(4), loop-control(4), lp(4), random(4), rtc(4), sd(4), smartpqi(4), st(4), tty(4), tty_ioctl(4), urandom(4), vcs(4), vcsa(4), arp(7), capabilities(7), namespaces(7), pipe(7), pty(7), signal(7), socket(7), tcp(7), termio(7), udp(7), unix(7), systemd-growfs(8), systemd-growfs.service(8), [email protected](8), systemd-makefs(8), systemd-makefs.service(8), [email protected](8), systemd-mkswap.service(8), [email protected](8)
Copyright and license for this manual page
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