ioctl - Man Page

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.

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)

apple2(6), apropos.man-db(1), arp(7), beep(1), capabilities(7), collectd.conf(5), cstream(1), curs_inopts.3x(3), drm(7), drm-memory(7), dsp56k(4), errno(3), fd(4), getifaddrs(3), getsockopt(2), grantpt(3), haveged(8), hwstamp_ctl(8), if_nameindex(3), if_nametoindex(3), inotify(7), 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_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_scrub_metadata(2), ioctl_xfs_scrubv_metadata(2), io_uring_enter(2), ksh93(1), landlock(7), lirc(4), loop(4), lp(4), man.man-db(1), mgettydefs(4), namespaces(7), nbdkit-release-notes-1.40(1), ntfsclone(8), open(2), openpty(3), pcap_set_immediate_mode.3pcap(3), perf_event_open(2), perlfunc(1), phosphor(6), pid_namespaces(7), pipe(7), pipesz(1), pmem2_badblock_context_new(3), procenv(1), proc_pid_io(5), pty(7), random(4), read(2), rmt(1), rtc(4), scrub(1), sd(4), seccomp_unotify(2), setterm(1), signal(7), smartpqi(4), sockatmark(3), socket(2), socket(7), sslsplit(1), st(4), stress-ng(1), swipl(1), syscalls(2), systemd-makefs@.service(8), tcgetsid(3), tcp(7), termio(7), tickit_term_build(3), tickit_term_get_size(3), timerfd_create(2), tput(1), tty(4), udp(7), unix(7), userfaultfd(2), user_namespaces(7), vcs(4), whatis.man-db(1), write(2).