ioctl(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

IOCTL(2)                Linux Programmer's Manual               IOCTL(2)

NAME         top

       ioctl - control device

SYNOPSIS         top

       #include <sys/ioctl.h>

       int ioctl(int fd, unsigned long request, ...);

DESCRIPTION         top

       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.

RETURN VALUE         top

       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 to
       indicate the error.

ERRORS         top

       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.

CONFORMING TO         top

       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.

NOTES         top

       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.

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                       IOCTL(2)

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