ioctl(2) — Linux manual page


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 appropriately.

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

       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 5.08 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

Linux                            2020-04-11                         IOCTL(2)

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