IOCTL(2)                  (2020-04-11)                   IOCTL(2)

     NAME
          ioctl - control device

     SYNOPSIS
          #include <sys/ioctl.h>

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

     DESCRIPTION
          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 tradi-
          tionally 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 argu-
          ment argp in bytes.  Macros and defines used in specifying
          an ioctl() request are located in the file <sys/ioctl.h>.
          See NOTES.

     RETURN VALUE
          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
          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

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          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
          In order to use this call, one needs an open file descrip-
          tor.  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 = aqTaq indicating the terminal driver, and CYGETTIMEOUT
          has value 0x00435906, with 0x43 0x59 = aqCaq aqYaq indicating
          the cyclades driver.

          Later (0.98p5) some more information was built into the num-
          ber.  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 disadvan-
          tages: it does not help in checking, but it causes varying
          values for the various architectures.

     SEE ALSO
          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),

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          sd(4), tty(4)

     COLOPHON
          This page is part of release 5.10 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/.

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