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ioctl(2)                      System Calls Manual                     ioctl(2)

NAME
       ioctl - control device

LIBRARY
       Standard C library (libc, -lc)

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  char-
       acter  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  ar-
       gument  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 pa-
       rameter 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
       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
       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.

STANDARDS
       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 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 con-
       stants 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 in-
       dicating 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  ar-
       chitectures.

SEE ALSO
       execve(2),  fcntl(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), open(2), sd(4), tty(4)

Linux man-pages 6.03              2023-02-05                          ioctl(2)

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