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

       ioctl - control device

       #include <sys/ioctl.h>

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

       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.

       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.

       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.

       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.

       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

       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-

       execve(2),  fcntl(2),  ioctl_console(2),  ioctl_fat(2),   ioctl_ficlon-
       erange(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)

       This  page  is  part of release 5.07 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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