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       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 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>.


       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 model).  See ioctl_list(2) for a list of many of the known
       ioctl() calls.  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.


       execve(2),    fcntl(2),     ioctl_console(2),     ioctl_fat(2),     ioctl_ficlonerange(2),
       ioctl_fideduperange(2),  ioctl_getfsmap(2),  ioctl_iflags(2),  ioctl_list(2), ioctl_ns(2),
       ioctl_tty(2), ioctl_userfaultfd(2), open(2), sd(4), tty(4)


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