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 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

       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

       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 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

       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

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       https://www.kernel.org/doc/man-pages/.