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
       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

       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.

       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

       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).

       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 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

       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)

       This page is part of release 5.13 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                              2021-03-22                           IOCTL(2)