IOCTL(2)                  NetBSD System Calls Manual                  IOCTL(2)

     ioctl -- control device

     Standard C Library (libc, -lc)

     #include <sys/ioctl.h>

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

     The ioctl() function manipulates the underlying device parameters of spe-
     cial files.  In particular, many operating characteristics of character
     special files (e.g. terminals) may be controlled with ioctl() requests.
     The argument d must be an open file descriptor.

     An ioctl() request has encoded in it whether the argument is an ``in'',
     ``out'', or ``inout'' parameter, and the size of the first variadic argu-
     ment in bytes.  Note that there can be only one variadic argument but
     cannot be represented as a void * argument in the prototype because this
     would require a cast to pass integral types without warnings.  Macros and
     defines used in specifying an ioctl() request are located in the header

     Some ioctls are applicable to any file descriptor.  These include:

             Set close-on-exec flag.  The file will be closed when exec(3) is
             invoked (This is equivalent to fcntl() F_SETFD FD_CLOEXEC and the
             fcntl() form should be preferred).

             Clear close-on-exec flag.  The file will remain open across
             exec(3) (This is equivalent to fcntl() F_SETFD 0 and the fcntl()
             form should be preferred).

     Some generic ioctls are not implemented for all types of file descrip-
     tors.  These include:

     FIONREAD int
             Get the number of bytes that are immediately available for read-

     FIONWRITE int
             Get the number of bytes in the descriptor's send queue.  These
             bytes are data which has been written to the descriptor but which
             are being held by the kernel for further processing.  The nature
             of the required processing depends on the underlying device.  For
             tty devices, these bytes are typically queued for delivery to the
             tty hardware.  For TCP sockets, these bytes have not yet been
             acknowledged by the other side of the connection.  For files,
             this operation always returns zero as files do not have send

     FIONSPACE int
             Get the free space in the descriptor's send queue.  This value is
             the size of the send queue minus the number of bytes being held
             in the queue.  Note: while this value represents the number of
             bytes that may be added to the queue, other resource limitations
             may cause a write not larger than the send queue's space to be
             blocked.  One such limitation would be a lack of network buffers
             for a write to a network connection.

     FIONBIO int
             Set non-blocking I/O mode if the argument is non-zero.  In non-
             blocking mode, read(2) or write(2) calls return -1 and set errno
             to EAGAIN immediately when no data is available (This is equiva-
             lent to fcntl() F_SETFL O_NONBLOCK and the fcntl() form should be

     FIOASYNC int
             Set asynchronous I/O mode if the argument is non-zero (This is
             equivalent to fcntl() F_SETFL O_ASYNC and the fcntl() form should
             be preferred).  In asynchronous mode, the process or process
             group specified by FIOSETOWN will start receiving SIGIO signals
             when data is available.  The SIGIO signal will be delivered when
             data is available on the file descriptor.

             Set/get the process or the process group (if negative) that
             should receive SIGIO signals when data is available (This is
             equivalent to fcntl() F_SETOWN pid_t and the fcntl form should be

     If an error has occurred, a value of -1 is returned and errno is set to
     indicate the error.

     ioctl() will fail if:

     [EBADF]            d is not a valid descriptor.

     [EFAULT]           argp points outside the process's allocated address

     [EINVAL]           request or argp is not valid.

     [ENOTTY]           d is not associated with a character special device;
                        or the specified request does not apply to the kind of
                        object that the descriptor d references.

     mt(1), execve(2), fcntl(2), intro(4), tty(4)

     An ioctl() function call appeared in Version 7 AT&T UNIX.

NetBSD 8.1                     December 19, 2010                    NetBSD 8.1

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