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, void *argp);

     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''
     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>.

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

             Set close-on-exec flag.  The file will be closed when exec(3) is

             Clear close-on-exec flag.  The file will remain open across

     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
             acknolwedged 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.

     FIOASYNC int
             Set asynchronous I/O mode if the argument is non-zero.  In asyn-
             chronous mode, the process or process group specified by
             FIOSETOWN will start receiving SIGIO signals when data is avail-
             able.  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.

     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.

     [ENOTTY]           d is not associated with a character special device.

     [ENOTTY]           The specified request does not apply to the kind of
                        object that the descriptor d references.

     [EINVAL]           request or argp is not valid.

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

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

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

NetBSD 5.1                     November 6, 2004                     NetBSD 5.1

You can also request any man page by name and (optionally) by section:


Use the DEFAULT collection to view manual pages for third-party software.

©1994 Man-cgi 1.15, Panagiotis Christias <>
©1996-2015 Modified for NetBSD by Kimmo Suominen