plucky (2) ioctl.2freebsd.gz

NAME

     ioctl — control device

LIBRARY

     Standard C Library (libc, -lc)

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 third argument to ioctl() is traditionally named char *argp.  Most uses of ioctl(), however, require
     the third argument to be a caddr_t or an int.

     An ioctl() request has encoded in it whether the argument is an “in” argument or “out” argument, 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>.

GENERIC IOCTLS

     Some generic ioctls are not implemented for all types of file descriptors.  These include:

     FIONREAD int
             Get the number of bytes that are immediately available for reading.

     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 TCP sockets, these bytes
             have not yet been acknowledged by the other side of the connection.

     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.

RETURN VALUES

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

ERRORS

     The ioctl() system call will fail if:

     [EBADF]            The fd argument is not a valid descriptor.

     [ENOTTY]           The fd argument is not associated with a character special device.

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

     [EINVAL]           The request or argp argument is not valid.

     [EFAULT]           The argp argument points outside the process's allocated address space.

SEE ALSO

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

HISTORY

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