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     ioctl — control device


     Standard C Library (libc, -lc)


     #include <sys/ioctl.h>

     ioctl(int d, 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 d 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>.


     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

     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.


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


     The ioctl() system call will fail if:

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

     [ENOTTY]           The d argument 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]           The request or argp argument is not valid.

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


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


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