The code here demonstrates the use of INDI, an Instrument-Neutral Device
Interface protocol. See http://www.clearskyinstitute.com/INDI/INDI.pdf.

Architecture:

    Typical INDI Client / Server / Driver / Device connectivity:


    INDI Client 1 ----|                  |---- INDI Driver A  ---- Dev X
                      |                  |
    INDI Client 2 ----|                  |---- INDI Driver B  ---- Dev Y
                      |                  |                     |
     ...              |--- indiserver ---|                     |-- Dev Z
                      |                  |
                      |                  |
    INDI Client n ----|                  |---- INDI Driver C  ---- Dev T


     Client       INET       Server       UNIX     Driver          Hardware
     processes    sockets    process      pipes    processes       devices



    Indiserver is the public network access point where one or more INDI Clients
    may contact one or more INDI Drivers. Indiserver launches each driver
    process and arranges for it to receive the INDI protocol from Clients on
    its stdin and expects to find commands destined for Clients on the
    driver's stdout. Anything arriving from a driver process' stderr is copied
    to indiserver's stderr.

    Indiserver only provides convenient port, fork and data steering services.
    If desired, a Client may run and connect to INDI Drivers directly.

Construction:

    An INDI driver typically consists of one .c file, eg, mydriver.c, which
    #includes indiapi.h to access the reference API declarations. It is
    compiled then linked with indidrivermain.o, eventloop.o and liblilxml.a to
    form an INDI process. These supporting files contain the implementation of
    the INDI Driver API and need not be changed in any way. Note that
    evenloop.[ch] provide a nice callback facility independent of INDI which
    may be used in other projects if desired.
    
    The driver implementation, again in our example mydriver.c, does not
    contain a main() but is expected to operate as an event-driver program.
    The driver must implement each ISxxx() function but never call them. The
    IS() functions are called by the reference implementation main() as messages
    arrive from Clients. Within each IS function the driver performs the
    desired tasks then may report back to the Client by calling the IDxxx()
    functions.

    The reference API provides IE() functions to allow the driver to add its
    own callback functions if desired. The driver can arrange for functions to
    be called when reading a file descriptor will not block; when a time
    interval has expired; or when there is no other client traffic in progress.

    The sample indiserver is a stand alone process that may be used to run one
    or more INDI-compliant drivers. It takes the names of each driver process
    to run in its command line args.

    To build indiserver type 'make indiserver';
    to build all the sample drivers type 'make drivers';
    to run the sample server with all drivers type 'make run'.
    Killing indiserver will also kill all the drivers it started.

Secure remote operation:

    Suppose we want to run indiserver and its clients on a remote machine, r,
    and connect them to our favorite INDI client, XEphem, running on the
    local machine.

    From the local machine log onto the remote machine, r, by typing:

	ssh2 -L 7624:s:7624 r

    after logging in, run indiserver on the remote machine:
    
	make run
	
    Back on the local machine, start XEphem, then open Views -> Sky View ->
    Telescope -> INDI panel. XEphem will connect to the remote INDI server
    securely and automatically begin running. Sweet.

Testing:

    A low-level way to test the socket, forking and data steering abilities of
    indiserver is to use the 'hose' command from the netpipes collection
    (http://web.purplefrog.com/~thoth/netpipes/netpipes.html):

    1. start indiserver using UNIX' cat program as the only INDI "device":

	% indiserver cat &

    2. use hose to connect to the "cat" device driver which just copies back:

	% hose localhost 7624 --slave
	hello world
	hello world
	more stuff
	more stuff

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