[Richard Suchenwirth] 2007-05-04 - [MJ] contributed this nice example of how to extend Tcl with a new subcommand to the [chan] ensemble (new from 8.5). First, write a self-contained function that implements what you want:
 /*
  *----------------------------------------------------------------------
  *
  * Tcl_ModeObjCmd --
  *
  *----------------------------------------------------------------------
 */

	/* ARGSUSED */
 int
 Tcl_ModeObjCmd(
    ClientData dummy,		/* Not used. */
    Tcl_Interp *interp,		/* Current interpreter. */
    int objc,			/* Number of arguments. */
    Tcl_Obj *const objv[])	/* Argument objects. */
 {
    Tcl_Channel chan;		/* The channel to puts on. */
    const char *channelId; /* Name of channel for puts. */
    int mode;			/* Mode in which channel is opened. */
	    
    if (objc != 2) {
        Tcl_WrongNumArgs(interp, 1, objv, "channelId");
        return TCL_ERROR;
    }

    channelId = Tcl_GetString(objv[1]);

    chan = Tcl_GetChannel(interp, channelId, &mode);
    if (chan == (Tcl_Channel) NULL) {
        return TCL_ERROR;
    }

    if ((mode & TCL_READABLE) != 0) {
        Tcl_AppendElement(interp, "r");
    }

    if ((mode & TCL_WRITABLE) != 0) {
        Tcl_AppendElement(interp, "w");
    }

    return TCL_OK;

 }
Then, make it available as a command to Tcl:
	    Tcl_CreateObjCommand(interp, "::tcl::chan::mode",
			    Tcl_ModeObjCmd, NULL, NULL);

Finally, in [init.tcl], register that new command as ensemble member of [chan]:
 # Set up the 'chan' ensemble (TIP #208).
    namespace eval chan {
        # TIP #219. Added methods: create, postevent.
	# TIP 287.  Added method: pending.
        namespace ensemble create -command ::chan -map {
            blocked     ::tcl::chan::blocked
            close       ::tcl::chan::close
            configure   ::tcl::chan::configure
            copy        ::tcl::chan::copy
            create      ::tcl::chan::rCreate
            eof         ::tcl::chan::eof
            event       ::tcl::chan::event
            flush       ::tcl::chan::flush
            gets        ::tcl::chan::gets
            mode        {::tcl::chan::mode ;############### here}
            names       {::file channels}
	    pending	::tcl::chan::Pending
            postevent   ::tcl::chan::rPostevent
            puts        ::tcl::chan::puts
            read        ::tcl::chan::read
            seek        ::tcl::chan::seek
            tell        ::tcl::chan::tell
            truncate    ::tcl::chan::Truncate
        }
    }

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[Category Example]