LHEATcl Plugin API

 
<embed src=http://weefle.com/myapp.tcl type=application/x-tcl>

 
<embed script='label .l -text "Hello World"; pack .l' type=application/x-tcl>

Loading the DLL creates a Tcl "interpreter" either as a thread or a separate process, depending on your configuration. The interpreter executes several initialization scripts (written in Tcl) and reads configuration parameters. The scripts are located in the user's .netscape/lheatclplug/2.0/* directories for UNIX and in the lheaplug/2.0/* directory for Windows. On the Mac, these scripts are located in the resource fork of the DLL. The configuration parameters are in files in the config/ directory and are used to setup security policies and a few other adjustable settings (on the Mac, these config files are in the extensions folder in the "Tool Command Language" folder).

For a great deal more information about security policies, see the Tcl plugin manual pages, particularily the policy and plugin architecture pages. All of the files dealing with policy and configuration are in the config subdirectory of the plugin's runtime library directory. Most of the files in the config directory are responsible for implementing various named security policy. The code in each of these files specifies what specific features of Tcl are turned on or off for that policy. So when a tclet wants to use a feature such as opening a socket to another computer save files on the browser's machine it will need to ask for a security policy that allows the use of the desired feature. For instance, a tclet wanting to send Javascript commands to the browser would need to include the line: policy javascript If the origin URL of the tclet is allowed to invoke the "javascript" policy, tclet execution will proceed, if not, the process will abort.

The file that actually sets up the behavior of your plugin config/plugin.cfg. Most of this file sets up which origin URLs can use which policies. For instance, by default, the original Tcl plugin allowed the "home" policy for any tclet, whereas the fairly dangerous features of the "javascript" policy are only allowed if the origin URL is located at SUN Microsystems. There is a "lhea" policy which allows several necessary features for our programs. Unfortunately, there is a bug in WASABI (our first major production tclet) that prevents it from running under the "lhea" policy and we haven't had time to figure out how to fix this; therefore, the LHEATcl plugin current installs configured so that any URL matching *.gsfc.nasa.gov for it's domain name is allowed to use the dangerous "trusted" policy.

The last few lines of "plugin.cfg" can be uncommented to set variables which allow some useful plugin features, primarily for debugging tclets:

TCL_PLUGIN_CONSOLE

If this is set to 1, a tk console window will pop up when a tclet loads. Typing Tcl commands into the console will execute them in the unrestricted *master* Tcl interpreter, not the restricted slave interpreter. If you want to send commands to the slave use the interp eval command. The interp slaves command can be used to find the name of the slave interpreter(s).

TCL_PLUGIN_LOGWINDOW

If set to 1, a window containing a fairly detailed log of what the plugin is doing pops up when a given tclet loads

TCL_PLUGIN_LOGFILE

Set this to the name of a file if you want to capture the log output from the plugin.

TCL_PLUGIN_WISH

Setting this to 1 causes the plugin to launch as a separate process instead of as a thread of your browser's process. On most platforms, this doesn't make much difference, but setting TCL_PLUGIN_WISH to 1 under Solaris is currently necessary because recent Solaris versions of Netscape were built under Solaris2.5 which had broken signal handling in threaded processes.

File IO in the Plugin

File IO in the LHEATcl plugin is somewhat complicated. By default, the plugin's security features disable writing files to the local machine entirely. However, fitsTcl's file writing does not implement any security features. The trusted policy allows unrestricted file writing on the local machine. The lhea policy allows the more limited persistent file writing. FITS file writing through fitsTcl doesn't work under Windows for the current version of the plugin. We will attempt to summarize the various possibilities here:

1. Writing non-FITS files can be done with the standard Tcl commands (open, puts, etc.). If you use a 'policy trusted' statement, the files will appear in the current working directory of the browser process.
2. If you use 'policy lhea' (or any policy which allows the "persist" feature) the files will appear in /tmp/username/persist/lhea/dir0. In Windows: '/tmp/username' will instead be the Windows temp directory; frequently this is C:/WINDOWS/temp. When using the persist feature, the total ammount of disk space used by persistent files is limited. See Tcl plugin documentation for details.
3. You cannot read files from the browser's computer unless they are present in the persistent file area or you are using the 'trusted' policy.
4. Using fitsTcl calls, you can read and write FITS files anywhere that the user is allowed to read or write files on the client machine. The default location is the current working directory of the browser process. There is no size limit.
5. You can use the Tcl built-in http package or standard extension packages that provide ftp, etc. (as long as they're written in straight Tcl) to fetch files from URLs.
6. For FITS files, you can pass a URL rather than a filename to the fitsTcl 'fits open' command and it will fetch the file using cfitsio's internal net drivers. Note: cfitsio net drivers currently only work on UNIX.

Known Problems and Considerations

There are a few known quirks in the plugin or its build that need to be allowed for.

On most Unix machines, if you resize the browser window containing a tclet, the browser will crash and core dump. This is a problem with the original Tcl plugin. We prevent this for WASABI by launching the tclet in it's own, non-resizable, browser window. See the example below for details on how to do this.

It would be safer, not so much to prevent malicious intent as to guard against catastrophic bugs, if we used the 'lhea' security policy rather than the 'trusted' security policy for WASABI. We are using some forbidden feature which we have not had the time to track down. "Your mileage may vary", so try to use the 'lhea' policy if you can. This does impose some strict limits on your code, the most obvious is probably the lack of the 'menu' widget. See Tcl plugin documents for details.

As mentioned above, if you want to use cfitsio net drivers, you are limited to Unix and for Solaris you must set TCL_PLUGIN_WISH equal to 1 in the plugin.cfg file. This is done by default in our prebuilt copies of the plugin.

If you need to rebuild and/or extend the LHEATcl plugin yourself, you are welcome to do so (read the SUN license agreement for full details on the legal status of the plugin, the terms are pretty generous). You may want to contact us to coordinate efforts if you're planning extensive improvements (send email to wasabi@olegacy.gsfc.nasa.gov). The UNIX build is fairly straightforward, we use proprietary compilers where possible since that is how Netscape is usually built, but gcc should work fine. The Windows build is fairly torturous at present and requires both Borland C++ and Microsoft Visual C++ and is not for the faint of heart, or even weak of stomach, I suspect. The MacIntosh build uses CodeWarrior.

Examples

While it's not strictly a LHEATcl question, it is handy to know how to launch a tclet in a new console window using Javascript. Here's the code to make a button that launches WASABI (a LHEATcl applet) on a the test file algol.fits:

<FORM>
<INPUT TYPE="button" VALUE="Launch WASABI" onClick='goober()'>
<SCRIPT LANGUAGE="JavaScript">
function goober() {
var newWindow
var winwidth
var content

if (screen.availWidth > 850 ) {
    winwidth = "850"
} else {	
    winwidth = "800"
}

newWindow = window.open("","WASABI","scrollbars,menubar,height=" +
screen.availHeight + ",width=" + winwidth)

content="<embed src=http://olegacy.gsfc.nasa.gov/wasabi/WASABI.tcl "+
"type=application/x-tcl height=2000 width=" + winwidth + 
" url=http://heasarc.gsfc.nasa.gov/wasabi/algol.fits graphname=Algol>"
newWindow.document.write(content)
newWindow.document.close()
}
</SCRIPT> 
</FORM>