an "Extension Root Directory" or "extroot" as part of the 
[./server/index.html|server setup].
If the Fossil server is itself run as 
[./server/any/cgi.md|CGI], then add a line to the 
[./cgi.wiki#extroot|CGI script file] that says:

<pre>
extroot: <i>DIRECTORY</i>
    extroot: <i>DIRECTORY</i>
</pre>

Or, if the Fossil server is being run using the 
"[./server/any/none.md|fossil server]" or
"[./server/any/none.md|fossil ui]" or 
"[./server/any/inetd.md|fossil http]" commands, then add an extra 
"--extroot <i>DIRECTORY</i>" option to that command.

The <i>DIRECTORY</i> is the DOCUMENT_ROOT for the CGI.
Files in the DOCUMENT_ROOT are accessed via URLs like this:

<pre>
https://example-project.org/ext/<i>FILENAME</i>
    https://example-project.org/ext/<i>FILENAME</i>
</pre>

In other words, access files in DOCUMENT_ROOT by appending the filename
relative to DOCUMENT_ROOT to the [/help?cmd=/ext|/ext]
page of the Fossil server.

Files that are readable but not executable are returned as static
content.  Files that are executable are run as CGI.
  *  Files that are readable but not executable are returned as static
     content.

  *  Files that are executable are run as CGI.

<h3>2.1 Example #1</h3>

The source code repository for SQLite is a Fossil server that is run
as CGI.  The URL for the source code repository is [https://sqlite.org/src].
The CGI script looks like this:

script.  (The extension mechanism is not required to use Wapp.  You can use
any kind of program you like.  But the creator of SQLite and Fossil is fond
of [https://www.tcl.tk|Tcl/Tk] and so he tends to gravitate toward Tcl-based
technologies like Wapp.)  The fileup1 script is a demo program that lets
the user upload a file using a form, and then displays that file in the reply.
There is a link on the page that causes the fileup1 script to return a copy
of its own source-code, so you can see how it works.

<h3>2.3 Example #3</h3>

For Fossil versions dated 2025-03-23 and later, the "--extpage FILENAME"
option to the [/help?cmd=ui|fossil ui] command is a short cut that treats
FILENAME as a CGI extension.  When the ui command starts up a new web browser
pages, it points that page to the FILENAME extension.  So if FILENAME is
a static content file (such as an HTML file or 
[/md_rules|Markdown] or [/wiki_rules|Wiki] document), then the
rendered content of the file is displayed.  Meanwhile, the user can be
editing the source text for that document in a separate window, and
periodically pressing "Reload" on the web browser to instantly view the
rendered results.

For example, the author of this documentation page is running
"<tt>fossil ui --extpage www/serverext.wiki</tt>" while editing this
very paragraph, and presses Reload from time to time to view his
edits.

A same idea applies when developing new CGI applications using a script
language (for example using [https://wapp.tcl.tk|Wapp]).  Run the
command "<tt>fossil ui --extpage SCRIPT</tt>" where SCRIPT is the name
of the application script, while editing that script in a separate
window, then press Reload periodically on the web browser to test the
script.

<h2 id="cgi-inputs">3.0 CGI Inputs</h2>

The /ext extension mechanism is an ordinary CGI interface.  Parameters
are passed to the CGI program using environment variables.  The following
standard CGI environment variables are supported:
standard CGI environment variables are supplied:

  *  AUTH_TYPE
  *  AUTH_CONTENT
  *  CONTENT_LENGTH
  *  CONTENT_TYPE
  *  DOCUMENT_ROOT
  *  GATEWAY_INTERFACE