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+<title>Adding Features To Fossil</title>
+
+<h2>1.0 Introduction</h2>
+
+This article provides a brief overview of how to write new C-code code that
+extends or enhances the core Fossil binary.
+
+New features can be added to a Fossil server using
+[./serverext.wiki|external CGI programs],
+but that is not what this article is about.
+This article focuses on how to make changes
+to Fossil itself.
+
+<h2>2.0 Programming Language</h2>
+
+Fossil is written in C-89.  There are specific [./style.wiki | style guidelines]
+that are required for any new code that will be accepted into the Fossil core.
+But, of course, if you are writing an extension just for yourself, you can
+use any programming style you want.
+
+The source code for Fossil is not sent directly into the C compiler.
+There are three separate code [./makefile.wiki#preprocessing|preprocessors]
+that run over the code first.
+
+  1.  The <b>mkindex</b> preprocessor scans all regular source files looking
+      for special comments that contain "help" text and which identify routines
+      that implement specific commands or which generate particular web pages.
+
+  2.  The <b>makeheaders</b> preprocessor generates all the ".h" files
+      automatically.  Fossil programmers write ".c" files only and let the
+      makeheaders preprocessor create the ".h" files.
+
+  3.  The <b>translate</b> preprocessor converts source code lines that
+      begin with "@" into string literals, or into print statements that
+      generate web page output, depending on context.
+
+The [./makefile.wiki|Makefile] for Fossil takes care of running these
+preprocessors with all the right arguments and in the right order.  So it is
+not necessary to understand the details of how these preprocessors work.
+(Though, the sources for all three preprocessors are included in the source
+tree and are well commented, if you want to dig deeper.)  It is only necessary
+to know that these preprocessors exist and hence will affect the way you
+write code.
+
+<h2>3.0 Adding New Source Code Files</h2>
+
+New source code files are added in the "src/" subdirectory of the Fossil
+source tree.  Suppose one wants to add a new source code file named
+"xyzzy.c".  The first step is to add this file to the various makefiles.
+Do so by editing the file tools/makemake.tcl and adding "xyzzy" (without
+the final ".c") to the list of source modules at the top of that script.
+Save the result and then run the makemake.tcl script using a TCL
+interpreter.  The command to run the makemake.tcl script is:
+
+<verbatim>
+tclsh makemake.tcl
+</verbatim>
+
+The working directory must be src/ when the command above is run.
+Note that TCL is not normally required to build Fossil, but
+it is required for this step.  If you do not have a TCL interpreter on
+your system already, they are easy to install.  A popular choice is the
+[http://www.activestate.com/activetcl|Active Tcl] installation from
+ActiveState.
+
+After the makefiles have been updated, create the xyzzy.c source file
+from the following template:
+
+<verbatim>
+/*
+** Copyright boilerplate goes here.
+*****************************************************
+** High-level description of what this module goes
+** here.
+*/
+#include "config.h"
+#include "xyzzy.h"
+
+#if INTERFACE
+/* Exported object (structure) definitions or #defines
+** go here */
+#endif /* INTERFACE */
+
+/* New code goes here */
+</verbatim>
+
+Note in particular the <b>#include "xyzzy.h"</b> line near the top.
+The "xyzzy.h" file is automatically generated by makeheaders.  Every
+normal Fossil source file must have a #include at the top that imports
+its private header file.  (Some source files, such as "sqlite3.c" are
+exceptions to this rule.  Don't worry about those exceptions.  The
+files you write will require this #include line.)
+
+The "#if INTERFACE ... #endif" section is optional and is only needed
+if there are structure definitions or typedefs or macros that need to
+be used by other source code files.  The makeheaders preprocessor
+uses definitions in the INTERFACE section to help it generate header
+files.  See [../tools/makeheaders.html | makeheaders.html] for additional
+information.
+
+After creating a template file such as shown above, and after updating
+the makefiles, you should be able to recompile Fossil and have it include
+your new source file, even before your source file contains any code.
+It is recommended that you try this.
+
+Be sure to [/help/add|fossil add] your new source file to the self-hosting
+Fossil repository and then [/help/commit|commit] your changes!
+
+<h2 id="newcmd">4.0 Creating A New Command</h2>
+
+By "commands" we mean the keywords that follow "fossil" when invoking
+Fossil from the command-line.  So, for example, in
+
+<verbatim>
+fossil diff xyzzy.c
+</verbatim>
+
+The "command" is "diff".  Commands may optionally be followed by
+arguments and/or options.  To create new commands in Fossil, add code
+(either to an existing source file, or to a new source file created as
+described above) according to the following template:
+
+<verbatim>
+/*
+** COMMAND: xyzzy
+**
+** Help text goes here.  Backslashes must be escaped.
+*/
+void xyzzy_cmd(void){
+  /* Implement the command here */
+  fossil_print("Hello, World!\n");
+}
+</verbatim>
+
+The example above creates a new command named "xyzzy" that prints the
+message "Hello, World!" on the console.  This command is a normal command
+that will show up in the list of command from [/help/help|fossil help].
+If you add an asterisk to the end of the command name, like this:
+
+<verbatim>
+** COMMAND: xyzzy*
+</verbatim>
+
+Then the command will only show up if you add the "--all" option to
+[/help/help|fossil help].  Or, if the command name starts with
+"test" then the command will be considered experimental and will only
+show up when the --test option is used with [/help/help|fossil help].
+
+The example above is a fully functioning Fossil command.  You can add
+the text shown to an existing Fossil source file, recompiling then test
+it out by typing:
+
+<verbatim>
+./fossil xyzzy
+./fossil help xyzzy
+./fossil xyzzy --help
+</verbatim>
+
+The name of the C function that implements the command can be anything
+you like (as long as it does not collide with some other symbol in the
+Fossil code) but it is traditional to name the function
+"<i>commandname</i><b>_cmd</b>", as is done in the example.
+
+You could also use "printf()" instead of "fossil_print()" to generate
+the output text, if desired.  But "fossil_print()" is recommended as
+it has extra logic to insert \r characters at the right times on
+Windows systems.
+
+Once you have the command running, you can then start adding code to
+make it do useful things.  There are lots of utility functions in
+Fossil for parsing command-line options and for
+opening and accessing and manipulating the repository and
+the working check-out.  Study implementations of existing commands
+to get an idea of how things are done.  You can easily find the implementations
+of existing commands by searching for "COMMAND: <i>name</i>" in the
+files of the "src/" directory.
+
+<h2 id="newpage">5.0 Creating A New Web Page</h2>
+
+As with commands, new webpages can be added simply by inserting a function
+that generates the webpage together with a special header comment.  A
+template follows:
+
+<verbatim>
+/*
+** WEBPAGE: helloworld
+*/
+void helloworld_page(void){
+  style_header("Hello World!");
+  @ <p>Hello, World!</p>
+  style_footer();
+}
+</verbatim>
+
+Add the code above to a new or existing Fossil source code file, then
+recompile fossil and run [/help/ui|fossil ui] then enter
+"http://localhost:8080/helloworld" in your web browser and the routine
+above will generate a web page that says "Hello World."
+It really is that simple.
+
+The special "WEBPAGE:" comment is picked up by the "mkindex" preprocessor
+and used to generate a table that maps the "helloworld" webpage name
+into a pointer to the "helloworld_page()" function.  The function that
+implements a webpage can be named anything you like (as long as it does
+not collide with another name) but the traditional name is
+"<i>pagename</i><b>_page</b>".
+
+HTML pages begin with a call to style_header() and end with the call to
+style_footer().  Content is generated by the "@" lines that are translated
+(by the "translate" preprocessor) into printf-like code that generates the
+content of the webpage.  Different techniques are used to generate
+non-HTML content.  In the unlikely event that you need to generate
+non-HTML content, look at existing webpage implementations
+(ex: "logo" or "style.css") to see how that is done.
+
+There are lots of other things that a real web-page implementation will
+need to do, such as verifying user credentials, parsing query parameters,
+and interacting with the repository.  But now that you have the general
+idea of how webpages are implemented, you can look at the many other
+webpage implementations already built into Fossil to see how all that
+works.
+
+<h2>6.0 See Also</h2>
+
+  *  [./makefile.wiki|The Fossil Build Process]
+  *  [./tech_overview.wiki|A Technical Overview Of Fossil]
+  *  [./contribute.wiki|Contributing To The Fossil Project]
+  *  [./serverext.wiki|Adding CGI Extensions To A Fossil Server]