The manual contains detailed documetation on the netrik code. It should be of interest to you if you want to hack on netrik, but also if you are only curious how it works.
The manual includes:
The program (except for the layout engine) presently doesn't obey any specific structure. It can be roughly divided into a copule of big modules: layout engine, file loading, and pager; the page handling, which is another part of the loading mechanism, may also be considered a module of its own. However, these modules aren't seperated clearly; they intersect at many places. The loading is tightly coupled to the layouting -- the page handling part invokes certain layouting passes, while the file loader offers it's service to feed the data into the layout engine. However, these two parts of the loading mechanism need to exchange much information (URLs, error conditions etc.), which can only be passed through the layout engine. The pager naturally has to make use of the layout engine to get the screen content; but it also uses some of it's data structures and helper functions for link handling. The main programm basically makes calls to both the pager and the page handling (which in turn invokes the layouting), but also has to use data structures and functions directly; actually, it contains part of the page handlig. Finally, the loading module affects and depends on some data used by the pager.
A problem arises from the fact that all control passing is done by direct function calls. This was perfectly OK at the time when netrik wasn't much more than a layouting engine; with all the link and anchor handling, various loading mechanisms (including error handling, user breaks etc.), form handling, and other functionality, this has gotten quite chaotic -- it's very hard to retrace the various intermingled calls between the modules or the data dependencies.
This probably could be helped somewhat by using a kind of object-oriented approach, i.e. trying to put all operations on a given data structure in one place. (Encapsulation.) However, I don't think it would be possible to make great progress without much hassle and considerable efficiency loss.
I've devised a much nicer, clearer solution instead (well, at least it looks nice in my imagination ;-) ), which involves a fundamental change in program structure. Basically it's about passing flow control to a main dispatcher, using state handles and an event queue.
We will probably implement that concept in one of the next major releases. It is inevitable for multitasking (displaying of partially loaded pages, multiwindowing) anyways, as we want to stick to explicit multitasking (real multithreading/multiprocessing is much more complicated, and terribly inefficient), and only this concept allows resuming and a fine grained scheduling. But again, it should also simplify and clearify the overall program structure, so we will probably implement it even before it's absolutely necessary.
As mentioned before, the layout engine, contrary to the other modules, has a fairly clear concept from the beginning. The main idea was to split HTML processing into a couple of independant passes. As the individual passes are quite simple, this makes understanding and altering the code quite easy.
On the other hand, this isn't terribly efficient the way it is done now. We think however that in the early stage netrik is now, it's more important to keep the code as simple as possible to faciliate fast developement.
Also, splitting up the processing is necessary to allow rendering of not completely loaded pages. As this is to be one of the major features of netrik (it's not implemented yet), we pay much attention to that.
However, the splitting into passes isn't presently optimal by any means. It's more or less the first which came to mind. In the new parser planned for the next major release the segmentation will be somewhat different.
Most of the processing steps of the layouting are working recursively due to the nature of SGML/XML. They arn't implemented recursively, though. This makes understanding a tick harder, but there is a couple of reasons for that. Efficiency is one of them, but not that important presently. The main reason is that a real recursive implementation wouldn't work with the planned multitasking system (s.a.) as it doesn't allow resuming.
Here is an overview how the different modules work together. Detailed descripitons of the modules are located in the various hacking-... files.
The main program is responsible for two things: Program initialization, and the main (pager) loop.
Initialization presently mostly comprises reading command line and config file options.
This is somewhat tricky, but not too complicated when you get the idea: The config file (~/.netrikrc) just contains options identical to the command line options, one per line. They are simply prepended to the command line options, and processed with getopt_long() together.
For that, netrik first reads the file. This is done by determining the file name (using the HOME environment variable), then using stat() to get the file size; this also tells us whether the file exists at all. Then it is opened and read into a memory buffer, all at once. The next step is the intersting one: The file contents is scanned, to extract the single options. Every encounterd newline is replaced by a '\0' (string end), and a pointer to the beginning of that line (option) is stored in the "argv_all" array . "argv_all" and "argc_all" are identical to "argv" and "argc", only we also store the config file options here. argv_all[0] is not filled, as argv[0] is always the command that started the programm; the real arguments start with 1. After extracting all config file options, the real command line options from "argv" are appended to "argv_all". This way, defaults can be set in the config file, but still overridden by command line options.
Having this, the arguments are scanned with getopt_long() in config_cmdln(), where options are extracted and stored in the global "cfg" struct, and non-option arguments singled out. (They are moved to the last argv-entries by getopt_long().) The last argument is used as the resource name for the document to load on startup; the other ones are ignored. (This allows setting a "home page" in the config file, which will be overwritten if some URL is specified on the command line.)
After scannig the arguments, the color map is initialized to the desired color scheme. (See Color Schemes in hacking-terminal.* for details.)
Now the first document is loaded and, unless netrik was invoked with the "--dump" option, the pager loop is entered.
At the beginning of each iteration, display() is called to start the pager. This one performs all pure viewing operations, including scrolling and link selection; it returns only when some page handling and/or leaving fullscreen mode is required. main() then performs the desired action (indicated by the return value of display()), before restarting the pager in the next iteration.
Most of those actions involve loading some new page -- actually, the main loop can be considered part of the Page Loading mechanism.
The new page (as well es the startup page, before entering the loop) is always loaded by calling load_page() in one mode or the other. The mode varies depending on the exact command: Following a link (or a form submit button), opening a new URL on the command prompt (s.b.), history commands. The precise actions are described in detail in hacking-load.*.
There are also situations that do not require loading a new page; in this case, the viewer is simply restarted with the same page in the next iteration.
For one, this includes handling of non-submit form items. This is described under Manipulating in hacking-links.*. Note that all form elements, including submit buttons, and also normal links are handled by one pager return value; they are distinguished by the type of the active link item, which is handled in another switch by main().
The second group of commands that do not involve loading a new page are the URL displaying commands ('u', 'U', 'c'), which just print some information on the screen before returning to the pager. Besides of that, they are also related to the page handling, and thus described in hacking-page.*.
The command prompt is somewhat special also, as it doesn't cause loading some new page immediately. readline() is used to get a command line from the user instead, and the action taken depends on that command. However, presently only the ":e" and ":E" commands are implemented, which also load an new page; they are described in hacking-page.* also.
Finally, if the pager returned because the terminal size changed (SIGWINCH received), main() calls resize() (described in hacking-layout.*) to prepare the page for the new screen width, and then restarts the pager as usual. This is detailed under a href=hacking-terminal.in#winch">SIGWINCH Handling
in hacking-terminal.*.A somewhat unobvious aspect is switching between curses fullscreen mode and normal scrolling mode. The viewer always operates in fullscreen mode, while everything in main() is done in scrolling mode.
Curses fullscreen mode is initialized once, just before entering the main loop. The program doesn't switch to fullscreen mode immediately, however; this is done only on the first refresh, which is induced by the getch() inside display() -- after rendering the screen content.
Fullscreen mode is turned off using endwin() before display() returns, and will be reactivated only when getch() is called again from display() in the next iteration.
Some commands need to display some information to the user before returning to the pager. Particularily the URL show commands, but also loading if some error occurs or if in --debug mode. This is accomplised by the "pager_wait" flag, which is set by any command that needs to display something. If the flag was set during an iteration, main() waits for a keypress before proceeding with the next one -- which will reactivate the pager.
Quitting netrik is also handled in the main loop: When the user typed 'q' in the pager, display() returns a special value, and the loop terminates. A few cleanup actions follow, whereafter main() returns.
The loading module consists of two parts: One is responsible for actually loading an HTML document from a local file or from a HTTP server. It consists of the files load.[ch], http.[ch], http-parse-header.[ch], the url handling functions in url.[ch], and interrupt.[ch] for user break handling. Some functions from forms.c are necessary also, when submitting HTML forms. This part is invoked from layout(), as part of the Layouting process.
The other part of the loading mechanism is the Page Loading system.
The main file of the file loader is load.c, which contains the init_load() and load() functions.
The loading is intialized by a call to init_load() with the desired URL as argument. A base URL is also passed, which is merged with the target URL to create the effective URL, if the target URL is a relative one. (Following links etc.) init_load() then decides whether it is a local file or an HTTP URL, and initializes the loading. (Opens file or establishes HTTP connection.)
The loading itself is done by calling load(). This function fills a buffer, which then can be processed. After ther buffer is processed, load() has to be called again, loading the next chunk.
The loading module is described in detail in hacking-load.*.
The second part of the loading mechanism is the page handling system. This one is responsible for getting a new document upon request, and preparing it so it can be displayed in the Viewer. It's also responsible for history handling, and everything else connected with handling the Page List (see hacking-page.*).
The page loading mechanism only has the page.c and page.h files for itself; however, part of it is also located in main().
The main function is load_page(), with the basic functionality of loading a new document and preparing it so it can (and will) be displayed in the Viewer, as soon as display() (see hacking-pager.*) is called from main(). For this, a new page list entry is created, and layout() (see hacking-layout.*) is used to load the page and prepare it for rendering.
load_page() also has other modes, depending on what kind of page is to be displayed: If the new page uses the same HTML document, and the page only has to jump to an anchor, the document isn't reloaded; a new page descriptor is created, but the layout date is taken from the old page.
If a page from the page history is to be reloaded, no new page descriptor is created; the old one is simply reactivated. The document is reloaded using layout(), if necessary.
load_page() is called from main() to load the start page given on the command line, and then in various modes from the main loop, when the user requests various functions in the Viewer (loading new URL, following link, going back/forward in page history).
The page loading mechanism is described in detail in hacking-page.*.
The layout engine consists of the files parse-syntax.c, syntax.h, facilities.c, dump-tree.c parse_elements.c, sgml.c, parse-struct.c, items.c, items.h, pre-render.c, render.c, render.h, layout.c, and layout.h.
As mentioned before, layouting is done in several passes.
The first passes are parsing syntax (parse_syntax()), looking up the element and attribute names (parse_elements()), (optionally) fixing a broken tree created by SGML documents (sgml_rework(), interpreting the elements (parse_struct()), and assigning positions and sizes to all items of the output page (pre_render()).
make_link_list() and make_anchor_list() are also called after parse_struct(), creating the link list and anchor list data structures necessary for link handling. (See hacking-links.*.)
All these passes are applied from layout(), immediately after the file is opened; load() is called from within parse_syntax().
After these processing steps, the page is ready for rendering. The actual rendering is done just in time by render(), which is called from the viewer, each time some new region of the output page needs to be displayed.
The layout engine is described in detail in hacking-layout.*.
The viewer module consists of the files pager.c and pager.h.
The pager uses curses to display the layouted page in an interactive manner.
Every time the visible output page region changes, render() is called to display the new region.
The viewer module is described in detail in hacking-pager.*.
Hyperlink (and anchor) handling is not a module in a classical sense; there is no central place, no source files specific for that. There are the links.c and links.h files, but they contain only some helper functions; most of the code necessary for handling links is distributed among almost all of the other modules.
The layout engine needs to extract all links and anchors while parsing the page, and assign coordinates to them while pre-rendering. The pager needs to highlight the selected link; provide commands for selecting and following links; and inform the main program about the link following. The main program needs to initiate loading of the link. The file loader needs to construct the target URL from the current page URL and the link URL.
All necessary steps are described in hacking-links.*, or else pointers to the specific module documentation are given.
HTML forms are very similar to links, and mostly they are handled together. Some special handling is required at certain places, though. These are mentioned in hacking-links.*. Some additional functions necessary for form handling reside in forms.c and form-file.c; these are also covered in hacking-links.*.
The search command presently doesn't reside in a file of its own; search.c only defines the global "search" struct, which is used to pass information between the search handling inside the pager and in main(), as well as keep information between multiple searches. Searching itself is performed inside main().
The search framework is very similar to the command prompt (see main.c). When typing the search character, display() quits with a special status. main() than requests a search string (via readline) and performs the search, resulting in a new cursor position; display() is then restarted in the next main loop iteration, and knows to jump to the match postion by the state stored in "search.type".
Details on how the search itself works can be found in hacking-search.*.
netrik uses the curses library for all terminal handling.
There are two distinct modes in which netrik operates: Full screen mode (where all output is managed by curses), and scrolling mode, where curses (more exactly, the terminfo handling functions of curses) is used to get some control strings (color setting etc.) and for setting text attributes like bold, but output is printed to the screen directly.
Other terminal related issues like keyboard input and signal handling are also done using curses.
Most input/output tasks (reading a single character from keyboard, printing a text string etc.) are trivial, and the curses functions are called directly from the functions needing them (mostly in pager.c and render.c). There are however a number of more complicated tasks like initialising the terminal or setting colors, which can't be done in a single function call. Those are handled by helper functions, which are located in screen.c (documented in hacking-terminal.*).
Another issue related to output (though only indirectly to terminal handling) is handling of text colors inside netrik. This is also described in hacking-terminal.*, under Color Handling.
Last but not least, handling of SIGWINCH is an important issue of terminal handling -- reacting to terminal size changes. Handling the signals itself is controlled by the helper functions in winch.c, called from pager.c. The real resizing action necessary to adopt to the new screen size is handled by main.c, which in turn uses the resize() function from pre-render.c. (Described in hacking-pager.*.)
SIGWINCH handling is described in more detail under SIGWINCH Handling, again in hacking-terminal.*.