A Web-Server in Forth

Bernd Paysan

Abstract:

An HTTP-Server in Gforth is presented as an opportunity to show that you can do string-oriented things with Forth as well. The development time (a few hours) shows that Forth is an appropriate tool for this kind of work and delivers fast results.

This is a translation of the paper presented at the Forth Tagung 2000 conference in Hamburg (proofreading and corrections by Chris Jakeman).

1. Introduction

Since I have always given bigFORTH/MINOS-related presentations in the last few years, I'll do something with Gforth this time. Gforth is another tool you can do neat things with, and in contrast to what you here elsewhere, Forth is suitable for almost anything. Even a web-server.

In this age of the ``new economy'', the Internet is important. Everybody is ``in there'' except Forth, which hides in the embedded control niche. There isn't any serious reason for that. The following code was created in just a few hours of work and mostly operates on strings. The old prejudice, that Forth was good at biting bits, but has troubles with strings, is thus disproved.

1.1 Motivation

What do you need a web-server for in Forth? Forth is used for measurement and control in remote locations such as the sea-bed or the crater of a volcano. Less remotely, Forth may be used in a refrigerator and, if that stops working, things soon get messy. So a communication thingy is built in.

How much better would it be if instead of "some communication thingy built in", there was a standard protocol. HTTP is accessible from the web-cafe in Mallorca, or from mobile yuppie toys such as PDAs or cell phones. Perhaps one should build such a web-server into each stove and into the bath, so that people can use their cell phone on holidays to check repeatedly (every three minutes?) if they really turned their stove off.

Anyway, the customer, boss or whoever buys the product, wants to hear that there is some Internet-thingy build in, especially if one isn't in e-Business already. And the costs must be zero too.

But let's take this slowly, step by step.

2. A Web Server, Step by Step

Actually, you had to study the RFC1-documents. The RFCs in question are RFC 945 (HTTP/1.0) and RFC 2068 (HTTP/1.1), which both refer to other RFCs. Since these documents alone are much longer than the source code presented below (and reading them would take longer than writing the sources), we will defer that for later. The web server thus won't be 100% RFC conforming (i.e. implement all features), and conforms only as far as necessary for a typical client like Netscape. However additions are easy to achieve.

A typical HTTP-Request looks like this:

GET /index.html HTTP/1.1 
Host: www.paysan.nom 
Connection: close 
 
(Note the empty line at the end). And the response is

HTTP/1.1 200 OK 
Date: Tue, 11 Apr 2000 22:27:42 GMT 
Server: Apache/1.3.12 (Unix) (SuSE/Linux) 
Connection: close 
Content-Type: text/html 
  
<HTML> 
...
This looks quite trivial, so let's start. The web server should run under Unix/Linux. That takes one problem out of our hands - how we get to our socket - since that's what inetd, the Internet daemon, does for us. We only need to tell it on which port our web server expects data, and enter that into the file /etc/inetd.conf:

# Gforth web server 
gforth stream tcp nowait.10000   wwwrun   /usr/users/bernd/bin/httpd
We won't replace the default web server just yet (something might not work straight away), so we shall need a new port and that one goes into the file /etc/services:

gforth  4444/tcp # Gforth web server
When we do a restart or a  killall -HUP inetd  inetd will realize the changes and start our web server for all requests on port 4444. What we need next is an executable program. Gforth supports scripting with #!, as common for scripting languages in Unix. In the line below, the blank is significant:

#! /usr/local/bin/gforth 
  
warnings off
We had better disable any warnings. Let's load a small string library (see attachment):

include string.fs
We shall need a few variables for the URL requested from the server, the arguments, posted arguments, protocol and states.

Variable url       \ stores the URL (string) 
Variable posted    \ stores arguments of POST (string) 
Variable url-args  \ stores arguments in the URL (string) 
Variable protocol  \ stores the protocol (string) 
Variable data      \ true, when data is returned 
Variable active    \ true for POST 
Variable command?  \ true in the request line
A request consist of two parts, the request line and the header. Spaces are separators. The first word in a line is a ``token'' indicating the protocol, the rest of the line, or one/two words are parameters.

Since we can process a request only once the whole header has been parsed, we save all the information. Therefore we define two small words which take a word representing the rest of a line and store it in a string variable:

: get ( addr - )  name rot $! ; 
: get-rest ( addr - ) 
  source >in @ /string dup >in +! rot $! ;
As told above, we have header values and request commands. To interpret them, we define two wordlists:

wordlist constant values 
wordlist constant commands
But before we can really start, the URL might contain spaces and other special characters, what to do with them? HTTP advises to transmit these special characters in the form %xx, where xx are two hex digits. We thus must replace these characters in the finished URL:

\ HTTP URL rework 
  
: rework-% ( add - ) { url }  base @ >r hex 
  0 url $@len 0 ?DO 
      url $@ drop I + c@ dup '% = IF 
          drop 0. url $@ I 1+ /string 
          2 min dup >r >number r> swap - >r 2drop 
      ELSE  0 >r  THEN  over url $@ drop + c!  1+ 
  r> 1+ +LOOP  url $!len 
  r> base ! ;
So, that's done. But stop! URLs consist of two parts: path and the optional arguments. Separator is `?'. So first split the string into two parts:

: rework-? ( addr - ) 
    dup >r $@ '? $split url-args $! nip r> $!len ;
So we've defined the basics and can start. Each requests fetches a URL and the protocol, splits the URL into path and arguments and replaces the special character glyphs by the real characters (but those in the arguments remain as we don't yet know what should happen to them). Finally, we must switch over to another vocabulary, since the header follows after the request.

: >values  values 1 set-order command? off ; 
: get-url ( - ) url get protocol get-rest 
  url rework-? url rework-% >values ;
So now we can define the commands. According to the RFC, we only need GET and HEAD, POST is then a bonus.

commands set-current 
  
: GET   get-url data on  active off ; 
: POST  get-url data on  active on  ; 
: HEAD  get-url data off active off ;
And now for the header values. Since we need a string variable for each value, and otherwise want only to store the string, we build that with CREATE-DOES>. Again: we need a variable and a word, which stores the rest of the line there. In two different vocabularies. The latter with a colon behind.

Fortunately, Gforth provides nextname, an appropriate tool for this. We construct exactly the string we need and call VARIABLE and CREATE afterwards.

: value:  ( - )  name 
  definitions 2dup 1- nextname Variable 
  values set-current nextname here cell - Create , 
  definitions  DOES> @ get-rest ;
And now we set to work and define all the necessary variables:

value: User-Agent: 
value: Pragma: 
value: Host: 
value: Accept: 
value: Accept-Encoding: 
value: Accept-Language: 
value: Accept-Charset: 
value: Via: 
value: X-Forwarded-For: 
value: Cache-Control: 
value: Connection: 
value: Referer: 
value: Content-Type: 
value: Content-Length:
There are some more (see RFC), but these are all we need for the moment.

3. Parsing a Request

Now we must parse the request. This should be completely trivial, we could just let the Forth interpreter chew it but for one little caveat:

  1. Each line ends with CR LF, while Gforth under Unix expects lines to end with an LF only. We thus must remove the CR. And
  2. each header ends with an empty line, not some executable Forth word. We must therefore read line by line with refill, remove CRs from the line end, and then check if the line was empty.
Variable maxnum 
  
: ?cr ( - ) 
  #tib @ 1 >= IF  source 1- + c@ #cr = #tib +!  THEN ; 
: refill-loop ( - flag ) 
  BEGIN  refill ?cr  WHILE  interpret  >in @ 0=  UNTIL 
  true  ELSE  maxnum off false  THEN ;
So, the key things are done now. Since we can't let the Forth interpreter loose on the raw input stream stdin, we pre-process the stream ourselves. We initialize a few variables which we need to interpret anyway, and steal some code from INCLUDED:

: get-input ( - flag ior ) 
  s" /nosuchfile" url $!  s" HTTP/1.0" protocol $! 
  s" close" connection $! 
  infile-id push-file loadfile !  loadline off  blk off 
  commands 1 set-order  command? on  ['] refill-loop catch
Waiiiit! The request isn't complete yet. The method POST, which was added as bonus, expects the data now. The length fortunately is stored as base 10 number in the field ``Content-Length:''.

  active @ IF  s" " posted $! Content-Length $@ snumber? drop 
      posted $!len  posted $@ infile-id read-file throw drop 
  THEN  only forth also  pop-file ;

4. Answer a Request

OK, we've handled a request, and now we must respond. The path of the URL is unfortunately not as we want it; we want to be somehow Apache-compatible, i.e. we have a ``global document root'' and a variable in the home directory of each user, where he can put his personal home page. Thus we can't do anything else but look at the URL again and finally check, if the requested file really is available:

Variable htmldir 
 
: rework-htmldir ( addr u - addr' u' / ior ) 
  htmldir $! 
  htmldir $@ 1 min s" " compare 0= 
  IF    s" /.html-data" htmldir dup $@ 2dup '/ scan 
        nip - nip $ins 
  ELSE  s" /usr/local/httpd/htdocs/" htmldir 0 $ins  THEN 
  htmldir $@ 1- 0 max + c@ '/ = htmldir $@len 0= or 
  IF  s" index.html" htmldir dup $@len $ins  THEN 
  htmldir $@ file-status nip ?dup ?EXIT 
  htmldir $@ ;
Next, we must decide how the client should render the file - i.e. which MIME type it has. The file suffix is all we need to decide, so we extract it next.

: >mime ( addr u - mime u' )  2dup tuck over + 1- ?DO 
  I c@ '. = ?LEAVE  1-  -1 +LOOP  /string ;
Normally, we'd transfer the file as is to the client (transparent). Then you tell the client how long the file is (otherwise, we'd have to close the connection after each request). We open a file, find its size and report that to the client.

: >file ( addr u - size fd ) 
  r/o bin open-file throw >r 
  r@ file-size throw drop 
  ." Accept-Ranges: bytes" cr 
  ." Content-Length: " dup 0 .r cr r> ; 
: transparent ( size fd - ) { fd } 
  $4000 allocate throw swap dup 0 ?DO 
      2dup over swap $4000 min fd read-file throw type 
      $4000 - $4000 +LOOP  drop 
  free fd close-file throw throw ;
We do all the work with transparent, using TYPE to send the file in chunks to support ``keep-alive'' connections, which modern web browsers prefer. The creation of a new connection is significantly more ``expensive'' than to continue with an established one. We benefit on our side also, since starting Gforth again isn't for free either. If the connection is keep-alive, we return that, reduce maxnum by one, and report to the client how often he may issue further requests. When it's the last request, or no further are pending, we send that back, too.

: .connection ( - ) 
  ." Connection: " 
  connection $@ s" Keep-Alive" compare 0= maxnum @ 0> and 
  IF  connection $@ type cr 
      ." Keep-Alive: timeout=15, max=" maxnum @ 0 .r cr 
      -1 maxnum +!  ELSE  ." close" cr maxnum off  THEN ;
Now we just need some means to recognise MIME file suffixes and sned the appropriate transmissions. For the response, we must also first send a header. We build it from back to front here, since the top definitions add their stuff ahead. To make the association between file suffixes and MIME types easy, we simply define one word per suffix. That gets the MIME type as string. transparent: does all that for all the file types that are handled using transparent:

: transparent: ( addr u - ) Create  here over 1+ allot place 
  DOES>  >r  >file 
  .connection 
  ." Content-Type: "  r> count type cr cr 
  data @ IF  transparent  ELSE  nip close-file throw  THEN ;
There are hundreds of MIME types, but who wants to enter all of them? Nothing could be easier than this, we steal the MIME types that are already known to the system, say from /etc/mime.types. The file lists the mime type on the left paired with the file suffixes on the right (sometimes none).

: mime-read ( addr u - )  r/o open-file throw 
  push-file loadfile !  0 loadline ! blk off 
  BEGIN  refill  WHILE  name 
      BEGIN  >in @ >r name nip  WHILE 
          r> >in ! 2dup transparent:  REPEAT 
      2drop rdrop 
  REPEAT  loadfile @ close-file pop-file throw ;
One more thing we need: for active content we want to use server side scripting (in Forth, of course). Since we don't know the size of these requests in advance, we don't report it but close the connection instead. That relieves us of the problem of cleaning up the trash the user is creating with his active content (that's Forth code!).

: lastrequest 
  ." Connection: close" cr maxnum off 
  ." Content-Type: text/html" cr cr ;
So let's start with the definition of MIME types. Get a new wordlist. Active content ends with shtml and is included. We provide a few special types and the rest we get from the system file mentioned above. For unknown file types, we need a default type, text/plain.

wordlist constant mime 
mime set-current 
 
: shtml ( addr u - )  lastrequest 
  data @ IF  included  ELSE  2drop  THEN ; 
 
s" application/pgp-signature" transparent: sig 
s" application/x-bzip2" transparent: bz2 
s" application/x-gzip" transparent: gz 
s" /etc/mime.types" mime-read 
 
definitions 
 
s" text/plain" transparent: txt

5. Error Reports

Sometimes a request goes wrong. We must be prepared for that and respond with an appropriate error message to the client. The client wants to know which protocol we speak, what happened (or if everything is OK), who we are, and in the error case, a error report in plain text (coded in HTML) would be nice:

: .server ( - )  ." Server: Gforth httpd/0.1 (" 
  s" os-class" environment? IF  type  THEN  ." )" cr ; 
: .ok  ( - ) ." HTTP/1.1 200 OK" cr .server ; 
: html-error ( n addr u - ) 
  ." HTTP/1.1 " 2 pick . 2dup type cr .server 
  2 pick &405 = IF ." Allow: GET, HEAD, POST" cr  THEN 
  lastrequest 
  ." <HTML><HEAD><TITLE>" 2 pick . 2dup type 
  ." </TITLE></HEAD>" cr 
  ." <BODY><H1>" type drop ." </H1>" cr ; 
: .trailer ( - ) 
  ." <HR><ADDRESS>Gforth httpd 0.1</ADDRESS>" cr 
  ." </BODY></HTML>" cr ; 
: .nok ( - ) command? @ IF  &405 s" Method Not Allowed" 
  ELSE  &400 s" Bad Request"  THEN  html-error 
  ." <P>Your browser sent a request that this server " 
  ." could not understand.</P>" cr 
  ." <P>Invalid request in: <CODE>" 
  error-stack cell+ 2@ swap type 
  ." </CODE></P>" cr .trailer ; 
: .nofile ( - ) &404 s" Not Found" html-error 
  ." <P>The requested URL <CODE>" url $@ type 
  ." </CODE> was not found on this server</P>" cr .trailer ;

6. Top Level Definitions

We are almost done now. We simply glue together all the pieces above to process a request in sequence - first fetch the input, then transform the URL, recognize the MIME type, work on it including error exits and default paths. We need to flush the output, so that the next request doesn't stall. And do that all over again \( n \) times, until we reach the last request.

: http ( - )  get-input  IF  .nok  ELSE 
  IF  url $@ 1 /string rework-htmldir 
      dup 0< IF  drop .nofile 
      ELSE  .ok  2dup >mime mime search-wordlist 
          0= IF  ['] txt  THEN  catch IF  maxnum off THEN 
  THEN  THEN  THEN  outfile-id flush-file throw ; 
 
: httpd  ( n - )  maxnum ! 
  BEGIN  ['] http catch  maxnum @ 0= or  UNTIL ;
To make Gforth run that at the start, we patch the boot message and then save the result as a new system image.

script? [IF]  :noname &100 httpd bye ; is bootmessage  [THEN] 

7. Scripting

As a special bonus, we can process active content. That's really simple: We just write our HTML file as usual and indicate the Forth code with ``<$'' and ``$> '' (the space for the closing parenthesis is certainly intentional!). Let's define two words, $>, and to get the whole thing started, <HTML>:

: $> ( - ) 
  BEGIN  source >in @ /string s" <$" search  0= WHILE 
      type cr refill  0= UNTIL  EXIT  THEN 
  nip source >in @ /string rot - dup 2 + >in +! type ; 
: <HTML> ( - )  ." <HTML>" $> ;
That's quite enough, we don't need more. The rest is all done by Forth, as in the following example:

<HTML> 
<HEAD> 
<TITLE>GForth <$ version-string type $>  presents</TITLE> 
</HEAD> 
<BODY> 
<H1>Computing Primes</H1><$ 25 Constant #prim $> 
<P>The first <$ #prim . $> primes are: <$ 
: prim?  0 over 2 max 2 ?DO  over I mod 0= or  LOOP  nip 0= ; 
: prims ( n - )  0 swap  2 
  swap 0 DO  dup prim? IF  swap  IF  ." , "  THEN  true swap 
  dup 0 .r 1+  1  ELSE  1+  0  THEN 
  +LOOP  drop ; 
#prim prims $> .</P> 
</BODY> 
</HTML>

8. Outlook

That was a few hundred lines of code - far too much. I have delivered an ``almost'' complete Apache clone. That won't be necessary for the sea-bed or the refrigerator. Error handling is ballast, too. And if you restrict to single connection (performance isn't the goal), you can ignore all the protocol variables. One MIME type (text/html) is sufficient -- we keep the images on another server. There is some hope that one can get a working HTTP protocol with server-side scripting in one screen.

Appendix: String Functions

Certainly we need some string functions, it doesn't work without. The following string library stores strings in ordinary variables, which then contain a pointer to a counted string stored allocated from the heap. Instead of a count byte, there's a whole count cell, sufficient for all normal use. The string library originates from bigFORTH and I've ported it to Gforth (ANS Forth). But now we consider the details of the functions. First we need two words bigFORTH already provides:

: delete   ( addr u n - ) 
  over min >r  r@ - ( left over )  dup 0> 
  IF  2dup swap dup  r@ +  -rot swap move  THEN  + r> bl fill ;
delete deletes the first \( n \) bytes from a buffer and fills the rest at the end with blanks.

: insert   ( string length buffer size - ) 
  rot over min >r  r@ - ( left over ) 
  over dup r@ +  rot move   r> move  ;
insert inserts as string at the front of a buffer. The remaining bytes are moved on.

Now we can really start:

: $padding ( n - n' ) 
  [ 6 cells ] Literal + [ -4 cells ] Literal and ;
To avoid exhausting our memory management, there are only certain string sizes; $padding takes care of rounding up to multiplies of four cells.

: $! ( addr1 u addr2 - ) 
  dup @ IF  dup @ free throw  THEN 
  over $padding allocate throw over ! @ 
  over >r  rot over cell+  r> move 2dup ! + cell+ bl swap c! ;
$! stores a string at an address, If there was a string there already, that string will be lost.

: $@ ( addr1 - addr2 u )  @ dup cell+ swap @ ;
$@ returns the stored string.

: $@len ( addr - u )  @ @ ;
$@len returns just the length of a string.

: $!len ( u addr - ) 
  over $padding over @ swap resize throw over ! @ ! ;
$!len changes the length of a string. Therefore we must change the memory area and adjust address and count cell as well.

: $del ( addr off u - )   >r >r dup $@ r> /string r@ delete 
  dup $@len r> - swap $!len ;
$del deletes \( u \) bytes from a string with offset \( off \).

: $ins ( addr1 u addr2 off - ) >r 
  2dup dup $@len rot + swap $!len  $@ 1+ r> /string insert ;
$ins inserts a string at offset \( off \).

: $+! ( addr1 u addr2 - ) dup $@len $ins ;
$+! appends a string to another.

: $off ( addr - )  dup @ free throw off ;
$off releases a string.

As a bonus, there are functions to split strings up.

: $split ( addr u char - addr1 u1 addr2 u2 ) 
  >r 2dup r> scan dup >r dup IF  1 /string  THEN 
  2swap r> - 2swap ;
$split divides a string into two, with one char as separator (e.g. '? for arguments)

: $iter ( .. $addr char xt - .. ) { char xt } 
  $@ BEGIN  dup  WHILE  char $split >r >r xt execute r> r> 
     REPEAT  2drop ;
$iter takes a string apart piece for piece, also with a character as separator. For each part a passed token will be called. With this you can take apart arguments -- separated with '& -- at ease.

Footnotes

... RFC1
Request For Comments -- Internet standards documents are all named like this.

Bernd Paysan
2000-07-22