Web51 - Simple http server

TCP part 2  software.html  Filesystem
Basic characteristis of the simple www server found in examples:

Using CGIs in HTML code


Let's take a closer look at the http source code (see http.a51) and explain the processing of a http request.

First, the client request is searched for "GET " (using a state machine controlled by state80table). When found, the state machine enters the port80s0 state, where fileID is filled with the hash value of the requested page. As detailed in filesystem description, three characters have a special meaning when calculating the hash sum. First, every "/" resets the sum to zero (filesystem ignores pathnames). Second and third, " " and "?" stop searching and pass the hash to exeurl, which searches the filesystem.

If a file with the same hash is found, an appropriate http header (according to file type) is sent and mode is switched to cgi or binary. In binary mode, the entire file is sent to the client using httpchar/sendchar.

Cgi mode is a little different. Input file is searched for occurences of "'". If found, another state machine (controlled by http80table) is activated. This one distinguishes double apostrophes ("''") that are converted to single apostrophy and output, and checks for "?" that indicates a "text1{text2}" sequence and causes flagIFcgi to be set. Again, the state machine tries to find a hash of the file name, which must end with a .cgi extension. After the extension, either "'", or "?" followed by a single-byte parameter (passed in state80) is expected. The respective CGI file is found according to its hash and passed to the P-Code interpreter for processing.

execgi: lcall   searchfile ;r4   file type
                           ;dptr file start
                           ;r2r3 file end
        jc      cginotfound
        mov     workreg,dph
        mov     workreg+1,dpl
        lcall   pcode      ;start p-code interpreter
        .pcode  pcall @workreg
        .byte   0          ;stop p-code interpreter
;end of exe cgi

Now, let's show a simple example of processing CGI file testP3 (see the LED1 example).

        .pcode  testP3
        .pcode  pret
testP3: mov     a,P3
        clr     Zflag
        anl     a,state80
        jnz     retP3
        setb    Zflag
retP3:  ret

Expand the CGI procedure testP3 call into its assembly equivalent, as the P-Code interpreter would expand it.

;;      lcall   pcode            ;;start p-code interpreter
;;      .pcode  pcall @workreg   ;;call @workreg
        mov     R6,#high workreg
        mov     R7,#low workreg
        mov     a,R7
        mov     R0,a
        mov     a,@R0
        mov     R6,a
        inc     R0
        mov     a,@R0
        mov     R7,a
        "push	pcall"
        ret                ;jmp pcall
        "push   pcodepointer"
        mov     DPH,R6     ;new Pcode pointer
        mov     DPL,R7

DPTR now contains a new pointer to testP3.cgi (dptr = workreg = searchfile("testP3.cgi")). The P-Code interpreter fetches next command.

;;      lcall   pcode
;;      .pcode  testP3
	"push   testP3"
        ret                ;jmp testP3
testP3: mov     a,P3
        clr     Zflag
        anl     a,state80
        jnz     retP3
        setb    Zflag
retP3:  ret                ;ret to p-code interpreter
;;      lcall   pcode      ;start p-code interpreter
;;      .pcode  pret
        "push	pret"
        ret                ;jmp pret
        "pop    pcodepointer"
;;      lcall   pcode
;;      .byte   0          ;stop p-code interpreter
        ret                ;return to asm

Http server sources are in http.a51. Examples of html pages and CGI usage are in example projects.

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(c)Copyright 2000 - 2002, HW server & Radek Benedikt
Web51@HW.cz, Web51.HW.cz
TCP part 2  Obsah  Filesystem