CGI: Tutorial 01: Supplying Dynamic Data to a Web Client

In the early '90s the NCSA released HTTPd 1.0 (a web server), a new concept was included; CGI. This feature allowed web content to be dynamically generated on the server. Up-to-date reports of stocks, scores, and weather were possible with CGI. Other uses include message boards, guest books, or e-stores.

Typically a CGI application will interface with a Mosaic type web browser; supplying HTML with the data. When the server recieves a request targeting a CGI program, it will lauch the application. Any data from the client will be piped to StdIn. The app's StdOut will then be sent back to the client.

Tools Needed

This tutorial is written for FAsm ({http://omega.im.uj.edu.pl/~grysztar/}). If you wish to assemble the program, you will need FAsm 1.13.4 (or later) or you can translate it to an assembler supporting 80x86 PE console.

For any CGI testing access to a web server is a must. I recommend Apache 1.3.20 ({http://httpd.apache.org/}). For starting out, you can place your assembled executable into the \Apache\cgi-bin\ directory. For the server name use "localhost" (excluding the quotes).

Knowledge of HTML (HyperText Markup Language) is usefull. The basics of HTML are easy to learn. CSS (Cascading Style Sheets) will prove invaluable if you use a lot of HTML. A list of books is provided at the end of this article.

A Win32 platform. My system consist of Win 98 SE on a Celeron 433 w/ 128MB RAM. Win 95 - NT should work without issues. A Linux box running WINE shoud also work for those with a strong stomach.

Win32 API

To access the Standard I/O, will need to use GetStdHandle. Under Win32, StdIO is not availiable under predefined handles. ReadFile and WriteFile is used to move data. ReadConsole and WriteConsole will not work; file redirection in not availiable.

CGI Environment

Content-Type: text/plain

Hello World. Example of HTTP 1.1 header and body.

If you don't write any data, the web server will report with the error: "Premature end of script headers". If you really don't want to supply data, you could just write: "Content-Type: text/plain" and two newlines.

The Example Program

~~~~~~~~~~~~~~~~~~~|||-------------------[code]-------------------||| format PE console
entry Start

include '\Asm_Win32\Include\Kernel.inc' include '\AsmWin32\Include\macro\stdcall.inc' include '\Asm_Win32\Include\macro\import.inc'

Cr = 0x0D
Lf = 0x0A
;***---------------------------------------------------------------*** section '.code' code readable executable Start:

pusha ;Save all of the Registers stdcall [GetStdHandle], STD_OUTPUT_HANDLE ;Retrive the actual handle

    mov     [StdOut], eax
cmp     eax, INVALID_HANDLE_VALUE            ;Error with handle
jz      Exit

Get Time

stdcall [GetSystemTime], Time ;Load SYSTEMTIME with UTC call Format_Time ;Convert Hex(bin) to ascii

; and Place into HTML

Write

stdcall [WriteFile], [StdOut], HTML, HTML._size, HTML.Len, 0

;Write the HTML to StdOut

Exit

popa ;Restore all of the Registers stdcall [ExitProcess], 0

;***-------------------------[Subroutine]--------------------------*** Format_Time:

    mov     ax, [Time.wYear]                     ;16b Data
mov     edi, HTML.Date_S + 9                 ;Ptr to LAST byte of dest
call    .ascii                               ;Convert and place into HTML
mov     ax, [Time.wDay]
mov     edi, HTML.Date_S + 4

call .ascii

    mov     ax, [Time.wMonth]
mov     edi, HTML.Date_S + 1

call .ascii

    mov     edi, HTML.Day_S                      ;Destination Ptr
mov     esi, Day.Wk                          ;Source Ptr (Array of Days)
xor     eax, eax
mov     ax, [Time.wDayOfWeek]                ;0 <= eax < 7
add     esi, eax                             ;esi =+ eax * 3
add     esi, eax                             ; Indexes the Array
add     esi, eax
mov     ecx, 3                               ;3B per Day String
cld                                          ;Copy Left to Right
rep                                          ;    (esi++, edi++)

movsb

    mov     ax, [Time.wHour]
cmp     al, 13                               ;Check for PM
jl      .wHour
sub     al, 12                               ;Correct Hour
mov     [HTML.Time_S + 9], 'P'               ; AM -> PM

.wHour:

mov edi, HTML.Time_S + 1
call .ascii

    mov     ax, [Time.wMinute]
mov     edi, HTML.Time_S + 4

call .ascii

    mov     ax, [Time.wSecond]
mov     edi, HTML.Time_S + 7

call .ascii
ret

;***----------------------[Import Table / IAT]---------------------*** .ascii:

    std                                          ;String OPs Right to Left
cmp     ax, 10                               ;Single Digit?
jl      .onex10
and     ah, ah                               ;Only Two Digits
jz      .twox16
mov     bh, 10                               ;Reduce 3x16 to 2x16
div     bh                                   ;  so that AAM can be used
or      ah, 0x30                             ;BCD -> ASCII
mov     [edi], ah
dec     edi
.twox16:
aam                                          ; AH / 10 = AH r AL
or      al, 0x30                             ;BCD -> ASCII
stosb
mov     al, ah
cmp     ah, 9
jg      .twox16
.onex10:
or      al, 0x30
stosb                                        ;Copy Last/Only Digit to Mem

ret

;***--------------------[Data used by this App]--------------------*** section '.data' data readable writeable

  StdIn         dd 0                             ;Standard I/O Handles
StdOut        dd 0

HTML

db 'Content-type: text/html', Cr, Lf, Cr, Lf db '<html><head><title>Hello World</title></head>', Cr, Lf db '<body bgcolor=Black text=Cyan><h1>Hello World</h1>', Cr, Lf db '<h2><font color=Lime>', Cr, Lf db 'This HTML is dynamicly generated by a PE console Application writen in' db '80x86 Assembler</font></h2>', Cr, Lf db '<h2><font color=Red>It is: </font><font color=Blue>'

.Day_S db 'WkD ' .Date_S db ' 0/00/0000</font> <font color=Magenta>' .Time_S db ' 0:00:00 AM</font> <font color=Lime>UTC</font></h2>', Cr, Lf

db '</body></html>', Cr, Lf HTML._size = $ - HTML - 1 HTML.Len dd 0 ;Number of bytes actually wrote

Time SYSTEMTIME Day.Wk db 'SunMonTueWedThuFriSat'

;***----------------------[Import Table / IAT]---------------------*** section '.idata' import data readable writeable

library kernel, 'KERNEL32.DLL'

kernel

import GetModuleHandle, 'GetModuleHandleA',\

GetCommandLine, 'GetCommandLineA',\ GetSystemTime, 'GetSystemTime',\ GetEnvVar, 'GetEnvironmentVariableA',\ GetStdHandle, 'GetStdHandle',\ CreateFile, 'CreateFileA',\ ReadFile, 'ReadFile',\ WriteFile, 'WriteFile',\ CloseHandle, 'CloseHandle',\ ExitProcess, 'ExitProcess' __________________|||-------------------[/code]-------------------|||

How to Run

Main > Text.html
For the real CGI, place Main.exe into the cgi-bin directory, launch Apache, and type "localhost/cgi-bin/Main.exe" in the address box of IE.

References

SAMS Teach Yourself CGI in 24 Hours

            SAMS 2000                                   $24.99US
Rafe Colburn                                ISBN: 0-672-31880-6

CGI by Example

            QUE 1996                                    $34.99US
Robert Niles & Jeffry Dwight                ISBN: 0-7897-0877-9

HTML in Plain English - 2nd Edition

            MIS Press 1998                              $19.95US
Sandra E. Eddy                              ISBN: 1-55828-587-3

Cascading Sytle Sheets - The Definitive Guide

            O'Reilly 2000                               $34.95US
Eric A. Meyer                               ISBN: 1-56592-622-6

Win32 Programming Reference (Win32 API Help file)

            Microsoft 1990-1995                         Free
{http://win32asm.rxsp.com/files/win32api.zip}

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