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|
CGI Programming
BR's crystallographic computing tutorials 
By Bernhard Rupp
CGI scripting
There is a little CGI script that decodes a
space group. Just enter a valid space goup symbol or a
spacegroup number (1-230) and voila! here we go:
Enter a spacegroup or spacegroup
number and hit enter
Cool, isn't it? So how does it work?
Introduction
The web pages you actually see are the client side of a client-server
connection through the internet. If your browser (the client)
wants to see a web page, for example, it sends a request to the host
you specify in the URL (Uniform Resource Locator) using the
specified protocol (HTTP in this case, Hyper Text Transfer
Protocol). Example for a URL : http://www.ruppweb.org/default.htm
(protocol://site.domain/document). The site name is nothing but a
name assigned to the IP address of the machine (example 128.115.150.112)
running the (web) server. (NOTE : in this
context the server is a program
running the web service on the machine, not
the computer itself - the computer is the host).
When the server receives such a request, it responds and sends
back a page, or whatever the request was for. The language in
which the server and client communicate is HTML (Hyper Text
Markup Language). Beyond static requests for web pages, one may
want the server to execute a certain task (such as searching a
data base or decode a space group). There must be a mechanism for
the server to receive information about what to do and to hand it
down to the application that handles the request. This interface
is the CGI (Common Gateway Interface). The
server essentially sets certain system environment variables,
which the called program can read and process. The output is
passed back to the server forwarding it through the internet to
the requesting browser which displays the result. You may want to
check out the Microsoft Internet Development CD or the other
references for more information on the subject.
Example
In this web page I used following script to create the input
box and define the action to be taken :
<form action="/cgi-bin/sexie32.exe" method="GET">
<p>Enter a spacegroup or spacegroup number
<input type="text" size="11" name="spcgrp">
and hit enter</p>
</form>
This minimal code creates the above input box, assigns the
input to the variable named 'spcgrp' and executes the program
sexie32 which does the job. How do I get the space group variable
into the program? By specifying GET as method, the server adds
the variable name and its contents after a question mark to the
program name and stores this string in an environment variable
QUERY_STRING. All the program does is read this environment
variable string, decode it, and after the calculation, write the
result as an HTML file to the standard output (console) which the
server redirects to the requester. Most scripting languages
provide libraries and one does not have to deal with the details
of reading the system environment.
FORTRAN
As scientists, we may have a useful program in FORTAN that
does a nice job and it would be desirable to make it available on
the web. The obvious benefit is that you do not have to deal with
platform and implementation issues - no unvalidated version
proliferation, no source code mess etc - the application runs on
your host computer, and the browsers take care of the platform
specific implementation. All of a sudden your old F66 code has a
graphical windows interface!
Subroutine WEBSPC reads the environment variables and returns
the spacegroup string to the main routine :
subroutine webspc (spcgrp,title,igr,ierr) WEBS0001
c ----------------------------------------------------------------------WEBS0002
c returns space group passed from web server through GET method WEBS0003
c ----------------------------------------------------------------------WEBS0004
USE MSFLIB WEBS0005
integer*4 lstr WEBS0006
integer igr(36) WEBS0007
character today*24,spcgrp*10,title*60,usradd*15,fname*60,qstr*80 WEBS0008
character usrnam*80,browsr*80 WEBS0009
WEBS0010
c --- Set spacegroup blank WEBS0011
spcgrp=' ' WEBS0012
ierr=0 WEBS0013
Here is how to read the environment variables which were set
by the server according to the action instructions in the HTML
code fragment above. You'll find a listing of other variables
that are set in most books on web programming (some of them are
read by webspc as well). The key is QUERY_STRING which contains
the variable part of the URL constructed by the action listed
above:
http://ruppweb.dyndns.org/cgi-bin/sexie32.exe?spcgrp=I+4%2Fm+m+m
QUERY_STRING contains: spcgrp=I+4%2Fm+m+m
(NOTE: if you enter the URL above or click it, the CGI script
will actually be executed). Notice also that the CGI interface
substitutes blanks with + signs and special characters with their
hexadecimal code preceeded by a % sign (e.g., the slash is %2F).
The actual input string in the box was 'I 4/m m m' in this case.
WEBS0014
c --- get the client name and the query_string from environment WEBS0015
ladd = GETENVQQ('REMOTE_ADDR', usradd) WEBS0016
lnam = GETENVQQ('REMOTE_USR', usrnam) WEBS0017
lbrs = GETENVQQ('HTTP_USER_AGENT', browsr) WEBS0018
lstr = GETENVQQ('QUERY_STRING', qstr) WEBS0019
WEBS0020
c --- read space group - if an integer decode by number WEBS0021
read(qstr,'(7x,i3)',err=0001) ispa WEBS0022
c --- if we get here, it was read successfully as a number WEBS0023
Note that I use the error label when reading the symbol as a
integer to distinguish whether a space group symbol or a space
group number was entered.
call spsymc (spcgrp,ispa) WEBS0024
c --- ispa returns the symbol now proceed with checks WEBS0025
goto 2002 WEBS0026
WEBS0027
c --- parse the input string if not a number WEBS0028
0001 continue WEBS0029
ilen=lstr-7 WEBS0030
j=0 WEBS0031
c --- begin loop over query_string WEBS0032
Notice that the CGI interface substitutes blanks with + signs
and special characters with hexadecimal code preceeded by a %
sign (e.g., / is %2F), so we need to parse that string:
i=8 WEBS0033
1000 continue WEBS0034
c --- replace + with blank WEBS0035
if (qstr(i:i).eq.'+') then WEBS0036
j=j+1 WEBS0037
spcgrp(j:j)=' ' WEBS0038
i=i+1 WEBS0039
c --- replace special character hex code with character WEBS0040
else if (qstr(i:i).eq.'%') then WEBS0041
c --- %2F is slash WEBS0042
if (qstr(i+1:i+2).eq.'2F') then WEBS0043
j=j+1 WEBS0044
spcgrp(j:j)='/' WEBS0045
i=i+3 WEBS0046
end if WEBS0047
else WEBS0048
j=j+1 WEBS0049
spcgrp(j:j)=qstr(i:i) WEBS0050
i=i+1 WEBS0051
end if WEBS0052
c --- repeat until finished with string WEBS0053
if (i.lt.(ilen+8)) goto 1000 WEBS0054
WEBS0055
c --- now check if a valid space group symbol WEBS0056
2002 call upstrg(spcgrp,10) WEBS0057
read(spcgrp,'(10a1)') (igr(i), i=1,10) WEBS0058
ispa=0 WEBS0059
call spsymc (spcgrp,ispa) WEBS0060
WEBS0061
c --- get a date string, format is :Wed Nov 20 15:33:29 1996 WEBS0062
call fdate (today) WEBS0063
title='Webjob on '//today(1:10)//today(20:24)//' at'// WEBS0064
& today(11:19) //' from '//usradd WEBS0065
The program also keeps a log of the users.
c --- log the job WEBS0066
fname='C:\inetsrv\wwwroot\webjobs.html' WEBS0067
open (66,file=fname,access='append',position='append') WEBS0068
c --- overwrite old </body> tag WEBS0069
backspace (66) WEBS0070
write(66,'(a,a,a)')' <p>Date : '//today//'<br>' WEBS0071
write(66,'(a,a,a)')' Client : '//usradd(1:ladd)//'<br>' WEBS0072
write(66,'(a,a,a)')' Browser : '//browsr(1:lbrs)//'<br>' WEBS0073
write(66,'(a,a,a)')' Spacegr: '//spcgrp//'<br>' WEBS0074
if (ispa.lt.0) then WEBS0075
write(66,'(a)')' Status : failed!</p>' WEBS0076
else WEBS0077
write(66,'(a)')' Status : ok</p>' WEBS0078
end if WEBS0079
write(66,'(a)') ' <hr>' WEBS0080
write(66,'(a)') ' </BODY>' WEBS0081
close(66) WEBS0082
c --- if something went wrong, return message and termination signal WEBS0083
if (ispa.lt.0) then WEBS0084
call webtxt (1,spcgrp) WEBS0085
ierr=1 WEBS0086
close(5) WEBS0087
return WEBS0088
else WEBS0089
close(5) WEBS0090
return WEBS0091
end if WEBS0092
return WEBS0093
end WEBS0094
The variable spcgrp gets returned to the main routine which
decodes the space group. The output is written to a file, and all
that is needed is a tiny routine to write this output as a HTML
file to the standard output (console).
subroutine htmake HTMA0001
HTMA0002
character*80 line HTMA0003
HTMA0004
open(6,file='sexie.out') HTMA0005
write(*,*)'Content-type: text/html' HTMA0006
write(*,*) HTMA0007
write(*,*)'<BODY bgcolor="#FFFDE6" TEXT="#0000FF">' HTMA0008
write(*,*)'<TITLE>Space group decoding</TITLE>' HTMA0009
write(*,*)'<pre>' HTMA0010
do i=1,100000 HTMA0011
read(6,'(1x,a)',end=9999) line HTMA0012
write(*,'(1x,a)') line HTMA0013
end do HTMA0014
9999 close(6) HTMA0015
open(6,file='xyz.txt') HTMA0016
write(*,'(//a//)') HTMA0017
& ' Alternate listing of symmetry operators follows' HTMA0018
do i=1,100000 HTMA0019
read(6,'(1x,a)',end=9990) line HTMA0020
write(*,'(1x,a)') line HTMA0021
end do HTMA0022
9990 close(6) HTMA0023
write(*,*) '</pre>' HTMA0024
write(*,*) '</BODY>' HTMA0025
return HTMA0026
end HTMA0027
So, that's all there is to basic CGI scripting. Just go ahead,
write a little code fragment yourself and play around a little. I
used Microsoft Frontpage [2] to create my site and the forms, and
Microsoft FORTRAN professional edition for the F90 programming.
Your FORTRAN compiler may use a different way to get the
environment variables, but you should be able to find this in
your documentation. Of course, this is NOT an encouragement to
write CGI scripts in Fortran. The executables are bulky and full
with overhead, and for many tasks, C, Pearl, J++ etc are faster.
But if you have a solid peace of code, hey, why not make it
useable on the web!
References
[1] CGI developer's guide, E.E.Kim,
SamsNet Publishing (1996)
[2] http://www.microsoft.com/frontpage/documents/technical11/default.htm
[3] Mastering Internet Development, Micosoft CD-ROM
 Back to Introduction
This World Wide Web site conceived and maintained by
Bernhard Rupp
Last revised
July 05, 2006 16:05
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