Operating in CGI mode


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Manual page for Operating_in_CGI_mode(PL)

INVOKING PL AS A CGI PROGRAM

As of version 2.00, pl may be invoked directly as a CGI program, to dynamically create plots without having to write images to temp files. CGI mode is not available for pltab .

To set this up, copy your pl executable to your cgi-bin (or make a link), and create a ploticus config file (see below). The specifics of local CGI bin and file naming conventions vary; see your local admin if having difficulty. Then, use a construct such as this in your HTML page:

<img src="http://abc.org/cgi-bin/pl?cgi=1&scat1.pl&-png&-scale&0.8& TITLE=Trailer%20Tongue%20Technologies">

This is equivalent to command line usage:

pl scat1.pl -png -scale 0.8 TITLE="Trailer Tongue Technologies"

cgi=1 should be the first parameter in your URL construct, as shown in the above example.

After that, the regular pl command line options and arguments , including those for prefabs , may be given in the usual order. However, to conform to URL syntax, a question mark (?) must separate the program name from the first argument, and ampersand (&) must be used to separate subsequent arguments. Also, arguments should not be enclosed in quotes. Use %20 to represent an embedded space, %26 for an embedded ampersand, etc (%XX may be used to represent any problematic character, where XX is its hex ascii value). Arguments are limited to a length of 250 chars each and may not contain embedded newlines.


CGI MODE BEHAVIOR

pl automatically goes into CGI mode if it detects the environment variable REQUEST_URI (set by your web server), and there are no conventional command line arguments given. pl behaves this way when in CGI mode:
  • a ploticus config file is required, and the config file must contain a projectroot setting; other settings are optional. Here is an example of such a config file:

projectroot: /home/steve/proj1

The config file must have the same name as your pl executable, plus a .cnf extension (if your pl executable name ends in .cgi the .cgi is ignored). If pl must be used in several different contexts within the same cgi-bin, you can make soft links to pl, using other names for the links, and have a separate config file corresponding with each one. Examples:

        pl executable name		config file name
    ---------------------------     ---------------------------
	/data/cgi-bin/pl		/data/cgi-bin/pl.cnf 
	/data/cgi-bin/plproj1		/data/cgi-bin/plproj1.cnf 
	/data/cgi-bin/pl.cgi		/data/cgi-bin/pl.cnf 

  • pl will operate from the projectroot directory. All filenames should be expressed relative to projectroot. The script file must be below projectroot. Script file names beginning with slash (/) or dot (.), or containing double-dot (..) are prohibited. This restriction does not apply to other files (eg data files).

  • command arguments are extracted from REQUEST_URI (HTML POST method is not supported). SCRIPT_FILENAME is used to build the config filename. These are environment variables that are set by your web server before it invokes CGI programs. If either of these are not available, pl will exit.

  • default output format is GIF, JPEG, or PNG depending on the pl build; this can also be controlled via the arguments list.

  • an appropriate Content-type header is generated based on output format type. This header as well as image content are written to standard output.

  • output is limited to one image or "page". To do multiple images, temp files will need to be used.


DEVELOPMENT HINTS

When using pl in CGI mode, debugging is difficult because diagnostics cannot be viewed, and the image content is being streamed to standard output. If you are having trouble getting pl to work in CGI mode, try the following: cd to your cgi bin, deploy a new term window, and in the new window manually set the REQUEST_URI and SCRIPT_FILENAME environment variables, eg:

export REQUEST_URI="/data/cgi-bin/pl?scat1.pl&-scale&0.8"

export SCRIPT_FILENAME="/data/cgi-bin/pl"

Then, in the new window, invoke pl by hand, with no command line arguments:

pl > out.gif

Any error messages should be visible on screen.


CGI SECURITY

You should be fully familiar with CGI security issues and how they relate to your platform and project, before putting pl into service as a CGI program. Some thoughts:
  • Anyone can easily view your HTML, including your invocation of CGI programs. It is also very easy for users to submit modified CGI invocations (especially altered arguments).
  • If you pass a data file name (or other filename) to pl via a varvalue argument, use the following within your pl script to guard against hacks on the file name:

  #if @DATAFILE inlike /*,.*,*..*
    #exit
  #endif
  • If you build a shell command from anything passed via a varvalue argument, remove shell metacharacters from the variables using the $shellstripchars function .
  • CGI security worries can be avoided by invoking ploticus via the shell (ie. the old way), capturing the results into a temp file, then using <img> to access the temp file. The tradeoff is that this is less efficient due the temp file i/o.




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Copyright Steve Grubb


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