Using Tcl to write CGI applications

Purpose: Discuss the various options for writing Tcl Common Gateway Interface CGI compliant applications.

It's really easy, even if you don't use one of the helper libraries out there. There are two parts of running as a CGI script. The first part is understanding the incoming request, which is just a matter of looking at the right environment variables and, possibly, reading some data from stdin. The second part is producing the output to go back to the client (via the webserver), which is really just producing (on stdout) HTML with a little header magic.

The things to watch out for are testing and error handling. If an error occurs, the chance is it will just end up in the server log and you won't find out what happened for ages. And constructing a test harness can be a pain as well (there are many environment variables to set to emulate a CGI environment!)

One trick I use is to generate the output to a string and only output that string at the last moment. I then have a wrapper round my whole script that detects (with the help of catch of course) whether an error occurred and, if so, produces a nice error page instead of output which is still in the string. Careful overriding of the puts command can make this easier still:

rename puts orig_puts
proc puts {args} {
   set newline 1
   if {[lindex $args 0] eq "-nonewline"} {
      set newline 0
      set args [lassign $args dummy]
   }
   if {[llength $args] == 1} {
      global accumulator
      append accumulator [lindex $args 0]
      if {$newline} {
         append accumulator \n
      }
   } elseif {[llength $args] == 2} {
      if {$newline} {
         orig_puts {*}$args
      } else {
         orig_puts -nonewline {*}$args
      }
   } else {
      error "wrong # args: should be \"puts ?-nonewline? ?channel? string\""
   }
}

See also

• Helga
• Miscellaneous Tcl procs (Dillinger)
• Writing simple CGI applications using vanilla Tcl

Projects

SnamlScript is a language to generate HTML5 for Web applications. It adds block command and inline command to integrate the Tcl with HTML5 for Web programming. Example:

# Use SnamlScript HTML5 package
package require HTML5

# output to a file. If no output or its first parameter is null, default output is to stdout.
output hello.html 

# declare document type as html 
__doctype html

# html header. quote command render the string.
_html "lang='en'"
_head
 _title; quote Hello; title_
head_

# html body
_body
 __img "src='hello.gif' alt='hello'"
 quote "Hello World!"
body_
html_