CGI Output

CGI program works as a filter. It gets input from client and output the processed result. Usually CGI program outputs a HTML file. It is possible to return binary data or send HTTP status codes. _cgi command is on the header of HTTP of a CGI program. The simplest header is 'Content-type: text/html'. It specifies that the output has a HTML format. The 'Content-length' header specifies the length (in bytes) of a binary data. 'Expires' header specifies the expire date and time of a document for reloading.

In addition, 'Status' specifies the status of a request. It includes three-digit numerical status code. Usually, value 200 indicates success; 204 is no response; 301 means document moved; 400 has a bad request; 401 is unauthorized; 403 is forbidden; 404 represents not found; 500 has internal server error; 501 means not implemented, and so on.

The rarely used 'Location' header specifies the server redirection; 'Pragma' header controls document cache; and 'Refresh' header lets client reload document. Finally, 'Set-Cookie' header stores a tag data on the client for tracing. The 'Location' and 'Status' must be used in a complete header specification.

When a CGI program returns a complete HTTP header, there is no overhead for server. Its output is directly sent to the client. Therefore, server can not intercept the output errors. In the NCSA server, CGI program must have a file name with nph- prefix for complete header.

#!/usr/local/bin/snaml
_cgi "Content-type: text/html"
_html
_head
  _title 
    _quote "CGI Environment Variables" 
  title_ 
head_
_body
  # list environment variables 
  foreach key [array::names ::env] {
    _quote "env($key) = $::env($key)"
    _br
  }
body_
html_

The status of Neatware web site is:

env(GATEWAY_INTERFACE) = CGI/1.1 
env(DOCUMENT_ROOT) = /home/neatware/HTML 
env(SERVER_PORT) = 80 
env(HTTP_HOST) = www.neatware.com 
env(REMOTE_ADDR) = 209.148.144.12 
env(HTTP_CONNECTION) = Keep-Alive 
env(HTTP_PRAGMA) = no-cache 
env(SERVER_NAME) = www.neatware.com 
env(SERVER_PROTOCOL) = HTTP/1.0 
env(REMOTE_PORT) = 1215 
env(REQUEST_METHOD) = GET 
env(SERVER_SOFTWARE) = Apache/1.2.6 FrontPage/3.0.4 
env(REMOTE_HOST) = 209.148.144.12 
env(QUERY_STRING) = 
env(HTTP_USER_AGENT) = Mozilla/3.0Gold (Win95; I) 
env(PATH) = /usr/local/bin:/usr/bin:/bin 
env(SCRIPT_NAME) = /cgi/status.cgi 
env(SCRIPT_FILENAME) = /home/neatware/HTML/cgi/status.cgi 
env(HTTP_ACCEPT) = image/gif, image/x-xbitmap, 
  image/jpeg, image/pjpeg, */*