CGI basics

Architecture of a CGI Application

In order to pass data about the information request from the server to the script, the server uses command-line arguments, as well as environment variables. These environment variables; set when the server executes the gateway program.

The client connects to the Web server via the network. Several applications can reside on the server. The CGI interface acts as the gateway between the Web server and the server-side processes. One of the server-side processes could be a relational database. Although CGI acts as a gateway between any type of Web server and server-side applications, it has been more commonly implemented with the HTTP server.

The client browser invokes the CGI script and waits until the CGI script completes its process. Thus, the CGI script must execute fast enough to have no perceived delay in the response time.

The drawback of using CGI is that every time a CGI script is invoked, the Web server spawns a new process. This setup becomes a problem when a given Web site gets frequently accessed by several users.

The Web server captures the HTML form data into environment variables. A CGI script reads the environment variables, processes the data, and sends the response back to the browser. The form data is received as a set of name-value pairs. These pairs are separated by the & sign. The name and value tags are separated by the = sign. Because each name=value pair is URL encoded, the following conversion must occur:

1. Convert all + characters to spaces.
2. Convert all %xx sequences to the single character whose ASCII value is xx in hexadecimal form.

The form data gets stored in the environment variable QUERY_STRING if the GET method is used in the FORM tag for submission. If the POST method is used in the FORM tag for submission, the form data is not stored in any environment variables. You get it from STDIN. Because the server does not send an EOF at the end of the data, use the environment variable CONTENT_LENGTH to determine the number of bytes to be read.

You can send different types of data back to the browser including HTML, GIF, and other MIME Content-Types. For example, to send HTML data, use the Content-Type of text/html. To send a GIF image, use the Content-Type of image/gif.

CGI Environment Variables

The following CGI environment variables are set for all requests:

• GATEWAY_INTERFACE: The revision of the CGI specification to which this server complies.
The copyright of the article CGI basics in VB Script is owned by Maxim Karetnikov. Permission to republish CGI basics in print or online must be granted by the author in writing.

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