How is the CGI program communicated?

* Question

How is the CGI program communicated?

* Answer

A CGI (Common Gateway Interface) program is typically communicated between the web server and the client (browser) through a set of input and output mechanisms. Here’s how the communication works:

1. Client Request:

• The client (usually a web browser) makes an HTTP request to the web server for a specific URL that invokes the CGI program.
• This request can be triggered by submitting a form or requesting a resource (like a .cgior .pl script).

2. Server Executes CGI Script:

• The web server receives the request and identifies that it should invoke a CGI program.
• The server passes the relevant environment variablesand input data (such as form data or query parameters) to the CGI script. These are passed via the standard input (stdin) or query strings.

The server sets up various environment variables, such as:

• QUERY_STRING: Any data passed via URL parameters (for GET requests).
• CONTENT_TYPE: Type of content in the HTTP request (for POST requests).
• CONTENT_LENGTH: Length of the data in the HTTP body (for POST).
• REQUEST_METHOD: HTTP method used (GET or POST).
• SCRIPT_NAME: The name of the script being executed.
• REMOTE_ADDR: The IP address of the client.

3. CGI Program Processing:

• The CGI program reads these environment variables and any input data (such as form data, cookies, etc.).
• It processes the input and generates an appropriate response. This could involve reading or writing data to databases, generating dynamic content, or performing calculations.

4. Output Generation:

• After processing, the CGI program generates an HTTP responsein the form of plain text (or other formats like HTML, JSON, XML, etc.).
• It sends this response back to the server’s standard output (stdout).
• The response typically includes:

• HTTP headers(e.g., Content-Type: text/html) to specify the type of content being returned.
• The actual body of the content, which could be HTML code, plain text, or other formats.

5. Server Sends Response to Client:

• The web server then sends this HTTP response back to the client (the web browser).
• The browser renders the content, displaying the result of the CGI program’s execution to the user.

Example Flow:

1. Usersubmits a form in the browser, sending data via a POST request.
2. Web serverreceives the request and triggers the CGI script.
3. CGI programprocesses the form data and generates an HTML response.
4. CGI programsends this HTML response back to the server’s standard output.
5. Web serversends the HTTP response to the browser.
6. Browserdisplays the response to the user.

In summary, CGI enables communication between a web server and a program by using environment variables and standard input/output for data transfer. The server executes the CGI program, processes input, and sends the generated output back to the client.