I never really understood: what is CGI?

Question

CGI is a Comman Gateway Interface. As the name says, it is a "common" gateway interface for everything. It is so trivial and naive from the name. I feel that I understood this and I felt this every time I encountered this word. But frankly, I didn't. I'm still confused.

I am a PHP programmer. I did lot of web development.

user (client) request for page ---> webserver(->embedded PHP interpreter) ----> Server side(PHP) Script ---> MySQL Server.

Now say my PHP Script can fetch results from MySQL Server && MATLAB Server && Some other server.

So, now PHP Script is the CGI? because its interface for the between webserver & All other servers? I don't know. Sometimes they call CGI, a technology & othertimes they call CGI a program or someother server.

• What exactly is CGI?

• Whats the big deal with /cgi-bin/*.cgi? Whats up with this? I don't know what is this cgi-bin directory on the server for. I don't know why they have *.cgi extensions.

• Why does Perl always comes in the way. CGI & Perl (language). I also don't know whats up with these two. Almost all the time I keep hearing these two in combination "CGI & Perl". This book is another great example CGI Programming with Perl Why not "CGI Programming with PHP/JSP/ASP". I never saw such things.

• CGI Programming in C this confuses me a lot. in C?? Seriously?? I don't know what to say. I"m just confused. "in C"?? This changes everything. Program needs to be compiled and executed. This entirely changes my view of web programming. When do I compile? How does the program gets executed (because it will be a machine code, so it must execute as a independent process). How does it communicate with the web server? IPC? and interfacing with all the servers (in my example MATLAB & MySQL) using socket programming? I'm lost!!

• They say that CGI is depreciated. Its no more in use. Is it so? What is its latest update?

Once, I ran into a situation where I had to give HTTP PUT request access to web server (Apache HTTPD). Its a long back. So, as far as I remember this is what I did:

1. Edited the configuration file of Apache HTTPD to tell webserver to pass all HTTP PUT requests to some put.php ( I had to write this PHP script)

2. Implement put.php to handle the request (save the file to the location mentioned)

People said that I wrote a CGI Script. Seriously, I didn't have clue what they were talking about.

• Did I really write CGI Script?

I hope you understood what my confusion is. (Because I myself don't know where I'm confused). I request you guys to keep your answer as simple as possible. I really can't understand any fancy technical terminology. At least not in this case.

EDIT:

I found this amazing tutorial "CGI Programming Is Simple!" - CGI Tutorial Which explains the concepts in simplest possible way. After reading this article you may want to read Getting Started with CGI Programming in C to supplement your understanding with actual code samples. I've also added these links to this tutorial to Wikipedia's article : http://en.wikipedia.org/wiki/Common_Gateway_Interface

Answer 1

CGI is an interface which tells the webserver how to pass data back and forth to and from an application. More specifically, it describes how request information is passed in environment variables (such as request type, remote IP address), how the reqeust body is passed in via standard input, and how the response is passed out via standard output. You can refer to the CGI specification for details.

To use your image:

user (client) request for page ---> webserver ---[CGI]----> Server side Program ---> MySQL Server.

Most, if not all, webservers can be configured to execute a program as a 'CGI'. This means that the webserver, upon receiving a request, will forward the data to a specific program, setting some environment variables and marshalling the parameters via standard input and standard output so the program can know where and what to look for.

The main benefit is that you can run ANY executable code from the web, given that both the webserver and the program know how CGI works. That's why you could write web programs in C or Bash or Brainfuck with a regular CGI-enabled webserver. That, and that most programming environments can easily use standard input, standard output and environment variables.

In your case you most likely used another, specific for PHP, means of communication between your scripts and the webserver, this, as you well mention in your question, is an embedded interpreter called mod_php.

So, answering your questions:

What exactly is CGI?

See above.

Whats the big deal with /cgi-bin/*.cgi? Whats up with this? I don't know what is this cgi-bin directory on the server for. I don't know why they have *.cgi extensions.

That's the traditional place for cgi programs, many webservers come with this directory pre configured to execute all binaries there as CGI programs. The .cgi extension denotes an executable that is expected to work through the CGI.

Why does Perl always comes in the way. CGI & Perl (language). I also don't know whats up with these two. Almost all the time I keep hearing these two in combination "CGI & Perl". This book is another great example CGI Programming with Perl Why not "CGI Programming with PHP/JSP/ASP". I never saw such things.

Because Perl is ancient (older than PHP, JSP and ASP which all came to being when CGI was already old, Perl existed when CGI was new) and became fairly famous for being a very good language to serve dynamic webpages via the CGI. Nowadays there are other alternatives to run Perl in a webserver, mainly mod_perl.

CGI Programming in C this confuses me a lot. in C?? Seriously?? I don't know what to say. I"m just confused. "in C"?? This changes everything. Program needs to be compiled and executed. This entirely changes my view of web programming. When do I compile? How does the program gets executed (because it will be a machine code, so it must execute as a independent process). How does it communicate with the web server? IPC? and interfacing with all the servers (in my example MATLAB & MySQL) using socket programming? I'm lost!!

You compile the executable once, the webserver executes the program and passes the data in the request to the program and outputs the received response. CGI specifies that one program instance will be launched per each request. This is why CGI is inefficient and kind of obsolete nowadays.

They say that CGI is deprecated. Its no more in use. Is it so? What is its latest update?

CGI is still used when performance is not paramount and a simple means of executing code is required. It is inefficient for the previously stated reasons and there are more modern means of executing any program in a web enviroment. Currently the most famous is FastCGI.

Answer 2

CGI essentially passes the request off to any interpreter that is configured with the web server - This coupd be Perl, Python, PHP, Ruby, C pretty much anything. Perl was the most common back in the day thats why you often see it in reference to CGI.

CGI is not dead. In fact most large hosting companies run PHP as CGI as opposed to mod_php because it offers user elvel config and some other things while it is slower than mod_php. Ruby and Python are also typically run as CGI. they key difference her eis that a server module runs as part of the actual server software - where as with CGI its totally outside the server The server just uses the CGI module to determine how to pass and recieve data to the outside interpreter.

Answer 3

Have a look at CGI in Wikipedia. CGI is a protocol between the web server and a external program or a script that handles the input and generates output that is sent to the browser.

CGI is a simply a way for web server and a program to communicate, nothing more, nothing less. Here the server manages the network connection and HTTP protocol and the program handles input and generates output that is sent to the browser. CGI script can be basically any program that can be executed by the webserver and follows the CGI protocol. Thus a CGI program can be implemented, for example, in C. However that is extremely rare, since C is not very well suited for the task.

/cgi-bin/*.cgi is a simply a path where people commonly put their CGI script. Web server are commonly configured by default to fetch CGI scripts from that path.

a CGI script can be implemented also in PHP, but all PHP programs are not CGI scripts. If webserver has embedded PHP interpreter (e.g. mod_php in Apache), then the CGI phase is skipped by more efficient direct protocol between the web server and the interpreter.

Whether you have implemented a CGI script or not depends on how your script is being executed by the web server.

Answer 4

What exactly is CGI?

A means for a web server to get its data from a program (instead of, for instance, a file).

Whats the big deal with /cgi-bin/*.cgi?

No big deal. It is just a convention.

I don't know what is this cgi-bin directory on the server for. I don't know why they have *.cgi extensions.

The server has to know what to do with the file (i.e. treat it as a program to execute instead of something to simply serve up). Having a .html extension tells it to use a text/html content type. Having a .cgi extension tells it to run it as a program.

Keeping executables in a separate directory gives some added protection against executing incorrect files and/or serving up CGI programs as raw data in case the server gets misconfigured.

Why does Perl always comes in the way.

It doesn't. Perl was just big and popular at the same time as CGI.

I haven't used Perl CGI for years. I was using mod_perl for a long time, and tend towards FastCGI these days.

This book is another great example CGI Programming with Perl Why not "CGI Programming with PHP/JSP/ASP".

CGI isn't very efficient. Better methods for talking to programs from webservers came along at around the same time as PHP. JSP and ASP are different methods for talking to programs.

CGI Programming in C this confuses me a lot. in C?? Seriously??

It is a programming language, why not?

When do I compile?

1. Write code
2. Compile
3. Access URL
4. Webserver runs program

How does the program gets executed (because it will be a machine code, so it must execute as a independent process).

It doesn't have to execute as an independent process (you can write Apache modules in C), but the whole concept of CGI is that it launches an external process.

How does it communicate with the web server? IPC?

STDIN/STDOUT and environment variables — as defined in the CGI specification.

and interfacing with all the servers (in my example MATLAB & MySQL) using socket programming?

Using whatever methods you like and are supported.

They say that CGI is depreciated. Its no more in use. Is it so?

CGI is inefficient, slow and simple. It is rarely used, when it is used, it is because it is simple.

What is its latest update?

1.1

Answer 5

CGI is a mechanism whereby an external program is called by the web server in order to handle a request, with environment variables and standard input being used to feed the request data to the program. The exact language the external program is written in does not matter, although it is easier to write CGI programs in some languages versus others.

Since CGI scripts need execute permissions, httpd by default only allows CGI programs in the cgi-bin directory to be run for (possibly now misguided) security purposes.

Most PHP scripts run in the web server process via mod_php. This is not CGI.

CGI is slow since the program (and related interpreter) must be started up per request. Modern alternatives are embedded execution, used by mod_php, and long-running processes, used by FastCGI. A given language may have its own way of implementing those mechanisms, so be sure to ask around before resorting to CGI.

Answer 6

The CGI is specified in RFC 3875, though that is a later "official" codification of the original NCSA document. Basically, CGI defines a protocol to pass data about a HTTP request from a webserver to a program to process - any program, in any language. At the time the spec was written (1993), most web servers contained only static pages, "web apps" were a rare and new thing, so it seemed natural to keep them apart from the "normal" static content, such as in a cgi-bin directory apart from the static content, and having them end in .cgi.

At this time, here also were no dedicated "web programming languages" like PHP, and C was the dominating portable programming language - so many people wrote their CGI scripts in C. But Perl quickly turned out to be a better fit for this kind of thing, and CGI became almost synonymous with Perl for a while. Then there came Java Servlets, PHP and a bunch of others and took over large parts of Perl's market share.

Answer 7

The idea behind CGI is that a program/script (whether Perl or even C) receives input via STDIN (the request data) and outputs data via STDOUT (echo, printf statements). The reason most php scripts don't qualify is because they are run under the PHP Apache module.

Answer 8

CGI is an interface specification between a web server (HTTP server) and an executable program of some type that is to handle a particular request.

It describes how certain properties of that request should be communicated to the environment of that program and how the program should communicate the response back to the server and how the server should 'complete' the response to form a valid reply to the original HTTP request.

For a while CGI was an IETF Internet Draft and as such had an expiry date. It expired with no update so there was no CGI 'standard'. It is now an informational RFC, but as such documents common practice and isn't a standard itself. rfc3875

Programs implementing a CGI interface can be written in any language runnable on the target machine. They must be able to access environment variables and usually standard input and they generate their output on standard output.

Compiled languages such as C were commonly used as were scripting languages such as perl, often using libraries to make accessing the CGI environment easier.

One of the big disadvantages of CGI is that a new program is spawned for each request so maintaining state between requests could be a major performance issue. The state might be handled in cookies or encoded in a URL, but if it gets to large it must be stored elsewhere and keyed from encoded url information or a cookie. Each CGI invocation would then have to reload the stored state from a store somewhere.

For this reason, and for a greatly simple interface to requests and sessions, better integrated environments between web servers and applications are much more popular. Environments like a modern php implementation with apache integrate the target language much better with web server and provide access to request and sessions objects that are needed to efficiently serve http requests. They offer a much easier and richer way to write 'programs' to handle HTTP requests.

Whether you wrote a CGI script rather depends on interpretation. It certainly did the job of one but it is much more usual to run php as a module where the interface between the script and the server isn't strictly a CGI interface.

Answer 9

You maybe want to know what is not CGI, and the answer is a MODULE for your web server (if I suppose you are runnig Apache). AND THAT'S THE BIG DIFERENCE, because CGI needs and external program, thread, whatever to instantiate a PERL, PHP, C app server where when you run as a MODULE that program is the web server (apache) per-se.

Because of all this there is a lot of performance, security, portability issues that come into play. But it's good to know what is not CGI first, to understand what it is.