What is Common Gateway Interface CGI

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CGI is a Common 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 with web development experience       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  amp  MATLAB server  amp  some other server   So  now PHP Script is the CGI  Because its interface for the between webserver  amp  All other servers  I don t know  Sometimes they call CGI  a technology  amp  other times they call CGI a program or some other server     What exactly is CGI  Whats the big deal with  cgi-bin   cgi  What s 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  amp  Perl  language   I also don t know what s up with these two  Almost all the time I keep hearing these two in combination  CGI  amp  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  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  amp  MySQL  using socket programming  I m lost   People say that CGI is deprecated and isn t in use anymore  Is that so  What is the 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          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    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 a 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

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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

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A real-life example  a complicated database that needs to be shown on a website  Since the database was designed somewhere around 1986      lots of data was packed in different ways to save on disk space    As the development went on  the developers could no longer solve complicated data requests in SQL alone  for example because the sorting algorythms were unusual    There are three sensible solutions     quick and dirty  send the unsored data to PHP  sort it there  Obviously a very expensive solution  because this would be repeated every time the page is called write a plugin to the database engine -- but the admin wasn t ready to allow foreign code to run on their server  or  you can process the data in a program  C  Perl  etc    and output HTML  The program itself goes into  cgi-bin  and is called by the web server  e g  Apache  directly  not through PHP     CGI runs your script in Solution  3 and outputs the effect to the browser  You have the speed of the compiled program  the flexibility of a language better than SQL  and no need to write plugins to the SQL server   Again  this is an example specific to SQL and C

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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

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A CGI is a program  or a Web API  you write  and save it on the Web Server site  CGI is a file   This file sits and waits on the Web Server   When the client browser sends a request to the Web Server to execute your CGI file  the Web Server runs your CGI file on the server site  The inputs for this CGI program  if any  are from the client browser  The outputs of this CGI program are sent to the browser   What language you use to write a CGI program  Other posts already mention c java  php  perl  etc

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A CGI script is a console shell program  In Windows  when you use a  Command Prompt  window  you execute console programs  When a web server executes a CGI script it provides input to the console shell program using environment variables or  standard input   Standard input is like typing data into a console shell program  in the case of a CGI script  the web server does the typing  The CGI script writes data out to  standard output  and that output is sent to the client  the web browser  as a HTML page  Standard output is like the output you see in a console shell program except the web server reads it and sends it out   A CGI script can be executed from a browser  The URI typically includes a query string that is provided to the CGI script  If the method is  get  then the query string is provided to the CGI Script in an environment variable called QUERY STRING  If the method is  post  then the query string is provided to the CGI Script using standard input  the CGI Script reads the query string from standard input    An early use of CGI scripts was to process forms  In the beginning of HTML  HTML forms typically had an  action  attribute and a button designated as the  submit  button  When the submit button is pushed the URI specified in the  action  attribute would be sent to the server with the data from the form sent as a query string  If the  action  specifies a CGI script then the CGI script would be executed and it then produces a HTML page   RFC 3875  The Common Gateway Interface  CGI   partially defines CGI using C  as in saying that environment variables  are accessed by the C library routine getenv   or variable environ    If you are developing a CGI script using C C   and use Microsoft Visual Studio to do that then you would develop a console program

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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

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CGI essentially passes the request off to any interpreter that is configured with the web server - This could 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 level config and some other things while it is slower than mod php  Ruby and Python are also typically run as CGI  they key difference here is 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

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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

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CGI is an interface which tells the webserver how to pass data 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 request 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 --- gt  webserver --- CGI ---- gt  Server side Program --- gt  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 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  amp  Perl  language   I also don t know whats up with these two  Almost all the time I keep hearing these two in combination  CGI  amp  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  amp  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

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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 that they are run under the PHP Apache module

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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 PSGI Plack with 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     Write code Compile Access URL 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  amp  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  If performance isn t a big deal  then simplicity is worth a lot      What is its latest update    1 1

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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 txt  rfc3875 html  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

[cgi] What is Common Gateway Interface (CGI)?

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