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CGI is the abbreviation for the common gateway interface, a benchmark for external gateway programs to interface with information servers like hypertext transfer protocol (HTTP) servers or web servers. Gateways such as CGI manage information requests and return suitable responses; when no suitable response is available, these programs create the needed documents instantly. Basic HTML documents that web servers transfer are static, meaning they remain in a constant state. Such items are documents that do not change. CGI programs act in real-time, creating their ability to yield dynamic information, such as current weather conditions, allowing website visitors to run programs that execute certain activities. For example, if someone wants to connect a database containing Census information for the years 1800 to 1900 to the internet and allow users to search it, that individual needs a CGI program that the web daemon will use and send requested information to the database search engine, accept the results, and present them to the searcher. Speed is of the essence in such situations, as while the programs are sending out, hunting down, and sending back requested information the user is sitting and staring at a blank screen. This is a gateway, and in essence the roots of CGI. Another important piece to understanding CGI is being familiar with daemons. Daemons are programs that run separately from other programs, such as browsers or mailers. Daemons complete a variety of organizational tasks such as creating indexes, overviews, and back linking. In systems such as UNIX, daemons are servers because they operate independently.

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Because web designers are essentially placing their program out for the entire internet public to have access to, security is of the utmost importance. The best way to do this is to hide the program in a unique directory only accessible to the webmaster. There are other methods available, but they depend specifically on the systemís organization and control settings, and as such require a great deal of consideration and detail. Generally, users with the NCSA HTTPd server distribution have the /cgi-bin directory; this is the unique directory where CGI programs are kept. The programs can be written in any language that the system will execute, including C/C++, Fortran, PERL, TCL, UNIX shells, Visual Basic, and AppleScript. Some languages, such as C/C++ and Fortran, require users compile the program before it will operate correctly. Some source code for some CGI programs is located in the /cgi-bin directory that the server came loaded with. On the other hand, scripting languages, like PERL and TCL, only the script will need to reside in the /cgi-bin directory because it has not associated source code. Scripts are easier to write than programs because they are simpler to repair, chance, and sustain than usual compiled programs.

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