Using C for CGI Programming

Perl, Python and PHP are the holy trinity of CGI application programming. Stores have shelves full of books about these languages, they're covered well in the computer press and there's plenty on the Internet about them. A distinct lack of information exists, however, on using C to write CGI applications. In this article, I show how to use C for CGI programming and lay out some situations in which it provides significant advantages.

I use C in my applications for three reasons: speed, features and stability. Although conventional wisdom says otherwise, my own benchmarks have found that C and PHP are equivalent in speed when the processing to be done is simple. When there is any complexity to the processing, C wins hands-down.

In addition, C provides an excellent feature set. The language itself comes with a bare-bones set of features, but a staggering number of libraries are available for nearly any job for which a computer is used. Perl, of course, is no slouch in this area, and I don't contend that C offers more extensibility, but both can fill nearly any bill.

Furthermore, CGI programs written in C are stable. Because the program is compiled, it is not as susceptible to changes in the operating environment as PHP is. Also, because the language is stable, it does not experience the dramatic changes to which PHP users have been subjected over the past few years.

My application is a simple event listing suitable for a business to list upcoming events, say, the meeting schedule for a day or the events at a church. It provides an administrative interface intended to be password-protected and a public interface that lists all upcoming events (but only upcoming events). This application also provides for runtime configuration and interface independence.

I use a database, rather than write my own data store, and a configuration file contains the database connection information. A collection of files is used to provide interface/code separation.

The administrative interface allows events to be listed, edited, saved and deleted. Listing events is the default action if no other action is provided. Both new and existing events can be saved. The interface consists of a grid screen that displays the list of events and a detail screen that contains the full record of a single event.

The database schema for this application consists of a single table, defined in Listing 1. This schema is MySQL-specific, but an equivalent schema can be created for any database engine.

CREATE TABLE event (
  event_no int(11) NOT NULL auto_increment,
  event_begin date NOT NULL default '0000-00-00',
  name varchar(80) NOT NULL default '',
  location varchar(80) NOT NULL default '',
  begin_hour varchar(10) default NULL,
  end_hour varchar(10) default NULL,
  event_end date NOT NULL default '0000-00-00',
  PRIMARY KEY  (event_no),
  KEY event_date (event_begin)
)

The following functions are the minimum necessary to implement the functionality of the administrative interface: list_events(), show_event(), save_event() and delete_event(). I also am going to abstract the reading and writing of database data into their own group of functions. This keeps each function simpler, which makes debugging easier. The functions that I need for the data-storage interface are event_create(), event_destroy(), event_read(), event_write and event_delete. To make my life easier, I'm also going to add event_fetch_range(), so I can choose a range of events—something I need to do in at least two places.

Next, I need to abstract my records to C structures and abstract database result sets to linked lists. Abstraction lets me change database engines or data representation with relatively little expense, because only a little part of my code deals directly with the data store.

There isn't room here to print all of my source code. Complete source code and my Makefile can be downloaded from my Web site (see the on-line Resources).

The first hurdle to overcome when using C is acquiring the set of tools you need. At bare minimum, you need a CGI parser to break out the CGI information for you. Chances are good that you're also looking for some database connectivity. A little bit of logic/interface independence is good too, so you aren't rewriting code every time the site needs a makeover.

For CGI parsing, I recommend the cgic library from Thomas Boutell (see Resources). It's shockingly easy to use and provides access to all parts of the CGI interface. If you're a C++ person, the cgicc libraries also are suitable (see Resources), although I found the Boutell library to be easier to use.

MySQL is pretty much the standard for UNIX Web development, so I stick with it for my sample application. Every significant database engine has a functional C interface library, though, so you can use whatever database you like.

I'm going to provide my own interface-independence routines, but you could use libxml and libxslt to do the same thing with a good deal more sophistication.