Using C for CGI Programming

At runtime, I need to be able to configure the database connection. Given a filename and an array of character strings for the configuration keys, my configuration function populates a corresponding array of configuration values, as shown in Listing 2. Now I can populate a string array with whatever keys I've chosen to use and get the results back in the value array.

The user interface has two parts. As a programmer, I'm concerned primarily with the input forms and URL strings. Everybody else cares how the page around my form looks and takes the form itself for granted. The solution to keep both parties happy is to have the page exist separately from the form and my program.

Templating libraries abound in PHP and Perl, but there are no common HTML templating libraries in C. The easiest solution is to include only the barest minimum of the output in my C code and keep the rest in HTML files that are output at the appropriate time. A function that can do this is found in Listing 3.

void html_get(char* path, char* file) {

  struct stat sb;
  FILE* html;
  char* buffer;
  char fullpath[1024];

  /* File & path name exceed system limits */
  if (strlen(path) + strlen(file) > 1024) return;

  sprintf(fullpath, "%s/%s", path, file);
  if (stat(fullpath, &sb)) return;

  buffer = (char*)calloc(1, sb.st_size + 1);
  if (!buffer) return;
  html = fopen(fullpath, "r");
  fread((void*)buffer, 1, sb.st_size, html);
  fclose(html);
  puts(buffer);
  free(buffer);

}

Before generating output, I need to tell the Web server and the browser what I'm sending; cgiHeaderContentType() accomplishes this task. I want a content type of text/html, so I pass that as the argument. The general steps to follow for any page I want to display are:

Now that I can generate a page and print a form, I need to be able to process that form. I need to read both numeric and text elements, so I use a couple of functions from the cgic library: cgiFormStringNoNewlines() and cgiFormInteger(). The cgic library implements the main function and requires that I implement int cgiMain(void). cgiMain() is where I put the bulk of my form processing.

To display a single record in my show_event function, I get the event_no (my primary key) from the CGI eventno parameter. cgiFormInteger() retrieves an integer value and sets a default value if no CGI parameter is provided.

I also need to get a whole raft of data from the form in save_event. Dates are thorny things to input because they consist of three pieces of data: year, month and date. I need both a begin and an end date, which gives me six fields to interpret. I also need to input the name of the event, begin and end times (which are strings because they might be events themselves, such as sunrise or sunset) and the location. Listing 4 shows how this works in code.

Listing 4 also demonstrates cgiHeaderLocation(), a function that redirects the user to a new page. After I've saved the submitted data, I want to show the event listing page. Instead of a literal string, I use one of the variables that libcgic provides, cgiScriptName. Using this variable instead of a literal one means the program name can be changed without breaking the program.