How to do CGI using C++

Introduction

For this document, I will assume that you have a working knowledge of html including html forms. There are two methods of doing cgi, "get" and "post" and this document will concentrate solely on the post method. (So on your html page make sure that the form tag contains 'method="post"')

Step 1: Getting the Data length

With the data passed to the cgi script, before you can read it in you need to know how much of it there is. The length of the data is stored in an environmental variable known as "CONTENT_LENGTH". To get the length, use the function "getenv" in stdlib.h. This will return a string containing that environmental variable. So, to get the length of the data and store it in the variable called "data-len", requires the following lines
(NOTE: the below line will not compile. Replace the variable data-len with your own variable defined previously as a string i.e. char X[?])

strcpy(data-len,getenv("CONTENT_LENGTH"));

Step 2: Converting the Data Length

So, now we've got the data length, we're set? Not quite. The data length as you may have noticed from before, is a string. To make it usable, we need to convert it to an int. There are a number of functions to do this (To see them type man string and they will be in that list). The function I frequently use is "atoi()" which takes the string as a parameter and returns the integer equivalent.

Step 3: Reading in the Data

Finally now we can read in the data. We know the length of it from our atoi function (you did store the result in a variable didn't you?). So all we need is a basic for loop which runs length times and in the loop we use "cin.get(c)" where c is a character (most likely a character in a string i.e. string[i])

for (int i=0; i< data-len; i++)
  cin.get(str[i]);

Step 4: Data Parsing

Now we have the data passed from a form stored in a string variable. All that remains is to use it. Sadly, there are still a few problems to be overcome. If you have a form with two input fields, lets call them field1 and field2, declared as follows

< INPUT TYPE=TEXT MAXLENGTH=30 NAME="field1">
< INPUT TYPE=TEXT MAXLENGTH=20 NAME="field2">

Lets say that into field1 the user types "hello word" and that into field2 he types "username@host.domain". What will then be read in by your program is

field1=hello+world&field2=username%40host.domain

So as you can see, the data from the fields is not in a particularly usable form. I shall leave it up to you, the programmer to work out the details of how to extract the correct substrings from the data string, but the following information is useful:

Data String Formatting

The string consists of the name of the input followed by the value that input takes.
The field name is separated from the data value by an "="
One set of field and data is separated from the next by an "&"
Spaces in the input data are replaced by "+"
Non letters and numbers are replaced by "%xx" where "xx" is the hex value corresponding to that character.

All of the above can be seen in the sample data input given in the previous section.