Sending Input to a CGI Script

Table of contents:

Though preset environment variables provide much information, we would like to be able to supply any type of data to our CGI scripts, such as a user's name or a search-engine query. The environment variable QUERY_STRING provides a mechanism to do just that. The QUERY_STRING variable contains information that is appended to a URL in a get request. For example, the URL

causes the Web browser to request a CGI script (cgi-bin/script.cgi) with query string (state=California) from The Web server stores the query string following the ? in the QUERY_STRING environment variable. The query string provides parameters that customize the request for a particular client. Note that the question mark (?) is not part of the resource requested, nor is it part of the query string. It serves as a delimiter (or separator) between the two.

Figure 19.9 shows a simple example of a CGI script that reads data passed through the QUERY_STRING. The data in the query string can be formatted in a variety of ways. The CGI script reading the query string must know how to interpret the formatted data. In the example in Fig. 19.9, the query string contains a series of name-value pairs delimited by ampersands (&), as in name=Jill&age=22.

Figure 19.9. Reading input from QUERY_STRING.

(This item is displayed on pages 929 - 930 in the print version)

 1 // Fig. 19.9: querystring.cpp
 2 // Demonstrating QUERY_STRING.
 3 #include 
 4 using std::cout;
 6 #include 
 7 using std::string;
 9 #include 
10 using::getenv;
12 int main()
13 {
14 string query = "";
16 if ( getenv( "QUERY_STRING" ) ) // QUERY_STRING variable exists 
17  query = getenv( "QUERY_STRING" ); // retrieve QUERY_STRING value
19 cout << "Content-Type: text/html

"; // output HTTP header
21 // output XML declaration and DOCTYPE
22 cout << ""
23 << "
24 << """>";
26 // output html element and some of its contents
27 cout << ""
28 << "Name/Value Pairs;
29 cout << "

Name/Value Pairs

"; 30 31 // if query contained no data 32 if ( query == "" ) 33 cout << "Please add some name-value pairs to the URL above. Or" 34 << " try <a href="" name="Joe&age=29">this</a>."; 35 else // user entered query string 36 cout << "

The query string is: " << query << "

"; 37 38 cout << ""; 39 return 0; 40 } // end main

In line 16 of Figure 19.9, we pass "QUERY_STRING" to function getenv, which returns the query string or a null pointer if the server has not set a QUERY_STRING environment variable. [Note: The Apache HTTP Server sets QUERY_STRING even if a request does not contain a query stringin this case, the variable contains an empty string. However, some servers, such as Microsoft's IIS, set this variable only if a query string actually exists.] If the QUERY_STRING environment variable exists (i.e., getenv does not return a null pointer), line 17 invokes getenv again, this time assigning the returned query string to string variable query. After outputting a header, some XHTML start tags and the title (lines 1929), we test if query contains data (line 32). If not, we output a message instructing the user to add a query string to the URL. We also provide a link to a URL that includes a sample query string. Query string data may be specified as part of a hyperlink in a Web page when encoded in this manner. The contents of the query string are output by line 36.

This example simply demonstrated how to access data passed to a CGI script in the query string. Later chapter examples show how to break a query string into useful pieces of information that can be manipulated using separate variables.

Introduction to Computers, the Internet and World Wide Web

Introduction to C++ Programming

Introduction to Classes and Objects

Control Statements: Part 1

Control Statements: Part 2

Functions and an Introduction to Recursion

Arrays and Vectors

Pointers and Pointer-Based Strings

Classes: A Deeper Look, Part 1

Classes: A Deeper Look, Part 2

Operator Overloading; String and Array Objects

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism


Stream Input/Output

Exception Handling

File Processing

Class string and String Stream Processing

Web Programming

Searching and Sorting

Data Structures

Bits, Characters, C-Strings and structs

Standard Template Library (STL)

Other Topics

Appendix A. Operator Precedence and Associativity Chart

Appendix B. ASCII Character Set

Appendix C. Fundamental Types

Appendix D. Number Systems

Appendix E. C Legacy Code Topics

Appendix F. Preprocessor

Appendix G. ATM Case Study Code

Appendix H. UML 2: Additional Diagram Types

Appendix I. C++ Internet and Web Resources

Appendix J. Introduction to XHTML

Appendix K. XHTML Special Characters

Appendix L. Using the Visual Studio .NET Debugger

Appendix M. Using the GNU C++ Debugger


C++ How to Program
C++ How to Program (5th Edition)
ISBN: 0131857576
EAN: 2147483647
Year: 2004
Pages: 627 © 2008-2020.
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