std::free(x); new double;
} ///:~
The trace file created in MemCheck.cpp causes the generation of one "pointer not in memory list" message, apparently from the creation of the file pointer on the heap. [[ This may not still be true – test it ]]
CGI programming in C++
The World-Wide Web has become the common tongue of connectivity on planet earth. It began as simply a way to publish primitively-formatted documents in a way that everyone could read them regardless of the machine they were using. The documents are created in hypertext markup language (HTML) and placed on a central server machine where they are handed to anyone who asks. The documents are requested and read using a web browser that has been written or ported to each particular platform.
Very quickly, just reading a document was not enough and people wanted to be able to collect information from the clients, for example to take orders or allow database lookups from the server. Many different approaches to client-side programming have been tried such as Java applets, JavaScript, and other scripting or programming languages. Unfortunately, whenever you publish something on the Internet you face the problem of a whole history of browsers, some of which may support the particular flavor of your client-side programming tool, and some which won’t. The only reliable and well-established solution27 to this problem is to use straight HTML (which has a very limited way to collect and submit information from the client) and common gateway interface (CGI) programs that are run on the server. The Web server takes an encoded request submitted via an HTML page and responds by invoking a CGI program and handing it the encoded data from the request. This request is classified as either a “GET” or a “POST” (the meaning of which will be explained later) and if you look at the URL window in your Web browser when you push a “submit” button on a page you’ll often be able to see the encoded request and information.
CGI can seem a bit intimidating at first, but it turns out that it’s just messy, and not all that difficult to write. (An innocent statement that’s true of many things – after you understand them.) A CGI program is quite straightforward since it takes its input from environment variables and standard input, and sends its output to standard output. However, there is some decoding that must be done in order to extract the data that’s been sent to you from the client’s web page. In this section you’ll get a crash course in CGI programming, and we’ll develop tools that will perform the decoding for the two different types of CGI submissions
27 Actually, Java Servlets look like a much better solution than CGI, but – at least at this writing – Servlets are still an up-and-coming solution and you’re unlikely to find them provided by your typical ISP.
Appendix B: Programming Guidelines
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(GET and POST). These tools will allow you to easily write a CGI program to solve any problem. Since C++ exists on virtually all machines that have Web servers (and you can get GNU C++ free for virtually any platform), the solution presented here is quite portable.
Encoding data for CGI
To submit data to a CGI program, the HTML “form” tag is used. The following very simple HTML page contains a form that has one user-input field along with a “submit” button:
//:! C10:SimpleForm.html <HTML><HEAD>
<TITLE>A simple HTML form</TITLE></HEAD> Test, uses standard html GET
<Form method="GET" ACTION="/cgi-bin/CGI_GET.exe"> <P>Field1: <INPUT TYPE = "text" NAME = "Field1" VALUE = "This is a test" size = "40"></p> <p><input type = "submit" name = "submit" > </p> </Form></HTML>
///:~
Everything between the <Form and the </Form> is part of this form (You can have multiple forms on a single page, but each one is controlled by its own method and submit button). The “method” can be either “get” or “post,” and the “action” is what the server does when it receives the form data: it calls a program. Each form has a method, an action, and a submit button, and the rest of the form consists of input fields. The most commonly-used input field is shown here: a text field. However, you can also have things like check boxes, drop-down selection lists and radio buttons.
CGI_GET.exe is the name of the executable program that resides in the directory that’s typically called “cgi-bin” on your Web server.28 (If the named program is not in the cgi-bin directory, you won’t see any results.) Many Web servers are Unix machines (mine runs Linux) that don’t traditionally use the .exe extension for their executable programs, but you can call the program anything you want under Unix. By using the .exe extension the program can be tested without change under most operating systems.
If you fill out this form and press the “submit” button, in the URL address window of your browser you will see something like:
http://www.pooh.com/cgi-bin/CGI_GET.exe?Field1= This+is+a+test&submit=Submit+Query
28 Free Web servers are relatively common and can be found by browsing the Internet; Apache, for example, is the most popular Web server on the Internet.
Appendix B: Programming Guidelines
540