Magic Web Stuff

Now it’s time to start surfing the web! Perhaps you know that web pages are written in something called HTML (HyperText Markup Language), and that the http you see in web addresses like http://pike.lysator.liu.se/ means HyperText Transfer Protocol. The HyperText Transfer Protocol is a description of how web browsers communicate with web servers.

It is actually a fairly complicated operation to connect to a web server, to tell it to send you a web page, and then to receive that page as the server sends it. Fortunately for us, someone has already done this for us. There is a module called Protocols.HTTP, which handles the communication with the web server. A module is a package of Pike code that can easily be used by other programmers.

We rewrite the method handle_url to actually try to fetch the web page, using the module Protocols.HTTP:

void handle_url(string this_url)
{
  write("Fetching URL '" + this_url + "'...");
  Protocols.HTTP.Query web_page;
  web_page = Protocols.HTTP.get_url(this_url);
  if (web_page == 0)
  {
    write(" Failed!\n");
    return;
  }
  write(" Done.\n");
} // handle_url

The interesting part here is the lines

  Protocols.HTTP.Query web_page;
  web_page = Protocols.HTTP.get_url(this_url);

First we define the variable web_page, with the data type Protocols.HTTP.Query. Actually, the data type is called Query, and is defined in the module Protocols.HTTP, but we must write it as Protocols.HTTP.Query so Pike knows where to find it.

A data item of the type Protocols.HTTP.Query contains the result of a web page retrieval: the text of the web page, but also some more information, such as the time when the page was created.

The actual work is done by the method Protocols.HTTP.get_url, which is actually the method get_url in the module Protocols.HTTP. It talks to the web server, fills a Query object with everything it finds, and returns it. If it cannot find the web page, it returns zero (0) instead.

Some other things that might need to be explained in this example:

We can use single quotes (‘) inside a string. If we want to put a double quote (“) in a string, we can do so by prefixing the double quote with a backslash:
```
"This string contains a \"."
```
If the web page couldn’t be found, we use the statement
```
return;
```
to stop executing the method handle_url, and instead return to where it was called. This is the same as the
```
return 0;
```
we have seen in main, except that handle_url doesn’t return a value.
return just returns from the method we are in. If we want to terminate the program, we can use the built-in function exit:
```
exit(0);
```
This has the same effect as returning 0 from main.

When we run this version of the web browser, it may look something like this. The user’s command is shown in italics:

> webbrowser.pike pike.lysator.liu.se
Welcome to the Very Simple WWW Browser!
Fetching URL 'pike.lysator.liu.se'... Done.

If we try to retrieve a web page that doesn’t exist, the web browser fails:

> webbrowser.pike cod.lysator.liu.se
Welcome to the Very Simple WWW Browser!
Fetching URL 'cod.ida.liu.se'... Failed!