Detecting an Error

In the web browser, we detected the first two possible errors by checking the data: the number of command-line arguments, and the length of the web address that the user types.

In the third case, when a web page couldn't be downloaded, the problem is detected somewhere inside the method get_url in the module Protocols.HTTP. The problem must then be reported to the program that called get_url, and this is done with the return value. The method get_url usually returns an object with the data from the web page, but if it fails to retrieve the web page, it returns zero (0):

web_page = Protocols.HTTP.get_url(this_url);
if(web_page == 0)
{
  write(" Failed.\n");
  return;
}

The value zero is just the normal integer 0, but it is used in many contexts to mean "no value" or "there was a problem". Many of the built-in methods in Pike return 0 if they fail to do what they are supposed to do. Most Pike programmers use the same convention.

To work this way, the integer 0 has to be special. It can be stored in a variable of any type, and it can be returned from any method. Every new variable that you define will contain the value 0, until you give it another value.