671: Mainline

First, however, you should know a little about the proper and improper uses of global variables. Here are a few improper uses:

• It is possible to use a global variable to hold a constant such as pi. This practice is usually a bad idea, because constants are not supposed to change, yet you can certainly reassign a global variable. You should introduce macro symbols using #define instead.
• It is possible to use a global variable to return information from a function when there is more than one value to return. This practice is usually a bad idea, because passing information out of a function using global variable makes it hard to understand and control the interaction between the calling function and the one that is called. Instead, you should pass out information using pointer parameters, or you should combine multiple values into a single structure object that is returned in the ordinary way.
• It is possible to use a global variable to deliver information from a calling function to a called function. This practice is usually a bad idea, because passing information into a function using global variable makes it hard to understand and control the interaction between the calling function and the one that is called. Instead, you should use ordinary arguments.