https://msdn.microsoft.com/en-us/library/windows/desktop/ms682653(v=vs.85).aspx
Every process has an environment block that contains a set of environment variables and their values. There are two types of environment variables: user environment variables (set for each user) and system environment variables (set for everyone).
By default, a child process inherits the environment variables of its parent process. Programs started by the command processor inherit the command processor‘s environment variables. To specify a different environment for a child process, create a new environment block and pass a pointer to it as a parameter to the CreateProcess function.
The command processor provides the set command to display its environment block or to create new environment variables. You can also view or modify the environment variables by selecting System from the Control Panel, selecting Advanced system settings, and clicking Environment Variables.
原文：http://www.cnblogs.com/yuanjiangw/p/6225370.html