User Environment Variables
Environment variables specify search paths for files, directories for temporary files, application-specific options, and other similar information. The system maintains an environment block for each user and one for the computer. The system environment block represents environment variables for all users of the particular computer. A user's environment block represents the environment variables the system maintains for that particular user, including the set of system environment variables.
By default, each process receives a copy of the environment block for its parent process. Typically, this is the environment block for the user who is logged on. A process can specify different environment blocks for its child processes using the CreateProcess or CreateProcessAsUser function.
To add or modify environment variables, the user selects System from the Control Panel, then selects the Environment tab. The user can also add or modify environment variables at a command prompt using the set command. Environment variables created with the set command apply only to the command window in which they are set, and to its child processes. For more information, type set /? at a command prompt.
To retrieve a copy of the environment block for a given user, use the CreateEnvironmentBlock function. To free an environment block created by CreateEnvironmentBlock, use the DestroyEnvironmentBlock function. These functions reference a pointer to an environment block. The environment block is an array of null-terminated Unicode strings. The list ends with two nulls (\0\0).
To expand a string that contains environment variables by using the environment block for a specified user, use the ExpandEnvironmentStringsForUser function.