ANSI C Compliance
The naming convention for all Microsoft-specific identifiers in the run-time system (such as functions, macros, constants, variables, and type definitions) is ANSI-compliant. In this documentation, any run-time function that follows the ANSI/ISO C standards is noted as being ANSI compatible. ANSI-compliant applications should only use these ANSI compatible functions.
The names of Microsoft-specific functions and global variables begin with a single underscore. These names can be overridden only locally, within the scope of your code. For example, when you include Microsoft run-time header files, you can still locally override the Microsoft-specific function named _open by declaring a local variable of the same name. However, you cannot use this name for your own global function or global variable.
The names of Microsoft-specific macros and manifest constants begin with two underscores, or with a single leading underscore immediately followed by an uppercase letter. The scope of these identifiers is absolute. For example, you cannot use the Microsoft-specific identifier _UPPER for this reason.