Names starting with a single underscore and an uppercase letter, or names beginning with a double underscore and a lowercase letter, can be used in a "conforming" C program translated by a "conforming" implementation; these names, and the meaning and syntax associated with each name, are reserved by the implementation. ANSI C also specifies the requirements for a "strictly conforming" program, in which these constructions are not permitted.

Other compilers may ignore such names that exist in the implementation space, or produce any desired behavior, and the program can still be defined as conforming.

The Microsoft C compiler does not have a strictly conforming mode of operation. As a result, if a developer uses another vendor's compiler, which is a strictly conforming implementation, the Microsoft headers will not work. In this case, the vendor of the strictly conforming compiler must provide the appropriate headers for the system.

Because the standard headers are part of the implementation, there is no requirement for the header mechanisms provided by one implementation to be interoperable with the mechanisms of another. Another implementation MUST provide its own compatible set of standard headers and their associated inclusion mechanisms.