Reserved identifiers

Identifiers with two initial underscores or an initial underscore followed by an uppercase letter are reserved globally for use by the compiler.

Identifiers that begin with a single underscore are reserved as identifiers with file scope in both the ordinary and tag namespaces.

Identifiers that begin with a single underscore are reserved in the global namespace.

Although the names of system calls and library functions are not reserved words if you do not include the appropriate headers, avoid using them as identifiers. Duplication of a predefined name can lead to confusion for the maintainers of your code and can cause errors at link time or run time. If you include a library in a program, be aware of the function names in that library to avoid name duplications. You should always include the appropriate headers when using standard library functions.