Some identifiers are reserved for use by the implementation. Some of the more important identifiers that the C/C++ standards reserve for any use are:

• Identifiers that contain a double underscore (__)

• Identifiers that begin with an underscore followed by an uppercase letter

In addition to this, the IAR tools reserve for any use:

• Identifiers that contain a double dollar sign ($$)

• Identifiers that contain a question mark (?)