9.1 Identifiers

A C/C++ variable identifier (the following is also true for function identifiers) is a sequence of letters and digits, where the underscore character “_” counts as a letter. Identifiers cannot start with a digit. Although they may start with an underscore, such identifiers are reserved for the compiler’s use and should not be defined by your programs. Such is not the case for assembly domain identifiers, which often begin with an underscore

Identifiers are case sensitive, so main is different than Main.

All characters are significant in an identifier, although identifiers longer than 31 characters in length are less portable.