The ANSI standard of C adds several features to traditional versions of C and helps to unify its development. The most notable changes are: function prototyping, void types, the const and volatile qualifiers, and miscellaneous library changes.
The Language Reference in the TC User's Manual describes the differences between Think C 4.0 and ANSI C. Most significantly: TC does not support ANSI C's 'const' and 'volatile' type qualifiers nor the 'signed' type specifier; TC does not support the new "tentative definition" feature for global variables*; TC does not implement ANSI C trigraph sequences.