The term C99 used in this book refers to the ISO/IEC 9899:1999 C programming language. The term C90 refers to the ISO/IEC 9899:1990 C programming language.
Because the compiler also supports traditional K&R C (Kernighan and Ritchie, or pre-ANSI C), it can ease your migration to ISO C.
For information on C90 implementation-specific behavior, see D.1.17 _Pragma.
For more information on C99 features, see Table C-6.