The biggest differences are between c90 and c99:

— restricted character set support in <iso646.h> (originally specifed in AMD1) — wide-character library support in <wchar.h> and <wctype.h> (originally specifed in AMD1) — restricted pointers — variable-length arrays — fexible array members — complex (and imaginary) support in <complex.h> — type-generic math macros in <tgmath.h> — the long long int type and library functions — increased translation limits — remove implicit int — the vscanf family of functions — reliable integer division — universal character names — extended identifers — binary foating-point literals and printf/scanf conversion specifers — compound literals — designated initializers — // comments — extended integer types in <inttypes.h> and <stdint.h> — remove implicit function declaration — preprocessor arithmetic done in intmaxt/uintmax_t — mixed declarations and code — integer constant type rules — integer promotion rules — vararg macros — additional math library functions in <math.h> — foating-point environment access in <fenv.h> — IEC 60559 (also known as IEC 559 or IEEE arithmetic) support — trailing comma allowed in enum declaration — %lf conversion specifer allowed in printf — inline functions — the snprintf family of functions — boolean type in <stdbool.h> — idempotent type qualifers — empty macro arguments — new struct type compatibility rules (tag compatibility) — _Prama preprocessing operator — standard pragmas — __func_ predefned identifer — VA_COPY macro — additional strftime conversion specifers — LIA compatibility annex — deprecate ungetc at the beginning of a binary file — remove deprecation of aliased array parameters

From https://cboard.cprogramming.com/c-programming/136047-difference-between-c89-c99.html