Comment by archargelod

I downloaded a Lua 5.4.8 source tarball and checked all the uses of LUA_USE_C89 (manually, without AI):

luaconf.h:50-655

- windows builds always use C89 (quote: "broadly, Windows is C89")

- in C99 Lua uses 'strtod' and 'sprintf' for hex number conversions. Otherwise, Lua provides its own implementation.

- no math function variants with l_ and f_ prefixes in C89

- optional lua_KContext type is not available with C89

llimits.h:79

- type definition C99: uintptr.t vs C89: size_t

llimits.h:184

- in C99 or GCC Lua has a pragma for inlining functions, otherwise it's macro'ed to nothing

lmathlib.c:176

- math_log has an additional optimization in C99

    if (base == l_mathop(2.0))
      res = l_mathop(log2)(x);
    else

lmathlib.c:285

- LUA_RAND32 define might fail to find 64-bit type (comment says it's for testing)

loslib.c:36

- `strftime()` only supports one-char options in C89

lprefix.h:14

- no _XOPEN_SOURCE with C89 (POSIX/XSI stuff) - no _LARGEFILE_SOURCE with C89 (manipulation of large files in gcc and other compilers)

both of these defines don't appear anywhere else in Lua source code