lualib, 把字符串从utf8解成utf-16;并返回长度，解码失败，返回nil;

lua-utf8.c

#include <stdio.h>
#include <assert.h>
#include <stdint.h>

#include "lua.h"
#include "lauxlib.h"

inline int
_bytes(uint8_t c) {
    if(c < 0x80) return 1;
    if(c < 0xc0) return 0;
    if(c < 0xe0) return 2;
    if(c < 0xf0) return 3;
    if(c < 0xf8) return 4;
    return 0;
}

inline uint32_t
_value(const char *str, int i, int step) {
    uint8_t c = str[i];
    uint32_t v = c & (0xff >> step);
    int j = 1;
    for(;j<step; j++) {
        v = v << 6;
        v = v | (str[i+j] & 0x3f);
    }
    return v;
}

static int
_utf8(lua_State *L) {
    size_t len;
    const char* str = luaL_checklstring(L, 1, &len);
    luaL_checktype(L, 2, LUA_TTABLE);

    int count = 0;

    int i, step;
    uint8_t c;
    for(i=0;i<len;) {
        c = str[i];
        step = _bytes(c);
        if(step == 0 || len < i + step) {
            count = 0;
            break;
        }
        lua_pushnumber(L, _value(str, i, step));
        count = count + 1;
        lua_rawseti(L, 2, count);

        i = i + step;
    }

    if(count == 0) {
        lua_pushnil(L);
    } else {
        lua_pushinteger(L, count);
    }
    return 1;
}

int
luaopen_utf8_c(lua_State *L) {
    luaL_checkversion(L);

    lua_pushcfunction(L, _utf8);
    return 1;
}