luaL_checklstring

luaL_checklstring[-0, +0, v]

const char *luaL_checklstring (lua_State *L, int arg, size_t *l);

Checks whether the function argument arg is a string and returns this string; if l is not NULL fills *l with the string's length.

This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.