Function generic_reader

3rd/lua-5.4.3/src/lbaselib.c:349–363

** Reader for generic 'load' function: 'lua_load' uses the ** stack for internal stuff, so the reader cannot change the ** stack top. Instead, it keeps its resulting string in a ** reserved slot inside the stack. */

** reserved slot inside the stack.
*/
static const char *generic_reader (lua_State *L, void *ud, size_t *size) {
  (void)(ud);  /* not used */
  luaL_checkstack(L, 2, "too many nested functions");
  lua_pushvalue(L, 1);  /* get function */
  lua_call(L, 0, 1);  /* call it */
  if (lua_isnil(L, -1)) {
    lua_pop(L, 1);  /* pop result */
    *size = 0;
    return NULL;
  }
  else if (l_unlikely(!lua_isstring(L, -1)))
    luaL_error(L, "reader function must return a string");
  lua_replace(L, RESERVEDSLOT);  /* save string in reserved slot */
  return lua_tolstring(L, RESERVEDSLOT, size);
}


static int luaB_load (lua_State *L) {