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      I am encountering this error "attempt to concatenate a userdata
value" when I am trying to write a tostring metamethod for userdata
"realvector". This is what the function looks like:

static int realvector_tostring(lua_State* L)
  lua_pushfstring(L, "realvector: %p", realvector_check(L,1));
  return 1;

and realvector_check verifies that the argument is realvector by
verifying its metatable.

realvector *realvector_check(lua_State *L, int index)
  void *p = luaL_checkudata(L, index, "realvector_meta");
  luaL_argcheck(L, p != NULL, index, "realvector expected");
  return (realvector *)p;

the definition of realvector is

typedef struct
  double val[3];

I am pretty sure that realvector_tostring is called when print() or
tostring() is called in lua. What is weired is everything would work
OK if the address of the object is not included in the format string
of lua_pushfstring. For instance, it would print " realvector: 1 2 3
"if I were to change my code to

realvector* p = realvector_check(L,1)
lua_pushfstring(L, "realvector: %f %f %f", p->val[0], p->val[1], p->val[2]);

assuming the content of realvector is 1 2 3. I searched the archive
for a while and didn't find much. So I am wondering if I could get any
helpful suggestion here.