metatable

typedef struct Table {
  CommonHeader;
  lu_byte flags;  /* 1<<p means tagmethod(p) is not present */
  lu_byte lsizenode;  /* log2 of size of ‘node‘ array */
  unsigned int sizearray;  /* size of ‘array‘ array */
  TValue *array;  /* array part */
  Node *node;
  Node *lastfree;  /* any free position is before this position */
  struct Table *metatable;
  GCObject *gclist;
} Table;

getmetatable (object)

If object does not have a metatable, returns nil. 
Otherwise, if the object‘s metatable has a "__metatable" field, returns the associated value. 
Otherwise, returns the metatable of the given object.

setmetatable (table, metatable)

Sets the metatable for the given table. 
(You cannot change the metatable of other types from Lua, only from C.) 
If metatable is nil, removes the metatable of the given table. 
If the original metatable has a "__metatable" field, raises an error.

typedef enum {
  TM_INDEX,
  TM_NEWINDEX,
  TM_GC,
  TM_MODE,
  TM_LEN,
  TM_EQ,  /* last tag method with fast access */
  TM_ADD,
  TM_SUB,
  TM_MUL,
  TM_MOD,
  TM_POW,
  TM_DIV,
  TM_IDIV,
  TM_BAND,
  TM_BOR,
  TM_BXOR,
  TM_SHL,
  TM_SHR,
  TM_UNM,
  TM_BNOT,
  TM_LT,
  TM_LE,
  TM_CONCAT,
  TM_CALL,
  TM_N        /* number of elements in the enum */
} TMS;