Re: Question about the TKey union definition

[Roberto Ierusalimschy <roberto@...>]

> Hi all,In Lua 5.1.4 source,the Tkey union definition is:
> 323 typedef union TKey {
> 324   struct {
> 325     TValuefields;
> 326     struct Node *next;  /* for chaining */
> 327   } nk;
> 328   TValue tvk;
> 329 } TKey;
> 
> and the TValue definition is:
>  73 typedef struct lua_TValue {
>  74   TValuefields;
>  75 } TValue;
> 
> so my question is, why just define TKey union like this:
> typedef struct TKey {
>    TValue tvk;
>     struct Node *next;
> } Tkey;
> 
> ?

Because of alignment, the size of a 'TValue' can be larger than the size
of its elements. (That code in Lua is particularly relevant in a 32-bit
machine that aligns 'double' in 8-byte boundaries.)

-- Roberto