Re: [Tinycc-devel] [PATCH] Parentheses handling within struct/union initialization

[Dave Dodge]

On Fri, Sep 28, 2007 at 06:47:09PM +0200, Marc Andre Tanner wrote:
> struct in6_addr {
>       union {
>               uint8_t u6_addr8 [ 16 ] ;
>               uint16_t u6_addr16 [ 8 ] ;
>               uint32_t u6_addr32 [ 4 ] ;
>       } in6_u ;
> } ;
[...]
> IPADDR ip4_any = {
>       2,
>       { ( ( in_addr_t ) 0x00000000 ) } // broken
> //    {  ( in_addr_t ) 0x00000000  }   // this would work
> };

Two things to note about this:

- It's making use of a gcc language extension.  Proper C would require
  at least some additional braces around that initializer value.

- Even with gcc's initializer extension, it only produces the
  "correct" result as a side effect.  The lack of a designated
  initializer means that the initializer value gets assigned only to
  the very first thing in the union.  The 32-bit (in_addr_t)0 is
  truncated to 8 bits and assigned to u6_addr8[0].  The remaining 15
  elements of u6_addr8[] are implicitly zero-filled just like any
  other integer array that only had its first element initialized.
  The cast to in_addr_t is misleading, because it's not going to store
  32 bits in the union and if you try to use any value _other_ than 0
  you may not get what you want.  For example if you try something
  like 0xffffffff it will assign 0xff to only the first byte of the
  union and zero-fill the other 15.

                                                  -Dave Dodge