From: erik.alapaa@bredband.net   
      
   Den tisdagen den 30:e oktober 2012 kl. 20:37:15 UTC+1 skrev Martin   
   Bonner:   
   > Just because the address is the same, doesn't mean you are allowed   
   > to break the rules about using the right members.  Consider the   
   > following code.   
   > #include    
   > int main() {   
   >    union { int i; float f } x;   
   >    float *pf = &x.f;   
   >    x.i = 1;   
   >    *pf = 2.0;   
   >    printf( "2.0 as int == %d\n", x.i );   
   >    return 0;   
   > }   
   >   
   > This code invokes undefined behaviour by using x.i when x.f was the   
   > last member to be written.  An optimizing compiler may well optimize   
      
   I thought that the standards (both C and C++) explicitly allowed   
   writing to one union member, and then reading from another. For   
   example, from ISO/IEC 9899:TC2:   
      
   'An object shall have its stored value accessed only by an lvalue   
   expression that has one of the following types:   
      
   an aggregate or union type that includes one of the aforementioned   
   types among its members (including, recursively, a member of a   
   subaggregate or contained union), or' <...>   
      
   So, swapping the low and high 16-bit parts of a 32-bit word could be   
   done, fully standards-compliant, portable and no UB, by the (C) code   
   below. Correct or not?   
   (As a sidenote, similar code, without using a union, seems to work   
   well with gcc on x86 and x86_64, but not with gcc on modern RISC   
   processors.  Without the union, gcc gives strict aliasing warnings on   
   RISC CPUs, as expected.)   
      
   typedef union   
   {   
        uint32_t u32;   
        uint16_t u16[2];   
   } U32;   
   uint32_t   
   swap_words(uint32_t arg)   
   {   
        U32 in;   
        uint16_t lo;   
        uint16_t hi;   
      
        in.u32    = arg;   
        hi        = in.u16[0];   
        lo        = in.u16[1];   
        in.u16[0] = lo;   
        in.u16[1] = hi;   
      
        return (in.u32);   
   }   
      
   Regards,   
      
   /Erik Alapää   
      
      
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