From: noone@noone.net   
      
   On Mon 1/12/2026 6:28 AM, Michael S wrote:   
   >   
   > According to C Standard, access to p->table[4] in foo1() is UB.   
   > ...   
   > Now the question.   
   > What The Standard says about foo2() ? Is there UB in foo2() as well?   
      
   Yes, in the same sense as in `foo1`.   
      
   > gcc code generator does not think so.   
      
   It definitely does. However, since this is the trailing array member of   
   the struct, GCC does not want to accidentally suppress the classic   
   "struct hack". It assumes that quite possibly the pointer passed to the   
   function points to a struct object allocated through the "struct hack"   
   technique.   
      
   Add an extra field after the trailing array and `foo2` will also fold   
   into `return 1`, just like `foo1`.   
      
   Perhaps there's a switch in GCC that would outlaw the classic "struct   
   hack"... But in any case, it is not prohibited by default for   
   compatibility with pre-C99 code.   
      
   --   
   Best regards,   
   Andrey   
      
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