XPost: alt.folklore.computers   
   From: Peter@Iron-Spring.com   
      
   On 1/2/26 17:52, rbowman wrote:   
   > On Fri, 2 Jan 2026 15:00:07 -0700, Peter Flass wrote:   
   >   
   >> On 1/2/26 14:33, Lawrence D’Oliveiro wrote:   
   >>> On Fri, 2 Jan 2026 08:49:25 -0800, John Ames wrote:   
   >>>   
   >>>> ... it's not guaranteed that the compiler won't take liberties in   
   >>>> arranging members of a struct for optimization purposes ...   
   >>>   
   >>> The C23 spec (section 6.2.5, “Types”) does say the member objects of a   
   >>> struct type need to be “sequentially allocated”. The only freedom the   
   >>> compiler has (section 6.2.6) is to add “padding bytes”.   
   >>   
   >> It defeats the purpose of a structure if the compiler is free to   
   >> rearrange it. Local variables (PL/I AUTOMATIC) can, in most languages,   
   >> be stored however the compiler wants.   
   >   
   > That can lead to interesting bugs. The root cause is overflowing a local   
   > variable, say writing 6 characters to a char[4]. Which adjacent local   
   > variable gets whacked depends on the compiler's ordering. Whether it   
   > manifests as a bug depends on how the corrupt variable is used in the   
   > function and where it is initialized.   
   >   
      
   Indeed. A few times I suspected this I put a character string before and   
   after where I suspected the problem was, and did lots of checking to   
   find out where it was being clobbered.   
      
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