From: Keith.S.Thompson+u@gmail.com   
      
   David Brown  writes:   
   > On 22/10/2025 15:56, Janis Papanagnou wrote:   
   [...]   
   >>    switch (cmd) {   
   >>    case 'C': ...;   
   >>    case 'M': ...;   
   >>    default: printf ("Error: uncaught cmd '%c'\n", cmd);   
   >>    }   
   >> It's good to take precautions and define the 'default' case. YMMV.   
   >   
   > That's not "taking precautions".  If the "...optionally verify cmd"   
   > part does a good job, then the default line is worse than useless   
   > because it is code that never runs.  If that part doesn't exist (since   
   > it is "optional"), then the default line is not "taking precautions",   
   > it is normal handling of a realistic situation.   
      
   Error handling can be complicated.   
      
   You're right that code that prints a message for a condition that   
   should never happen (equivalently, that can only happen as a result   
   of a programming error) is difficult to test.  (I suppose you could   
   tweak the code to introduce an error so the message is triggered,   
   but that's ugly and difficult to automate.)   
      
   For example, the failure of the first Ariane 5 launch involved   
   an unexpected error diagnostic message being interpreted as data.   
   If the error had been quietly ignored, the rocket might have survived.   
      
   If a condition is really expected never to happen, something   
   like gcc's _builtin_unreachable might be useful, or more portably   
   an assert.   
      
   If you want to test for a condition that should never happen, you   
   need to think about how you would want to handle it.  If the best   
   way to handle it is to abort the application, that's easy enough.   
   If it's a safety critical system, though, it might be better to   
   attempt to log an error message (in a way that won't itself cause   
   a problem) and try to continue running.   
      
   And yes, handling a condition that can actually happen (say, in the   
   presence of bad input) is quite different.   
      
   --   
   Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com   
   void Void(void) { Void(); } /* The recursive call of the void */   
      
   --- SoupGate-Win32 v1.05   
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