Assigning to a dereferenced pointer

jsaxin
2020-10-13
2020-10-15
  • jsaxin

    jsaxin - 2020-10-13

    I have made an array of pointers to places in memory I would like to write to while treating them as a continuous block. But I cannot find the syntax for doing this as all examples of dereferencing has the pointer to the value being copied and not the one being overwritten.

    This triggers a runtime error at the assignment:

    IF xFirstCycle THEN
        apwPointers[0] := ADR(LagerD.Evaporator1.iDefrostHour1);
        xFirstCycle := FALSE;
    END_IF
    
    
    IF Slave.udiWriteAccessCounter <> udiWriteCounter THEN
        apwPointers[Slave.wWriteAddress]^ := awHoldingRegisters[Slave.wWriteAddress];
        udiWriteCounter := Slave.udiWriteAccessCounter;     
    END_IF
    
     

    Last edit: jsaxin 2020-10-13
  • jsaxin

    jsaxin - 2020-10-15

    I have found that I never actually initiated the pointer. Please delete this thread.

     
    • h-hermsen

      h-hermsen - 2020-10-15

      Some tips after seeing your code snippet:

      To ensure you always work with a valid pointer you should take care of the following:
      Always call the code which references the pointer and not do this only once(!) Doing it continuously ensures correct working during a warm download after a code change, besides the overhead is minimal compared to a crashing application ;-)

      //IF xFirstCycle THEN
          apwPointers[0] := ADR(LagerD.Evaporator1.iDefrostHour1);
      //    xFirstCycle := FALSE;
      //END_IF
      

      Plus always test the validity of the pointer before dereferencing it, even if you are sure that it is initialised properly.
      An escape is to rewrite your code using References, they are "equal" to pointers but are typesafe.

      Hope this helps you

       
      • jsaxin

        jsaxin - 2020-10-15

        Thank you. I will look into references.

         
      • jsaxin

        jsaxin - 2020-10-15

        Alas, it turns out references cannot be put into arrays.

         
        • h-hermsen

          h-hermsen - 2020-10-15

          Ah, I never considered that. But pointers should do fine if you check them <> 0 before trying to dereference them.

           

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