Why does volatile exist?

by

volatile is needed if you are reading from a spot in memory that, say, a completely separate process/device/whatever may write to.

I used to work with dual-port ram in a multiprocessor system in straight C. We used a hardware managed 16 bit value as a semaphore to know when the other guy was done. Essentially we did this:

void waitForSemaphore()
{
   volatile uint16_t* semPtr = WELL_KNOWN_SEM_ADDR;/*well known address to my semaphore*/
   while ((*semPtr) != IS_OK_FOR_ME_TO_PROCEED);
}

Without volatile, the optimizer sees the loop as useless (The guy never sets the value! He’s nuts, get rid of that code!) and my code would proceed without having acquired the semaphore, causing problems later on.

More Related Contents:

  1. Why is volatile not considered useful in multithreaded C or C++ programming?
  2. What does the explicit keyword mean?
  3. What is a smart pointer and when should I use one?
  4. Where and why do I have to put the “template” and “typename” keywords?
  5. Why should I not #include ?
  6. What is this weird colon-member (” : “) syntax in the constructor?
  7. C++ delete – It deletes my objects but I can still access the data?
  8. What is external linkage and internal linkage?
  9. What is a “span” and when should I use one?
  10. What does T&& (double ampersand) mean in C++11?
  11. Which kind of pointer do I use when?
  12. What is std::move(), and when should it be used?
  13. Why aren’t my include guards preventing recursive inclusion and multiple symbol definitions?
  14. Why is enum class preferred over plain enum?
  15. Polymorphism in C++
  16. What is the correct answer for cout
  17. What are template deduction guides and when should we use them?
  18. Why is ‘this’ a pointer and not a reference?
  19. Template specialization of particular members?
  20. What are the advantages of using nullptr?
  21. Why would one replace default new and delete operators?
  22. Volatile in C++11
  23. Does std::list::remove method call destructor of each removed element?
  24. Does the C++ volatile keyword introduce a memory fence?
  25. Why is the volatile qualifier used through out std::atomic?
  26. Why is volatile deprecated in C++20?
  27. Why am I not provided with a default copy constructor from a volatile?
  28. Conversion from Derived** to Base**
  29. What Rules does compiler have to follow when dealing with volatile memory locations?
  30. What is std::move(), and when should it be used and does it actually move anything?

Leave a Comment