Why does volatile exist?

c++ volatile c++-faq
theschmitzer picture theschmitzer · Sep 16, 2008 · Viewed 44.7k times · Source

What does the volatile keyword do? In C++ what problem does it solve?

In my case, I have never knowingly needed it.

Answer

Doug T. picture Doug T. · Sep 16, 2008

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.

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