Re: AW: [avr-gcc-list] Gcc bug??

[Dave Hansen]

<x-flowed>
From: Simon Han <address@hidden>
[...]
> Am I correct on this?  Could some compiler experts help to explain the
> detail about volatile keyword in the context of embedded system?  A
> pointer to a page might work as well.

The "volatile" keyword is actually pretty simple.  It tells the compiler 
that

  1) Something you can't see might change its value.
  2) Writing the variable may cause side effects you don't know about.

Or even more simply, "Do exactly what I tell you to do with this variable in 
exactly the order I tell you to do it.  No more, no less."

Someone else pointed out the classical example of a loop controlled by a 
flag.  Given

  while (flag == 0)
     continue;

If "flag" is not volatile, he compiler is free to generate code functionally 
identical to

  if (flag == 0)
     while (1)
        continue;

Another example might be

  flag = 1;
  // code that does not read or write flag, or call any functions
  flag = 0;

If flag is not volatile, the compiler could optimize out the first 
assignment (of 1) to flag. If it matters that flag be 1 between the 
assignments, make it volatile.

Something "volatile" will _not_ do is protect access.  For example, if you 
have a counter that is reset by an interrupt but incremented in mainline 
code, e.g.

  void check(void)
  {
     if (condition)
        ++ctr;
  }

  SIGNAL(SOMETHING_OR_OTHER)
  {
     ctr = 0;
  }

This can fail _even_if_ ctr is volatile.  To fix the code, the "check" 
function must disable interrupts before incrementing ctr.  Otherwise the 
interrupt might occur in the middle of the increment operation, and the 
value of ctr might be incorrect or corrupt.

HTH,
  -=Dave

_________________________________________________________________
Frustrated with dial-up? Lightning-fast Internet access for as low as 
$29.95/month. http://click.atdmt.com/AVE/go/onm00200360ave/direct/01/


_______________________________________________
avr-gcc-list mailing list
address@hidden
http://www.avr1.org/mailman/listinfo/avr-gcc-list

</x-flowed>