On 11/08/15 17:25, Daniel Beer wrote:
Thanks guys I knew that I could count on you all for help. volatile, 
that's new to me I'll look it up.  Kernighan and Ritchie say what you 
all say but ANSI C says it's implementation dependent, as long as it 
works.  Thanks Ian.
> On Sun, Nov 08, 2015 at 05:01:34PM -0500, Ian Chapman wrote:
>> Hi all,
>>        I'm having difficulty understanding this situation.
>> flag = 1;
>> while ( 0 != flag )
>> {
>> code;
>> }
>>    generates strange assembly code till I commented out
>> // flag = 1;
>> it looked to me like the optimizer realized that from then on the code
>> would be planted in a " $  jmp  $; " situation.  I intended flag to be
>> cleared after an ADC interrupt.  ie set flag and wait for it to be cleared.
>> Test code snippet and Makefile follow.
> The compiler is allowed to assume that flag won't spontaneously change,
> because you haven't told it otherwise. Either declare flag as volatile
> (probably the simplest), or insert a memory barrier before checking its
> value.
> Cheers,
> Daniel

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