While Loop waiting for global variable not triggering

Question

I have a flag (as a global variable) which is waiting for a certain time to pass before being set. I have a while loop which waits for the flag to set before proceeding. The global variable does get set to '1' but the while loop does not exit, any ideas what I'm doing wrong:

while (TC3Flag == 0); //Global Flag Variabe TC3Flag, 0 = Not Set, 1 = Set
TurnOnFive();
TurnOnTwelve();
TC3Flag = 0;

Edit i also have tried with the same results, I do want the processor to do nothing while the flag is 0 and only call TurnOnFive and TurnOnTwleve after the flag is set

    while (TC3Flag == 0) {}
    TurnOnFive();
    TurnOnTwelve();
    TC3Flag = 0;

EDIT EDIT: Adding volitile to the TC3FLag declaration fixed it.

There's a spurious semicolon after the condition so that the loop has no body. Remove it. — M Oehm, Oct 23 '17 at 12:16
Is your TC3Flag variable marked `volatile` ? As that variable seems to be able to change outside of the regular program flow, it should be. — fvu, Oct 23 '17 at 12:19
I am interested in knowing how is `TC3Flag` getting changed? — Gaurav Sehgal, Oct 23 '17 at 12:23
@MOehm The while loop is a busy wait. The semicolon should be there. — klutt, Oct 23 '17 at 12:30
@klutt: Yes, I've unserstood that by now. Thanks for pointing it out again. — M Oehm, Oct 23 '17 at 13:14
Nothing guarantees atomic access either... unless you use the atomic types :D — Antti Haapala -- Слава Україні, Oct 23 '17 at 14:07

Vittorio Romeo · Answer 1 · 2017-10-23T12:32:12.023

11

Unless TC3Flag is marked as volatile, the compiler can aggressively optimize the loop away (or just assume it's an infinite loop). You should mark TC3Flag as volatile in order to force the compiler to read the value at its memory address on every iteration.

edited Oct 23 '17 at 12:32

answered Oct 23 '17 at 12:22

Vittorio Romeo

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While Loop waiting for global variable not triggering

1 Answers1