Why is this output waiting for something after it?

Question

void Wait(double Duration)
{
    clock_t End;
    End = clock() + (Duration*CLOCKS_PER_SEC);

    while (clock() < End)
    {
        // This loop just stalls the program.
    }
}

My function works perfectly half the time, but it occasionally stalls the program before it's even called. For example, take the following snippet:

cout << "This is\n";
Wait(2.5)
cout << "a test!";

You'd expect the first line to appear immediately and the second line to appear after 2.5 seconds, but it sometimes ALL appears after 2.5 seconds. What's the deal?

There is almost certainly a better way to do what you really want to do. Let us know your platform/compiler, and we can give better advice. — John Dibling, Nov 09 '10 at 18:50
Does this only happen for you with a cout? Keep in mind that it's buffered. So if you don't flush, I think you might get the behavior you're experiencing. — Bart, Nov 09 '10 at 18:51
I'm not sure if this is correct, but is it possible the compiler is optimizing away the loop as it doesn't appear to do anything? See this question: http://stackoverflow.com/questions/3592557/optimizing-away-a-while1-in-c0x — AshleysBrain, Nov 09 '10 at 18:57
Be carefull with the information you are giving away. They will first ask for the platform/compiler, then your address. Finally you may end up with a bunch of programmers in your kitchen trying help you with your beer. — vitaut, Nov 09 '10 at 19:07

score 4 · Accepted Answer · answered Nov 09 '10 at 18:51

4

try

cout.flush();

before your Wait

answered Nov 09 '10 at 18:51

John Boker

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score 4 · Answer 2 · answered Nov 09 '10 at 18:51

4

That might be because of I/O buffering. You should flush the output buffer (either try << endl instead of '\n' or writing cout.flush) manually.

answered Nov 09 '10 at 18:51

lx.

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score 2 · Answer 3 · answered Nov 09 '10 at 18:50

2

Try cout << "This is" << endl;

It looks like a buffering, not clock issue.

answered Nov 09 '10 at 18:50

vitaut

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score 2 · Answer 4 · answered Nov 09 '10 at 19:06

The flush()/std::endl has already been mentioned - but is your intention to really consume 100% of one core while you wait? This is what the while() loop is doing! If you want a nicer approach to "waiting", consider one of the following:

boost::thread::sleep() - millisecond granularity
alarms (1 second granularity)
select()
pthread_cond_timedwait()

etc.

Why is this output waiting for something after it?

4 Answers4