I want to create a function, let's say (qDebug() << "result" ;). I want to display the result 2 secs later, the delay must be in this function implemented. In other words :
void MainWindow::my_function()
{
// Here I need something to make a delay of 2 secs
qDebug() << "result";
}
Is there a method or something that allows to wait 2 secs and then executes the next line ?
I m looking for the easiest method on QT.
Do NOT (mis)use sleep functions
Sleep functions are a specialized tool which is incredibly easy to misuse, and Qt gives you a lot of better options. So do not use sleep functions unless you know what you are doing (if only to avoid starting a bad habit).
So, what are your options?
If you only want the 2 second delay (e.g. after the user presses a button) you could use a QTimer::singleShot() which will call the function after the timer expires e.g.
QTimer::singleShot(2000, this, &MainWindow::printResultFunction)
Or you might use a local QEventLoop which you you will exec and then quit (again) using a timer e.g.
QEventLoop loop;
QTimer::singleShot(2000, &loop, &QEventLoop::quit);
loop.exec();
Or you might start a separate thread, which executes your function and in this function use (gasp) QThread::msleep() As I said before sleep functions are a specialized tool - here you know what you are doing. You are not stalling your GUI thread and qt events. You are pausing the execution of a thread designed to do one thing: doing stuff, waiting 2 seconds, doing some more stuff and terminating.
my suggestion is to read first why sleep are a bad idea, specially when GUIS are involved..
anyways, you can since qt5
Functions of QThread
void msleep(unsigned long msecs);
void sleep(unsigned long secs);
void usleep(unsigned long usecs);
using this in your code, you can do
void MainWindow::my_function(){
// Here I need something to make a delay of 2 secs
sleep(2);
qDebug() << "result";
}
but as I said before, read 1st, because this will work, but will freeze the main window too, which is a not so nice idea when considering User Experience etc
Related
When I create a QTimer object in Qt 5, and start it using the start() member function, is a separate thread created that keeps track of the time and calls the timeout() function at regular intervals?
For example,
QTimer *timer = new QTimer;
timer->start(10);
connect(timer,SIGNAL(timeout()),someObject,SLOT(someFunction()));
Here, how does the program know when timeout() occurs? I think it would have to run in a separate thread, as I don't see how a sequential program could keep track of the time and continue its execution simultaneously. However, I have been unable to find any information regarding this either in the Qt documentation or anywhere else to confirm this.
I have read the official documentation, and certain questions on StackOverflow such as this and this seem very related, but I could not get my answer through them.
Could anyone explain the mechanism through which a QTimer object works?
On searching further, I found that as per this answer by Bill, it is mentioned that
Events are delivered asynchronously by the OS, which is why it appears that there's something else going on. There is, but not in your program.
Does it mean that timeout() is handled by the OS? Is there some hardware that keeps track of the time and send interrupts at appropriate intervals? But if this is the case, as many timers can run simultaneously and independently, how can each timer be separately tracked?
What is the mechanism?
Thank you.
When I create a QTimer object in Qt 5, and start it using the start()
member function, is a separate thread created that keeps track of the
time and calls the timeout() function at regular intervals?
No; creating a separate thread would be expensive and it isn't necessary, so that isn't how QTimer is implemented.
Here, how does the program know when timeout() occurs?
The QTimer::start() method can call a system time function (e.g. gettimeofday() or similar) to find out (to within a few milliseconds) what the time was that start() was called. It can then add ten milliseconds (or whatever value you specified) to that time and now it has a record indicating when the timeout() signal is supposed to be emitted next.
So having that information, what does it then do to make sure that happens?
The key fact to know is that QTimer timeout-signal-emission only works if/when your Qt program is executing inside Qt's event loop. Just about every Qt program will have something like this, usually near the bottom its main() function:
QApplication app(argc, argv);
[...]
app.exec();
Note that in a typical application, almost all of the application's time will be spent inside that exec() call; that is to say, the app.exec() call will not return until it's time for the application to exit.
So what is going on inside that exec() call while your program is running? With a big complex library like Qt it's necessarily complicated, but it's not too much of a simplification to say that it's running an event loop that looks conceptually something like this:
while(1)
{
SleepUntilThereIsSomethingToDo(); // not a real function name!
DoTheThingsThatNeedDoingNow(); // this is also a name I made up
if (timeToQuit) break;
}
So when your app is idle, the process will be put to sleep inside the SleepUntilThereIsSomethingToDo() call, but as soon as an event arrives that needs handling (e.g. the user moves the mouse, or presses a key, or data arrives on a socket, or etc), SleepUntilThereIsSomethingToDo() will return and then the code to respond to that event will be executed, resulting in the appropriate action such as the widgets updating or the timeout() signal being called.
So how does SleepUntilThereIsSomethingToDo() know when it is time to wake up and return? This will vary greatly depending on what OS you are running on, since different OS's have different APIs for handling this sort of thing, but a classic UNIX-y way to implement such a function would be with the POSIX select() call:
int select(int nfds,
fd_set *readfds,
fd_set *writefds,
fd_set *exceptfds,
struct timeval *timeout);
Note that select() takes three different fd_set arguments, each of which can specify a number of file descriptors; by passing in the appropriate fd_set objects to those arguments you can cause select() to wake up the instant an I/O operations becomes possible on any one of a set of file descriptors you care to monitor, so that your program can then handle the I/O without delay. However, the interesting part for us is the final argument, which is a timeout-argument. In particular, you can pass in a struct timeval object here that says to select(): "If no I/O events have occurred after (this many) microseconds, then you should just give up and return anyway".
That turns out to be very useful, because by using that parameter, the SleepUntilThereIsSomethingToDo() function can do something like this (pseudocode):
void SleepUntilThereIsSomethingToDo()
{
struct timeval now = gettimeofday(); // get the current time
struct timeval nextQTimerTime = [...]; // time at which we want to emit a timeout() signal, as was calculated earlier inside QTimer::start()
struct timeval maxSleepTimeInterval = (nextQTimerTime-now);
select([...], &maxSleepTimeInterval); // sleep until the appointed time (or until I/O arrives, whichever comes first)
}
void DoTheThingsThatNeedDoingNow()
{
// Is it time to emit the timeout() signal yet?
struct timeval now = gettimeofday();
if (now >= nextQTimerTime) emit timeout();
[... do any other stuff that might need doing as well ...]
}
Hopefully that makes sense, and you can see how the event loop uses select()'s timeout argument to allow it to wake up and emit the timeout() signal at (approximately) the time that it had previously calculated when you called start().
Btw if the app has more than one QTimer active simultaneously, that's no problem; in that case, SleepUntilThereIsSomethingToDo() just needs to iterate over all of the active QTimers to find the one with the smallest next-timeout-time stamp, and use only that minimum timestamp for its calculation of the maximum time-interval that select() should be allowed to sleep for. Then after select() returns, DoTheThingsThatNeedDoingNow() also iterates over the active timers and emits a timeout signal only for those whose next-timeout-time stamp is not greater than the current time. The event-loop repeats (as quickly or as slowly as necessary) to give a semblance of multithreaded behavior without actually requiring multiple threads.
Looking at the documentation about timers and at the source code of QTimer and QObject we can see that the timer is running in the thread/event loop that is assigned to the object. From the doc:
For QTimer to work, you must have an event loop in your application; that is, you must call QCoreApplication::exec() somewhere. Timer events will be delivered only while the event loop is running.
In multithreaded applications, you can use QTimer in any thread that has an event loop. To start an event loop from a non-GUI thread, use QThread::exec(). Qt uses the timer's thread affinity to determine which thread will emit the timeout() signal. Because of this, you must start and stop the timer in its thread; it is not possible to start a timer from another thread.
Internally, QTimer simply uses the QObject::startTimer method to fire after a certain amount of time. This one itself somehow tells the thread it's running on to fire after the amount of time.
So your program is fine running continously and keeping track of the timers as long as you don't block your event queue. If you are worried of your timer being not 100% accurate try to move long-running callbacks out of the event queue in their own thread, or use a different event queue for the timers.
QTimer object registers itself into EventDispatcher (QAbstractEventDispatcher) which than takes care to send events of type QTimerEvent every time there is timeout for a particular registered QTimer. For example, on GNU/Linux there is a private implementation of QAbstractEventDispatcher called QEventDispatcherUNIXPrivate that makes calculations taking in consideration the platform api for the time. The QTimerEvent are sent from QEventDispatcherUNIXPrivate into the queue of the event loop of the same thread where QTimer object belongs, i.e. was created.
QEventDispatcherUNIXPrivate doesn't fire a QTimerEvent because of some OS system event or clock, but because it periodically checkes the timeout when processEvents is called by the thread event loop where QTimer lives too. Se here: https://code.woboq.org/qt5/qtbase/src/corelib/kernel/qeventdispatcher_unix.cpp.html#_ZN27QEventDispatcherUNIXPrivateC1Ev
So I know in Qt, you can just put everything you want after a signal to happen into a slot, but when editting code later on; that can require a lot of restructuring, and may complicate things.
Is there a more simple way to hold your program until a signal is emitted in Qt, like say for example:
downloadImage();
Qt::waitForSignal(downloadImageFinished());
editImage();
Also; why doesn't something like this work:
// bool isFinished();
downloadImage(); // When done, isFinished() returns true;
while (!isFinished()) {}
editImage();
? Thanks.
Basically, you should do this:
QEventLoop loop;
connect(this, &SomeObject::someSignal, &loop, &QEventLoop::quit);
// here you can send your own message to signal the start of wait,
// start a thread, for example.
loop.exec(); //exec will delay execution until the signal has arrived
Wait inside a cycle will put your execution thread into a doom of spinlock - this shouldn't happen in any program. Avoid spinlocks at all cost - you don't want to deal with the consequences. Even if everything works right, remember that you will take whole processor core just for yourself, significantly delaying overall PC performance while in this state.
I have loop inside my main window code, that is simply changing the colour of some text-boxes on screen.
It is simply for(int i=0; i<200; i++), but I'd like to make every colour change visible to the user, so inside the loop I've tried to add sth like a 10ms pause, so every execution is visible on screen.
I used this:
QTimer t;
t.start(10);
QEventLoop loop;
connect(&t, SIGNAL(timeout()), &loop, SLOT(quit()));
loop.exec();
The problem is, that I'd like to have this 10ms pace constantly, so the whole operation will take about ~2 seconds. Unfortunately, it slows down gradually, so hard, that the last ~20 executions takes even about 1 second each
It looks rather decently when i<20~50, adding more makes it significantly slowing...
I thought about my not-really-brand-new PC, but it is really simple operation to be done, so I don't really think it is because of my slow pc.
I'm assume my approach is wrong
PS. During the execution, ram usage for my app is about ~21MB, and cpu about 20-30%
It is not good way to achieve something. QTimer is enough to this task. For example:
QTimer *t = new QTimer;//without loops and sleeping
connect(t, SIGNAL(timeout()), this, SLOT(someSlot()));
t->start(10);
Create someSlot and in this slot change color and do other tasks. To stop timer after 2 seconds, you can use counter instead of using system time.
void MainWindow::someSlot()
{
//do something
}
Also consider that 10 ms is very very fast, human eyes not able to catch so fast changing. Try to use longer value.
I'm using the GCC compiler and C++ and I want to make a timer that triggers an interruption when the countdown is 0.
Any Ideas? Thanks in advance.
EDIT
Thanks to Adam, I know how to do it.
Now. What about multiple timers running in parallel?
Actually, these timers are for something very basic. In NCURSES, I have a list of things. When I press a key, one of the things will change colors for 5 seconds. If I press another key, another thing in the list will do the same. It's like emphasize strings depending on the user input. Is there a simpler way to do that?
An easy, portable way to implement an interrupt timer is using Boost.ASIO. Specifically, the boost::asio::deadline_timer class allows you to specify a time duration and an interrupt handler which will be executed asynchronously when the timer runs out.
See here for a quick tutorial and demonstration.
One way to do it is to use the alarm(2) system call to send a SIGALRM to your process when the timer runs out:
void sigalrm_handler(int sig)
{
// This gets called when the timer runs out. Try not to do too much here;
// the recommended practice is to set a flag (of type sig_atomic_t), and have
// code elsewhere check that flag (e.g. in the main loop of your program)
}
...
signal(SIGALRM, &sigalrm_handler); // set a signal handler
alarm(10); // set an alarm for 10 seconds from now
Take careful note of the cautions in the man page of alarm:
alarm() and setitimer() share the same timer; calls to one will interfere with use of the other.
sleep() may be implemented using SIGALRM; mixing calls to alarm() and sleep() is a bad idea.
Scheduling delays can, as ever, cause the execution of the process to be delayed by an arbitrary amount of time.
I'm implementing a checking system in C++. It runs executables with different tests. If the solution is not correct, it can take forever for it to finish with certain hard tests. That's why I want to limit the execution time to 5 seconds.
I'm using system() function to run executables:
system("./solution");
.NET has a great WaitForExit() method, what about native C++?. I'm also using Qt, so Qt-based solutions are welcome.
So is there a way to limit external process' execution time to 5 seconds?
Thanks
Use a QProcess with a QTimer so you can kill it after 5 seconds. Something like;
QProcess proc;
QTimer timer;
connect(&timer, SIGNAL(timeout()), this, SLOT(checkProcess());
proc.start("/full/path/to/solution");
timer.start(5*1000);
and implement checkProcess();
void checkProcess()
{
if (proc.state() != QProcess::NotRunning())
proc.kill();
}
Use a separate thread for doing your required work and then from another thread, issue the
pthread_cancle () call after some time (5 sec) to the worker thread. Make sure to register proper handler and thread's cancelability options.
For more details refer to: http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_cancel.3.html
Check out Boost.Thread to allow you to make the system call in a separate thread and use the timed_join method to restrict the running time.
Something like:
void run_tests()
{
system("./solution");
}
int main()
{
boost::thread test_thread(&run_tests);
if (test_thread.timed_join(boost::posix_time::seconds(5)))
{
// Thread finished within 5 seconds, all fine.
}
else
{
// Wasn't complete within 5 seconds, need to stop the thread
}
}
The hardest part is to determine how to nicely terminate the thread (note that test_thread is still running).
void WaitForExit(void*)
{
Sleep(5000);
exit(0);
}
And then use it (Windows specific):
_beginthread(WaitForExit, 0, 0);
Solution testing system on Windows should use Job objects to restrict it's access to the system and execution time (not the real time, BTW).
If you are working with Posix compliant systems (of which MacOS and Unix generally are), use fork execv and ``waitpidinstead ofsystem`.An example can be found here. The only really tricky bit now is how to get a waitpid with a timeout. Take a look here for ideas.