The C `clock()` function just returns a zero
clock() function always returning 0
why C clock() returns 0
I looked up all these questions and answers
And I learned that clock() returns clock ticks per some constant which differs per systems
And time() returns the number of seconds.
First, I was trying to measure the execution time of my sorting algorithm using clock() like this:
#include <iostream>
#include <ctime>
... Some other headers and codes
a = clock();
exchange_sort();
a = clock() - a;
... Rest of the code
I tried many different data types with a like int, clock_t, long, float.
And I sorted a pretty big size array int arr[1000] with already increased order.
But the value of a was always 0, so I tried to find the reason using gdb and I set a breakpoint to the line where the sorting algorithm is located so that I can check the value of a = clock(); and there has to be some number inside the variable but there was only 0.
So after that, I tried to check whether the function was the problem itself or something else like this:
#include <iostream>
#include <iostream>
int main()
{
int a;
clock_t b;
float c;
long d;
a = clock();
b = clock();
c = clock();
d = clock();
return 0;
}
And I checked the value of each variable through gdb and there were just garbage numbers before I put the return value of clock() but after I put there were only 0s inside the variables.
So apparently clock() just returns 0 all the time in my conclusion
I really don't know how can I fix this
My g++ version is 4.4.7.
I ran this in Linux
My processor is x86_64-redhat-linux
The clock() function is a coarse measure of CPU time used. Your code doesn't use enough CPU time to measure with such a coarse measure. You should probably switch to something like getrusage instead.
My code used enough CPU time. But it seems the clock is only ticking with a step of 15625ms.
My advice is to use <chrono> instead.
Related
How can one multiply a chrono timepoint by a scalar? It works for durations, but timepoints can't be multiplied by a scalar ("error: invalid operands to binary expression").
Context:
I have some code that in real life will run for a long time. For testing purposes, I want to be able to speed it up by a factor, so everything happens similarly, but just in fast forward.
I thought of making my own ScaledClock class, that returns values from chrono::steady_clock, but with a scaling parameter that can be set to something greater than 1 to achieve a speed up. Here is some code:
steady_clock::time_point ScaledClock::now() {
return steady_clock::now() * speedUp; // <--- error
}
void ScaledClock::sleep_for(steady_clock::duration duration) {
std::this_thread::sleep_for(duration / speedUp);
}
void ScaledClock::sleep_until(steady_clock::time_point time) {
std::this_thread::sleep_until(time / speedUp); // <--- error
}
If the speedUp is 2, for instance, then the program will always think that twice as much time has passed. It will also sleep for half as long. As long as I am disciplined about not using this class for all timing, I think it should work.
(Alternatively, if someone has a much better way of achieving this, I'd love to hear it).
Edit: copy of comment, because I think it is useful clarification:
en.cppreference.com/w/cpp/chrono/time_point:
Class template std::chrono::time_point represents a point in time. It
is implemented as if it stores a value of type Duration indicating the
time interval from the start of the Clock's epoch.
So I want all the times since the epoch doubled. If the epoch is not start of program execution, and my code happens to think that it is running in 4036, I'm not really bothered
You will need to store a starting point (now() e.g. at program start) and then determine the time passed since that starting point as a duration. You can then add this duration multiplied with your factor to the start point and return it as time point in your ScaledClock::now() function. Just like this:
#include <chrono>
#include <unistd.h>
int main() {
auto start = std::chrono::steady_clock::now();
sleep(1);
auto actualNow = std::chrono::steady_clock::now();
auto timePassed = actualNow - start;
auto timePassedScaled = timePassed * 2.0;
auto scaledNow = start + timePassedScaled;
return 0;
}
First and foremost, let me say that I just starting using this library yesterday, so my understanding of it is still fairly basic. I'm trying to capture the FPS of a vision processing program I'm creating and output it to a screen using the chrono library. In my case, I need to cast the elapsed time taken after I start a steady_clock to a double (or some other numerical typedef I could treat like a double). I looked through reference documentation and tried working with the duration_cast and time_point_cast functions, but neither of those seem to be what I'm looking for.
My question is; is there any way to simply cast the numerical value of a clock's current state in seconds to a primitive data type?
Any help would be appreciated.
Like this:
#include <chrono>
#include <iostream>
#include <thread>
int main()
{
using namespace std::literals;
// measure time now
auto start = std::chrono::system_clock::now();
// wait some time
std::this_thread::sleep_for(1s);
// measure time again
auto end = std::chrono::system_clock::now();
// define a double-precision representation of seconds
using fsecs = std::chrono::duration<double, std::chrono::seconds::period>;
// convert from clock's duration type
auto as_fseconds = std::chrono::duration_cast<fsecs>(end - start);
// display as decimal seconds
std::cout << "duration was " << as_fseconds.count() << "s\n";
}
example output:
duration was 1.00006s
You could do it using the duration::count function.
For example you could get the duration in the number of milliseconds, and then divide the count by 1000.0 to get the number of seconds as a double.
Am I doing it correctly? At times, my program will print 2000+ for the chrono solution and it always prints 1000 for the CLOCKS_PER_SEC..
What is that value I'm actually calculating? Is it Clocks Per Sec?
#include <iostream>
#include <chrono>
#include <thread>
#include <ctime>
std::chrono::time_point<std::chrono::high_resolution_clock> SystemTime()
{
return std::chrono::high_resolution_clock::now();
}
std::uint32_t TimeDuration(std::chrono::time_point<std::chrono::high_resolution_clock> Time)
{
return std::chrono::duration_cast<std::chrono::nanoseconds>(SystemTime() - Time).count();
}
int main()
{
auto Begin = std::chrono::high_resolution_clock::now();
std::this_thread::sleep_for(std::chrono::milliseconds(1));
std::cout<< (TimeDuration(Begin) / 1000.0)<<std::endl;
std::cout<<CLOCKS_PER_SEC;
return 0;
}
In order to get the correct ticks per second on Linux, you need to use the return value of ::sysconf(_SC_CLK_TCK) (declared in the header unistd.h), rather than the macro CLOCKS_PER_SEC.
The latter is a constant defined in the POSIX standard – it is unrelated to the actual ticks per second of your CPU clock. For example, see the man page for clock:
C89, C99, POSIX.1-2001. POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.
However, note that even when using the correct ticks-per-second constant, you still won't get the number of actual CPU cycles per second. "Clock tick" is a special unit used by the CPU clock. There is no standardized definition of how it relates to actual CPU cycles.
In boost's library, there is a timer class, use CLOCKS_PER_SEC to calculate the maximum time the timer can elapse. It said that on Windows CLOCKS_PER_SEC is 1000 and on Mac OS X, Linux it is 1000000. So on the latter OSs, the accuracy is higher.
Is there cross-platform solution to get seconds since epoch, for windows i use
long long NativesGetTimeInSeconds()
{
return time (NULL);
}
But how to get on Linux?
You're already using it: std::time(0) (don't forget to #include <ctime>). However, whether std::time actually returns the time since epoch isn't specified in the standard (C11, referenced by the C++ standard):
7.27.2.4 The time function
Synopsis
#include <time.h>
time_t time(time_t *timer);
Description
The time function determines the current calendar time. The encoding of the value is unspecified. [emphasis mine]
For C++, C++11 and later provide time_since_epoch. However, before C++20 the epoch of std::chrono::system_clock was unspecified and therefore possibly non-portable in previous standards.
Still, on Linux the std::chrono::system_clock will usually use Unix Time even in C++11, C++14 and C++17, so you can use the following code:
#include <chrono>
// make the decltype slightly easier to the eye
using seconds_t = std::chrono::seconds;
// return the same type as seconds.count() below does.
// note: C++14 makes this a lot easier.
decltype(seconds_t().count()) get_seconds_since_epoch()
{
// get the current time
const auto now = std::chrono::system_clock::now();
// transform the time into a duration since the epoch
const auto epoch = now.time_since_epoch();
// cast the duration into seconds
const auto seconds = std::chrono::duration_cast<std::chrono::seconds>(epoch);
// return the number of seconds
return seconds.count();
}
In C.
time(NULL);
In C++.
std::time(0);
And the return value of time is : time_t not long long
The native Linux function for getting time is gettimeofday() [there are some other flavours too], but that gets you the time in seconds and nanoseconds, which is more than you need, so I would suggest that you continue to use time(). [Of course, time() is implemented by calling gettimeofday() somewhere down the line - but I don't see the benefit of having two different pieces of code that does exactly the same thing - and if you wanted that, you'd be using GetSystemTime() or some such on Windows [not sure that's the right name, it's been a while since I programmed on Windows]
The Simple, Portable, and Proper Approach
#include <ctime>
long CurrentTimeInSeconds()
{
return (long)std::time(0); //Returns UTC in Seconds
}
I am trying to add a timed delay in a C++ program, and was wondering if anyone has any suggestions on what I can try or information I can look at?
I wish I had more details on how I am implementing this timed delay, but until I have more information on how to add a timed delay I am not sure on how I should even attempt to implement this.
An updated answer for C++11:
Use the sleep_for and sleep_until functions:
#include <chrono>
#include <thread>
int main() {
using namespace std::this_thread; // sleep_for, sleep_until
using namespace std::chrono; // nanoseconds, system_clock, seconds
sleep_for(nanoseconds(10));
sleep_until(system_clock::now() + seconds(1));
}
With these functions there's no longer a need to continually add new functions for better resolution: sleep, usleep, nanosleep, etc. sleep_for and sleep_until are template functions that can accept values of any resolution via chrono types; hours, seconds, femtoseconds, etc.
In C++14 you can further simplify the code with the literal suffixes for nanoseconds and seconds:
#include <chrono>
#include <thread>
int main() {
using namespace std::this_thread; // sleep_for, sleep_until
using namespace std::chrono_literals; // ns, us, ms, s, h, etc.
using std::chrono::system_clock;
sleep_for(10ns);
sleep_until(system_clock::now() + 1s);
}
Note that the actual duration of a sleep depends on the implementation: You can ask to sleep for 10 nanoseconds, but an implementation might end up sleeping for a millisecond instead, if that's the shortest it can do.
In Win32:
#include<windows.h>
Sleep(milliseconds);
In Unix:
#include<unistd.h>
unsigned int microsecond = 1000000;
usleep(3 * microsecond);//sleeps for 3 second
sleep() only takes a number of seconds which is often too long.
#include <unistd.h>
usleep(3000000);
This will also sleep for three seconds. You can refine the numbers a little more though.
Do you want something as simple like:
#include <unistd.h>
sleep(3);//sleeps for 3 second
Note that this does not guarantee that the amount of time the thread sleeps will be anywhere close to the sleep period, it only guarantees that the amount of time before the thread continues execution will be at least the desired amount. The actual delay will vary depending on circumstances (especially load on the machine in question) and may be orders of magnitude higher than the desired sleep time.
Also, you don't list why you need to sleep but you should generally avoid using delays as a method of synchronization.
You can try this code snippet:
#include<chrono>
#include<thread>
int main(){
std::this_thread::sleep_for(std::chrono::nanoseconds(10));
std::this_thread::sleep_until(std::chrono::system_clock::now() + std::chrono::seconds(1));
}
You can also use select(2) if you want microsecond precision (this works on platform that don't have usleep(3))
The following code will wait for 1.5 second:
#include <sys/select.h>
#include <sys/time.h>
#include <unistd.h>`
int main() {
struct timeval t;
t.tv_sec = 1;
t.tv_usec = 500000;
select(0, NULL, NULL, NULL, &t);
}
`
I found that "_sleep(milliseconds);" (without the quotes) works well for Win32 if you include the chrono library
E.g:
#include <chrono>
using namespace std;
main
{
cout << "text" << endl;
_sleep(10000); // pauses for 10 seconds
}
Make sure you include the underscore before sleep.
Yes, sleep is probably the function of choice here. Note that the time passed into the function is the smallest amount of time the calling thread will be inactive. So for example if you call sleep with 5 seconds, you're guaranteed your thread will be sleeping for at least 5 seconds. Could be 6, or 8 or 50, depending on what the OS is doing. (During optimal OS execution, this will be very close to 5.) Another useful feature of the sleep function is to pass in 0. This will force a context switch from your thread.
Some additional information:
http://www.opengroup.org/onlinepubs/000095399/functions/sleep.html
The top answer here seems to be an OS dependent answer; for a more portable solution you can write up a quick sleep function using the ctime header file (although this may be a poor implementation on my part).
#include <iostream>
#include <ctime>
using namespace std;
void sleep(float seconds){
clock_t startClock = clock();
float secondsAhead = seconds * CLOCKS_PER_SEC;
// do nothing until the elapsed time has passed.
while(clock() < startClock+secondsAhead);
return;
}
int main(){
cout << "Next string coming up in one second!" << endl;
sleep(1.0);
cout << "Hey, what did I miss?" << endl;
return 0;
}
to delay output in cpp for fixed time, you can use the Sleep() function by including windows.h header file
syntax for Sleep() function is Sleep(time_in_ms)
as
cout<<"Apple\n";
Sleep(3000);
cout<<"Mango";
OUTPUT. above code will print Apple and wait for 3 seconds before printing Mango.
Syntax:
void sleep(unsigned seconds);
sleep() suspends execution for an interval (seconds).
With a call to sleep, the current program is suspended from execution for the number of seconds specified by the argument seconds. The interval is accurate only to the nearest hundredth of a second or to the accuracy of the operating system clock, whichever is less accurate.
Many others have provided good info for sleeping. I agree with Wedge that a sleep seldom the most appropriate solution.
If you are sleeping as you wait for something, then you are better off actually waiting for that thing/event. Look at Condition Variables for this.
I don't know what OS you are trying to do this on, but for threading and synchronisation you could look to the Boost Threading libraries (Boost Condition Varriable).
Moving now to the other extreme if you are trying to wait for exceptionally short periods then there are a couple of hack style options. If you are working on some sort of embedded platform where a 'sleep' is not implemented then you can try a simple loop (for/while etc) with an empty body (be careful the compiler does not optimise it away). Of course the wait time is dependant on the specific hardware in this case.
For really short 'waits' you can try an assembly "nop". I highly doubt these are what you are after but without knowing why you need to wait it's hard to be more specific.
On Windows you can include the windows library and use "Sleep(0);" to sleep the program. It takes a value that represents milliseconds.