Easily measure elapsed time

Question

I am trying to use time   to measure various points of my program   What I don t understand is why the values in the before and after are the same   I understand this is not the best way to profile my program  I just want to see how long something take     printf    MyProgram  before time   ld n   time NULL     doSomthing    doSomthingLong     printf    MyProgram  after time   ld n   time NULL      I have tried   struct timeval diff  startTV  endTV   gettimeofday  amp startTV  NULL     doSomething    doSomethingLong     gettimeofday  amp endTV  NULL     timersub  amp endTV   amp startTV   amp diff    printf    time taken    ld  ld n   diff tv sec  diff tv usec     How do I read a result of   time taken   0 26339   Does that mean 26 339 nanoseconds   26 3 msec   What about   time taken   4 45025  does that mean 4 seconds and 25 msec

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I have created a class to automatically measure elapsed time, Please check the code (c++11) in this link: https://github.com/sonnt174/Common/blob/master/time_measure.h

Example of how to use class TimeMeasure:

void test_time_measure(std::vector<int> arr) {
  TimeMeasure<chrono::microseconds> time_mea;  // create time measure obj
  std::sort(begin(arr), end(arr));
}

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I needed to measure the execution time of individual functions within a library  I didn t want to have to wrap every call of every function with a time measuring function because its ugly and deepens the call stack  I also didn t want to put timer code at the top and bottom of every function because it makes a mess when the function can exit early or throw exceptions for example  So what I ended up doing was making a timer that uses its own lifetime to measure time   In this way I can measure the wall-time a block of code took by just instantiating one of these objects at the beginning of the code block in question  function or any scope really  and then allowing the instances destructor to measure the time elapsed since construction when the instance goes out of scope  You can find the full example here but the struct is extremely simple   template  lt typename clock t   std  chrono  steady clock gt  struct scoped timer     using duration t   typename clock t  duration    const std  function lt void const duration t amp   gt  callback    const std  chrono  time point lt clock t gt  start     scoped timer const std  function lt void const duration t amp   gt  amp  finished callback          callback finished callback   start clock t  now          scoped timer std  function lt void const duration t amp   gt  amp  amp  finished callback          callback finished callback   start clock t  now           scoped timer     callback clock t  now   - start          The struct will call you back on the provided functor when it goes out of scope so you can do something with the timing information  print it or store it or whatever   If you need to do something even more complex you could even use std  bind with std  placeholders to callback functions with more arguments   Here s a quick example of  using it   void test bool should throw      scoped timer lt  gt  t    const scoped timer lt  gt   duration t amp  elapsed        auto e   std  chrono  duration cast lt std  chrono  duration lt double  std  milli gt  gt  elapsed  count        std  cout  lt  lt   took    lt  lt  e  lt  lt   ms   lt  lt  std  endl           std  this thread  sleep for std  chrono  seconds 1       if  should throw      throw nullptr     std  this thread  sleep for std  chrono  seconds 1        If you want to be more deliberate  you can also use new and delete to explicitly start and stop the timer without relying on scoping to do it for you

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They are they same because your doSomething function happens faster than the granularity of the timer  Try   printf     MyProgram  before time   ld n   time NULL     for i   0  i  lt  1000    i        doSomthing        doSomthingLong       printf     MyProgram  after time   ld n   time NULL

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C   std  chrono has a clear benefit of being cross-platform  However  it also introduces a significant overhead compared to POSIX clock gettime    On my Linux box all std  chrono  xxx clock  now   flavors perform roughly the same   std  chrono  system clock  now   std  chrono  steady clock  now   std  chrono  high resolution clock  now     Though POSIX clock gettime CLOCK MONOTONIC   amp time  should be same as steady clock  now   but it is more than x3 times faster   Here is my test  for completeness    include  lt stdio h gt   include  lt chrono gt   include  lt ctime gt   void print timediff const char  prefix  const struct timespec amp  start  const  struct timespec amp  end        double milliseconds   end tv nsec  gt   start tv nsec                            end tv nsec - start tv nsec    1e6    end tv sec - start tv sec    1e3                            start tv nsec - end tv nsec    1e6    end tv sec - start tv sec - 1    1e3      printf   s   lf milliseconds n   prefix  milliseconds      int main         int i  n   1000000      struct timespec start  end          Test stopwatch     clock gettime CLOCK MONOTONIC   amp start       for  i   0  i  lt  n    i            struct timespec dummy          clock gettime CLOCK MONOTONIC   amp dummy             clock gettime CLOCK MONOTONIC   amp end       print timediff  clock gettime   start  end           Test chrono system clock     clock gettime CLOCK MONOTONIC   amp start       for  i   0  i  lt  n    i          auto dummy   std  chrono  system clock  now        clock gettime CLOCK MONOTONIC   amp end       print timediff  chrono  system clock  now   start  end           Test chrono steady clock     clock gettime CLOCK MONOTONIC   amp start       for  i   0  i  lt  n    i          auto dummy   std  chrono  steady clock  now        clock gettime CLOCK MONOTONIC   amp end       print timediff  chrono  steady clock  now   start  end           Test chrono high resolution clock     clock gettime CLOCK MONOTONIC   amp start       for  i   0  i  lt  n    i          auto dummy   std  chrono  high resolution clock  now        clock gettime CLOCK MONOTONIC   amp end       print timediff  chrono  high resolution clock  now   start  end        return 0      And this is the output I get when compiled with gcc7 2 -O3   clock gettime  24 484926 milliseconds chrono  system clock  now  85 142108 milliseconds chrono  steady clock  now  87 295347 milliseconds chrono  high resolution clock  now  84 437838 milliseconds

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include  lt ctime gt   include  lt functional gt   using namespace std   void f       clock t begin   clock             code to measure time       clock t end   clock       function lt double double  double  gt  convtime      clock t begin  clock t end           return double end - begin    CLOCKS PER SEC          printf  Elapsed time    2g sec n   convtime begin  end         Similar example to one available here  only with additional conversion function   print out

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On linux  clock gettime   is one of the good choices  You must link real time library -lrt     include  lt stdio h gt   include  lt unistd h gt   include  lt stdlib h gt   include  lt time h gt    define BILLION  1000000000L   int main  int argc  char   argv           struct timespec start  stop      double accum       if  clock gettime  CLOCK REALTIME   amp start     -1           perror   clock gettime           exit  EXIT FAILURE               system  argv 1          if  clock gettime  CLOCK REALTIME   amp stop     -1           perror   clock gettime           exit  EXIT FAILURE               accum     stop tv sec - start tv sec                 stop tv nsec - start tv nsec                 BILLION      printf    lf n   accum        return  EXIT SUCCESS

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In answer to OP s three specific questions    What I don t understand is why the values in the before and after are the same    The first question and sample code shows that time   has a resolution of 1 second  so the answer has to be that the two functions execute in less than 1 second  But occasionally it will  apparently illogically  inform 1 second if the two timer marks straddle a one second boundary   The next example uses gettimeofday   which fills this struct  struct timeval       time t      tv sec         seconds        suseconds t tv usec        microseconds         and the second question asks   How do I read a result of   time taken   0 26339  Does that mean 26 339 nanoseconds   26 3 msec    My second answer is the time taken is 0 seconds and 26339 microseconds  that is 0 026339 seconds  which bears out the first example executing in less than 1 second   The third question asks   What about   time taken   4 45025  does that mean 4 seconds and 25 msec    My third answer is the time taken is 4 seconds and 45025 microseconds  that is 4 045025 seconds  which shows that OP has altered the tasks performed by the two functions which he previously timed

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include  lt ctime gt   void f       using namespace std    clock t begin   clock       code to time       clock t end   clock      double elapsed secs   double end - begin    CLOCKS PER SEC      The time   function is only accurate to within a second  but there are CLOCKS PER SEC  clocks  within a second   This is an easy  portable measurement  even though it s over-simplified

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Matlab flavored   tic starts a stopwatch timer to measure performance  The function records the internal time at execution of the tic command  Display the elapsed time with the toc function    include  lt iostream gt   include  lt ctime gt   include  lt thread gt  using namespace std   clock t START TIMER   clock t tic         return START TIMER   clock       void toc clock t start   START TIMER        cout          lt  lt   Elapsed time             lt  lt   clock   - start     double CLOCKS PER SEC  lt  lt   s           lt  lt  endl     int main         tic        this thread  sleep for 2s       toc         return 0

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You can abstract the time measuring mechanism and have each callable s run time measured with minimal extra code  just by being called through a timer structure  Plus  at compile time you can parametrize the timing type  milliseconds  nanoseconds etc     Thanks to the review by Loki Astari and the suggestion to use variadic templates  This is why the forwarded function call    include  lt iostream gt   include  lt chrono gt   template lt typename TimeT   std  chrono  milliseconds gt  struct measure       template lt typename F  typename    Args gt      static typename TimeT  rep execution F amp  amp  func  Args amp  amp     args                auto start   std  chrono  steady clock  now            std  forward lt decltype func  gt  func  std  forward lt Args gt  args               auto duration   std  chrono  duration cast lt  TimeT gt                                std  chrono  steady clock  now   - start           return duration count              int main         std  cout  lt  lt  measure lt  gt   execution functor dummy    lt  lt  std  endl      Demo  According to the comment by Howard Hinnant it s best not to escape out of the chrono system until we have to  So the above class could give the user the choice to call count manually by providing an extra static method  shown in C  14   template lt typename F  typename    Args gt  static auto duration F amp  amp  func  Args amp  amp     args        auto start   std  chrono  steady clock  now        std  forward lt decltype func  gt  func  std  forward lt Args gt  args           return std  chrono  duration cast lt TimeT gt  std  chrono  steady clock  now  -start          call  count   manually later when needed  eg IO  auto avg    measure lt  gt   duration func    measure lt  gt   duration func     2 0    and be most useful for clients that       want to post-process a bunch of durations prior to I O  e g  average       The complete code can be found here  My attempt to build a benchmarking tool based on chrono is recorded here     If C  17 s std  invoke is available  the invocation of the callable in execution could be done like this     invoke forward lt decltype func  gt  func   forward lt Args gt  args         to provide for callables that are pointers to member functions

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struct profiler       std  string name      std  chrono  high resolution clock  time point p      profiler std  string const  amp n            name n   p std  chrono  high resolution clock  now             profiler                 using dura   std  chrono  duration lt double gt           auto d   std  chrono  high resolution clock  now   - p          std  cout  lt  lt  name  lt  lt                    lt  lt  std  chrono  duration cast lt dura gt  d  count                lt  lt  std  endl             define PROFILE BLOCK pbn  profiler  pfinstance pbn    Usage is below           PROFILE BLOCK  Some time           your code or function     THis is similar to RAII in scope  NOTE this is not mine  but i thought it was relevant here

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From what is see  tv sec stores the seconds elapsed while tv usec stored the microseconds elapsed separately  And they aren t the conversions of each other  Hence  they must be changed to proper unit and added to get the total time elapsed   struct timeval startTV  endTV   gettimeofday  amp startTV  NULL     doSomething    doSomethingLong     gettimeofday  amp endTV  NULL     printf    time taken in microseconds    ld n        endTV tv sec   1e6   endTV tv usec -  startTV tv sec   1e6   startTV tv usec

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include lt time h gt     for clock  include lt math h gt     for fmod  include lt cstdlib gt    for system  include  lt stdio h gt    for delay  using namespace std   int main          clock t t1 t2      t1 clock       first time capture        Now your time spanning loop or code goes here       i am first trying to display time elapsed every time loop runs     int ddays 0     d prefix is just to say that this variable will be used for display    int dhh 0     int dmm 0     int dss 0      int loopcount   1000      just for demo your loop will be different of course     for float count 1 count lt loopcount count               t2 clock       we get the time now       float difference     float t2 -  float t1       gives the time elapsed since t1 in milliseconds         now get the time elapsed in seconds      float seconds   difference 1000     float value of seconds     if  seconds lt  60 60 24      a day is not over               dss   fmod seconds 60      the remainder is seconds to be displayed         float minutes  seconds 60      the total minutes in float         dmm  fmod minutes 60       the remainder are minutes to be displayed         float hours  minutes 60     the total hours in float         dhh  hours      the hours to be displayed         ddays 0            else    we have reached the counting of days               float days   seconds  24 60 60           ddays    int  days           float minutes  seconds 60      the total minutes in float         dmm  fmod minutes 60       the rmainder are minutes to be displayed         float hours  minutes 60     the total hours in float         dhh  fmod  hours 24       the hours to be displayed             cout lt  lt  Count Is     lt  lt count lt  lt  Time Elapsed     lt  lt ddays lt  lt   Days   lt  lt dhh lt  lt   hrs   lt  lt dmm lt  lt   mins   lt  lt dss lt  lt   secs            the actual working code here I have just put a delay function     delay 1000       system  cls           end for loop      end of main

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I usually use the following    include  lt chrono gt   include  lt type traits gt   using perf clock   std  conditional lt      std  chrono  high resolution clock  is steady      std  chrono  high resolution clock      std  chrono  steady clock  gt   type   using floating seconds   std  chrono  duration lt double gt    template lt class F  class    Args gt  floating seconds run test Func amp  amp  func  Args amp  amp     args       const auto t0   perf clock  now       std  forward lt Func gt  func  std  forward lt Args gt  args          return floating seconds perf clock  now   - t0        It s the same as  nikos-athanasiou proposed except that I avoid using of a non-steady clock and use floating number of seconds as a duration

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Windows only   The Linux tag was added after I posted this answer   You can use GetTickCount   to get the number of milliseconds that have elapsed since the system was started   long int before   GetTickCount        Perform time-consuming operation  long int after   GetTickCount

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As I can see from your question  it looks like you want to know the elapsed time after execution of some piece of code  I guess you would be comfortable to see the results in second s   If so  try using difftime   function as shown below  Hope this solves your problem       include  lt time h gt   include  lt stdio h gt   time t start end  time   amp start          lt your code gt        time   amp end   double dif   difftime  end start   printf   Elasped time is   2lf seconds    dif

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C  11 Style      include  lt chrono gt   std  chrono  steady clock  time point begin   std  chrono  steady clock  now    std  chrono  steady clock  time point end   std  chrono  steady clock  now     std  cout  lt  lt   Time difference      lt  lt  std  chrono  duration cast lt std  chrono  microseconds gt  end - begin  count    lt  lt      s    lt  lt  std  endl  std  cout  lt  lt   Time difference      lt  lt  std  chrono  duration cast lt std  chrono  nanoseconds gt   end - begin  count    lt  lt    ns    lt  lt  std  endl

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You can use SFML library  which is Simple and Fast Multimedia Library  It includes many useful and well-defined classes like Clock  Socket  Sound  Graphics  etc  It s so easy to use and highly recommended   This is an example for this question   sf  Clock clock      Time time1   clock getElapsedTime        Time time2   clock restart

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include  lt ctime gt   include  lt cstdio gt   include  lt iostream gt   include  lt chrono gt   include  lt sys time h gt  using namespace std  using namespace std  chrono   void f1       high resolution clock  time point t1   high resolution clock  now      high resolution clock  time point t2   high resolution clock  now      double dif   duration cast lt nanoseconds gt   t2 - t1   count      printf   Elasped time is  lf nanoseconds  n   dif       void f2       timespec ts1 ts2    clock gettime CLOCK REALTIME   amp ts1     clock gettime CLOCK REALTIME   amp ts2     double dif   double  ts2 tv nsec - ts1 tv nsec      printf   Elasped time is  lf nanoseconds  n   dif       void f3       struct timeval t1 t0    gettimeofday  amp t0  0     gettimeofday  amp t1  0     double dif   double   t1 tv usec-t0 tv usec  1000     printf   Elasped time is  lf nanoseconds  n   dif      void f4       high resolution clock  time point t1   t2    double diff   0    t1   high resolution clock  now       for int i   1  i  lt   10   i            t2   high resolution clock  now         diff   duration cast lt nanoseconds gt   t2 - t1   count        t1   t2        printf   high resolution clock   Elasped time is  lf nanoseconds  n   diff 10       void f5       timespec ts1 ts2    double diff   0    clock gettime CLOCK REALTIME   amp ts1     for int i   1  i  lt   10   i            clock gettime CLOCK REALTIME   amp ts2       diff   double  ts2 tv nsec - ts1 tv nsec        ts1   ts2        printf   clock gettime   Elasped time is  lf nanoseconds  n   diff 10       void f6       struct timeval t1 t2    double diff   0    gettimeofday  amp t1  0     for int i   1  i  lt   10   i            gettimeofday  amp t2  0       diff   double   t2 tv usec-t1 tv usec  1000       t1   t2        printf   gettimeofday   Elasped time is  lf nanoseconds  n   diff 10       int main           f1          f2          f3      f6      f4      f5      return 0

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The reason both values are the same is because your long procedure doesn t take that long - less than one second  You can try just adding a long loop  for  int i   0  i  lt  100000000  i        at the end of the function to make sure this is the issue  then we can go from there     In case the above turns out to be true  you will need to find a different system function  I understand you work on linux  so I can t help you with the function name  to measure time more accurately  I am sure there is a function simular to GetTickCount   in linux  you just need to find it

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As others have already noted  the time   function in the C standard library does not have a resolution better than one second  The only fully portable C function that may provide better resolution appears to be clock    but that measures processor time rather than wallclock time  If one is content to limit oneself to POSIX platforms  e g  Linux   then the clock gettime   function is a good choice   Since C  11  there are much better timing facilities available that offer better resolution in a form that should be very portable across different compilers and operating systems  Similarly  the boost  datetime library provides good high-resolution timing classes that should be highly portable   One challenge in using any of these facilities is the time-delay introduced by querying the system clock  From experimenting with clock gettime    boost  datetime and std  chrono  this delay can easily be a matter of microseconds  So  when measuring the duration of any part of your code  you need to allow for there being a measurement error of around this size  or try to correct for that zero-error in some way  Ideally  you may well want to gather multiple measurements of the time taken by your function  and compute the average  or maximum minimum time taken across many runs   To help with all these portability and statistics-gathering issues  I ve been developing the cxx-rtimers library available on Github which tries to provide a simple API for timing blocks of C   code  computing zero errors  and reporting stats from multiple timers embedded in your code  If you have a C  11 compiler  you simply  include  lt rtimers cxx11 hpp gt   and use something like   void expensiveFunction         static rtimers  cxx11  DefaultTimer timer  expensiveFunc        auto scopedStartStop   timer scopedStart           Do something costly        On program exit  you ll get a summary of timing stats written to std  cerr such as   Timer expensiveFunc    lt t gt    6 65289us  std   3 91685us  3 842us  lt   t  lt   63 257us  n 731    which shows the mean time  its standard-deviation  the upper and lower limits  and the number of times this function was called   If you want to use Linux-specific timing functions  you can  include  lt rtimers posix hpp gt   or if you have the Boost libraries but an older C   compiler  you can  include  lt rtimers boost hpp gt   There are also versions of these timer classes that can gather statistical timing information from across multiple threads  There are also methods that allow you to estimate the zero-error associated with two immediately consecutive queries of the system clock

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Internally the function will access the system s clock  which is why it returns different values each time you call it  In general with non-functional languages there can be many side effects and hidden state in functions which you can t see just by looking at the function s name and arguments

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The values printed by your second program are seconds  and microseconds   0 26339   0 026 339 s     26339   s 4 45025   4 045 025 s   4045025   s

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The time NULL  function call will return the number of seconds elapsed since epoc  January 1 1970  Perhaps what you mean to do is take the difference between two timestamps   size t start   time NULL   doSomthing    doSomthingLong     printf     MyProgram  time elapsed   lds n   time NULL  - start

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time NULL  returns the number of seconds elapsed since 01 01 1970 at 00 00  the Epoch   So the difference between the two values is the number of seconds your processing took    int t0   time NULL   doSomthing    doSomthingLong    int t1   time NULL    printf   time    d secs n   t1 - t0     You can get finer results with getttimeofday    which return the current time in seconds  as time   does and also in microseconds

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the time NULL  function will return the number of seconds elapsed since 01 01 1970 at 00 00  And because  that function is called at different time in your program  it will always be different Time in C

[c++] Easily measure elapsed time

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