C obtaining milliseconds time on Linux -- clock doesn t seem to work properly

Question

On Windows  clock   returns the time in milliseconds  but on this Linux box I m working on  it rounds it to the nearest 1000 so the precision is only to the  second  level and not to the milliseconds level   I found a solution with Qt using the QTime class  instantiating an object and calling start   on it then calling elapsed   to get the number of milliseconds elapsed   I got kind of lucky because I m working with Qt to begin with  but I d like a solution that doesn t rely on third party libraries     Is there no standard way to do this   UPDATE  Please don t recommend Boost     If Boost and Qt can do it  surely it s not magic  there must be something standard that they re using

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clock   doesn t return milliseconds or seconds on linux   Usually clock   returns microseconds on a linux system   The proper way to interpret the value returned by clock   is to divide it by CLOCKS PER SEC to figure out how much time has passed

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I also recommend the tools offered by Boost  Either the mentioned Boost Timer  or hack something out of Boost DateTime or there is new proposed library in the sandbox - Boost Chrono  This last one will be a replacement for the Timer and will feature    The C  0x Standard Library s time utilities  including    Class template duration Class template time point Clocks    system clock monotonic clock high resolution clock   Class template timer  with typedefs    system timer monotonic timer high resolution timer  Process clocks and timers    process clock  capturing real  user-CPU  and system-CPU times  process timer  capturing elapsed real  user-CPU  and system-CPU times  run timer  convenient reporting of  process timer  results   The C  0x Standard Library s compile-time rational arithmetic    Here is the source of the feature list

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include  lt sys time h gt   include  lt stdio h gt   include  lt unistd h gt  int main         struct timeval start  end       long mtime  seconds  useconds           gettimeofday  amp start  NULL       usleep 2000       gettimeofday  amp end  NULL        seconds    end tv sec  - start tv sec      useconds   end tv usec - start tv usec       mtime     seconds    1000   useconds 1000 0    0 5       printf  Elapsed time   ld milliseconds n   mtime        return 0

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This should work   tested on a mac      include  lt stdio h gt   include  lt sys time h gt   int main             struct timeval tv          struct timezone tz          struct tm  tm          gettimeofday  amp tv  amp tz           tm localtime  amp tv tv sec           printf  StartTime   d  02d  02d  d  n   tm- gt tm hour  tm- gt tm min  tm- gt tm sec  tv tv usec       Yeah   run it twice and subtract

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I ve written a Timer class based on CTT s answer  It can be used in the following way   Timer timer   Timer    timer start       perform task    double duration   timer stop    timer printTime duration     Here is its implementation    include  lt stdio h gt   include  lt stdlib h gt   include  lt sys time h gt  using namespace std   class Timer   private       timeval startTime   public       void start            gettimeofday  amp startTime  NULL              double stop            timeval endTime          long seconds  useconds          double duration           gettimeofday  amp endTime  NULL            seconds    endTime tv sec  - startTime tv sec          useconds   endTime tv usec - startTime tv usec           duration   seconds   useconds 1000000 0           return duration             static void printTime double duration           printf   5 6f seconds n   duration

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Please note that clock does not measure wall clock time  That means if your program takes 5 seconds  clock will not measure 5 seconds necessarily  but could more  your program could run multiple threads and so could consume more CPU than real time  or less  It measures an approximation of CPU time used  To see the difference consider this code   include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   int main         std  clock t a   std  clock        sleep 5      sleep 5s     std  clock t b   std  clock         std  cout  lt  lt   difference     lt  lt   b - a   lt  lt  std  endl      return 0      It outputs on my system    difference  0   Because all we did was sleeping and not using any CPU time  However  using gettimeofday we get what we want       include  lt iostream gt   include  lt ctime gt   include  lt unistd h gt   include  lt sys time h gt   int main         timeval a      timeval b       gettimeofday  amp a  0       sleep 5      sleep 5s     gettimeofday  amp b  0        std  cout  lt  lt   difference     lt  lt   b tv sec - a tv sec   lt  lt  std  endl      return 0      Outputs on my system    difference  5   If you need more precision but want to get CPU time  then you can consider using the getrusage function

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I like the Hola Soy method of not using gettimeofday    It happened to me on a running server the admin changed the timezone  The clock was updated to show the same  correct  local value  This caused the function time   and gettimeofday    to shift 2 hours and all timestamps in some services got stuck

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In the POSIX standard clock has its return value defined in terms of the CLOCKS PER SEC symbol and an implementation is free to define this in any convenient fashion   Under Linux  I have had good luck with the times   function

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I wrote a C   class using timeb     include  lt sys timeb h gt  class msTimer    public      msTimer        void restart        float elapsedMs    private      timeb t start       Member functions   msTimer  msTimer           restart        void msTimer  restart           ftime  amp t start       float msTimer  elapsedMs          timeb t now      ftime  amp t now       return  float  t now time - t start time    1000 0f               float  t now millitm - t start millitm       Example of use    include  lt cstdlib gt   include  lt iostream gt   using namespace std   int main int argc  char   argv         msTimer t      for  int i   0  i  lt  5000000  i                  std  cout  lt  lt  t elapsedMs    lt  lt  endl      return 0      Output on my computer is  19   Accuracy of the msTimer class is of the order of milliseconds  In the usage example above  the total time of execution taken up by the for-loop is tracked  This time included the operating system switching in and out the execution context of main   due to multitasking

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You could use gettimeofday at the start and end of your method and then difference the two return structs   You ll get a structure like the following   struct timeval     time t tv sec    suseconds t tv usec      EDIT  As the two comments below suggest  clock gettime CLOCK MONOTONIC  is a much better choice if you have it available  which should be almost everywhere these days   EDIT  Someone else commented that you can also use modern C   with std  chrono  high resolution clock  but that isn t guaranteed to be monotonic  Use steady clock instead

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gettimeofday - the problem is that will can have lower values if you change you hardware clock  with NTP for example  Boost - not available for this project clock   - usually returns a 4 bytes integer  wich means that its a low capacity  and after some time it returns negative numbers   I prefer to create my own class and update each 10 miliseconds  so this way is more flexible  and I can even improve it to have subscribers   class MyAlarm   static int64 t tiempo  static bool running  public  static int64 t getTime    return tiempo    static void callback  int sig       if running           tiempo  10L          static void run    running   true       int64 t MyAlarm  tiempo   0L  bool MyAlarm  running   false    to refresh it I use setitimer   int main    struct sigaction sa   struct itimerval timer    MyAlarm  run    memset   amp sa  0  sizeof  sa     sa sa handler    amp MyAlarm  callback    sigaction  SIGALRM   amp sa  NULL      timer it value tv sec   0   timer it value tv usec   10000      timer it interval tv sec   0   timer it interval tv usec   10000     setitimer  ITIMER REAL   amp timer  NULL            Look at setitimer and the ITIMER VIRTUAL and ITIMER REAL   Don t use the alarm or ualarm functions  you will have low precision when your process get a hard work

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I prefer the Boost Timer library for its simplicity  but if you don t want to use third-parrty libraries  using clock   seems reasonable

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As an update appears that on windows clock   measures wall clock time  with CLOCKS PER SEC precision    http   msdn microsoft com en-us library 4e2ess30 VS 71  aspx   while on Linux it measures cpu time across cores used by current process  http   www manpagez com man 3 clock   and  it appears  and as noted by the original poster  actually with less precision than CLOCKS PER SEC  though maybe this depends on the specific version of Linux

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With C  11 and std  chrono  high resolution clock you can do this    include  lt iostream gt   include  lt chrono gt   include  lt thread gt  typedef std  chrono  high resolution clock Clock   int main         std  chrono  milliseconds three milliseconds 3        auto t1   Clock  now        std  this thread  sleep for three milliseconds       auto t2   Clock  now         std  cout  lt  lt   Delta t2-t1                    lt  lt  std  chrono  duration cast lt std  chrono  milliseconds gt  t2 - t1  count                  lt  lt    milliseconds   lt  lt  std  endl      Output   Delta t2-t1  3 milliseconds   Link to demo  http   cpp sh 2zdtu

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clock   has a often a pretty lousy resolution  If you want to measure time at the millisecond level  one alternative is to use clock gettime    as explained in this question    Remember that you need to link with -lrt on Linux

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If you don t need the code to be portable to old unices  you can use clock gettime    which will give you the time in nanoseconds  if your processor supports that resolution   It s POSIX  but from 2001

[c++] C++ obtaining milliseconds time on Linux -- clock() doesn't seem to work properly

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