How to calculate a time difference in C

Question

What s the best way to calculate a time difference in C    I m timing the execution speed of a program  so I m interested in milliseconds  Better yet  seconds milliseconds    The accepted answer works  but needs to include ctime or time h as noted in the comments

User · Answer

Get the system time in milliseconds at the beginning, and again at the end, and subtract.

To get the number of milliseconds since 1970 in POSIX you would write:

struct timeval tv;

gettimeofday(&tv, NULL);
return ((((unsigned long long)tv.tv_sec) * 1000) +
        (((unsigned long long)tv.tv_usec) / 1000));

To get the number of milliseconds since 1601 on Windows you would write:

SYSTEMTIME systime;
FILETIME filetime;

GetSystemTime(&systime);
if (!SystemTimeToFileTime(&systime, &filetime))
    return 0;

unsigned long long ns_since_1601;
ULARGE_INTEGER* ptr = (ULARGE_INTEGER*)&ns_since_1601;

// copy the result into the ULARGE_INTEGER; this is actually
// copying the result into the ns_since_1601 unsigned long long.
ptr->u.LowPart = filetime.dwLowDateTime;
ptr->u.HighPart = filetime.dwHighDateTime;

// Compute the number of milliseconds since 1601; we have to
// divide by 10,000, since the current value is the number of 100ns
// intervals since 1601, not ms.
return (ns_since_1601 / 10000);

If you cared to normalize the Windows answer so that it also returned the number of milliseconds since 1970, then you would have to adjust your answer by 11644473600000 milliseconds. But that isn't necessary if all you care about is the elapsed time.

User · Answer

In Windows  use GetTickCount    GetTickCount defintition  include  lt windows h gt  int main          DWORD dw1   GetTickCount           Do something       DWORD dw2   GetTickCount         cout lt  lt  Time difference is   lt  lt  dw2-dw1  lt  lt   milliSeconds  lt  lt endl

User · Answer

boost 1 46 0 and up includes the Chrono library      thread clock class provides access to the real thread wall-clock  i e    the real CPU-time clock of the calling thread  The thread relative   current time can be obtained by calling thread clock  now      include  lt boost chrono thread clock hpp gt            using namespace boost  chrono      thread clock  time point start   thread clock  now                thread clock  time point stop   thread clock  now          std  cout  lt  lt   duration     lt  lt  duration cast lt milliseconds gt  stop - start  count    lt  lt    ms n

User · Answer

You can also use the clock gettime  This method can be used to measure    System wide real-time clock System wide monotonic clock Per Process CPU time Per process Thread CPU time   Code is as follows    include  lt  time h  gt   include  lt iostream gt  int main      timespec ts beg  ts end    clock gettime CLOCK PROCESS CPUTIME ID   amp ts beg     clock gettime CLOCK PROCESS CPUTIME ID   amp ts end     std  cout  lt  lt   ts end tv sec - ts beg tv sec     ts end tv nsec - ts beg tv nsec    1e9  lt  lt    sec

User · Answer

I would seriously consider the use of Boost  particularly boost  posix time  ptime and boost  posix time  time duration  at http   www boost org doc libs 1 38 0 doc html date time posix time html    It s cross-platform  easy to use  and in my experience provides the highest level of time resolution an operating system provides  Possibly also very important  it provides some very nice IO operators   To use it to calculate the difference in program execution  to microseconds  probably overkill   it would look something like this  browser written  not tested    ptime time start microsec clock  local time           execution goes here     ptime time end microsec clock  local time     time duration duration time end - time start   cout  lt  lt  duration  lt  lt    n

User · Answer

just in case you are on Unix  you can use time to get the execution time     g   myprog cpp -o myprog   time   myprog

User · Answer

See std  clock   function   const clock t begin time   clock       do something std  cout  lt  lt  float  clock    - begin time      CLOCKS PER SEC    If you want calculate execution time for self   not for user    it is better to do this in clock ticks   not seconds       EDIT  responsible header files -  lt ctime gt  or  lt time h gt

User · Answer

if you are using c  11  here is a simple wrapper  see this gist     include  lt iostream gt   include  lt chrono gt   class Timer   public      Timer     beg  clock   now           void reset     beg    clock   now          double elapsed   const            return std  chrono  duration cast lt second  gt               clock   now   - beg   count       private      typedef std  chrono  high resolution clock clock       typedef std  chrono  duration lt double  std  ratio lt 1 gt   gt  second       std  chrono  time point lt clock  gt  beg        Or for c  03 on  nix    include  lt iostream gt   include  lt ctime gt   class Timer   public      Timer     clock gettime CLOCK REALTIME   amp beg           double elapsed             clock gettime CLOCK REALTIME   amp end            return end  tv sec - beg  tv sec                end  tv nsec - beg  tv nsec    1000000000              void reset     clock gettime CLOCK REALTIME   amp beg       private      timespec beg   end        Example of usage   int main         Timer tmr      double t   tmr elapsed        std  cout  lt  lt  t  lt  lt  std  endl       tmr reset        t   tmr elapsed        std  cout  lt  lt  t  lt  lt  std  endl      return 0

User · Answer

I added this answer to clarify that the accepted answer shows CPU time which may not be the time you want  Because according to the reference  there are CPU time and wall clock time  Wall clock time is the time which shows the actual elapsed time regardless of any other conditions like CPU shared by other processes  For example  I used multiple processors to do a certain task and the CPU time was high 18s where it actually took 2s in actual wall clock time   To get the actual time you do    include  lt chrono gt   auto t start   std  chrono  high resolution clock  now       the work    auto t end   std  chrono  high resolution clock  now     double elapsed time ms   std  chrono  duration lt double  std  milli gt  t end-t start  count

User · Answer

This seems to work fine for intel Mac 10 7    include  lt time h gt   time t start   time NULL           Do your work   time t end   time NULL   std  cout lt  lt  Execution Time    lt  lt   double  end-start  lt  lt   Seconds  lt  lt std  endl

User · Answer

Just a side note  if you re running on Windows  and you really really need precision  you can use QueryPerformanceCounter  It gives you time in  potentially  nanoseconds

User · Answer

If you are using   tstart   clock           do something     tend   clock      Then you will need the following to get time in seconds   time    tend - tstart     double  CLOCKS PER SEC

User · Answer

For me  the most easy way is    include  lt boost timer hpp gt   boost  timer t  double duration   t restart       DO SOMETHING HERE       duration   t elapsed     t restart       DO OTHER STUFF HERE       duration   t elapsed      using this piece of code you don t have to do the classic end - start   Enjoy your favorite approach

[c++] How to calculate a time difference in C++

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