How to measure time in milliseconds using ANSI C

Question

Using only ANSI C  is there any way to measure time with milliseconds precision or more  I was browsing time h but I only found second precision functions

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include  lt time h gt  clock t uptime   clock      CLOCKS PER SEC   1000

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The accepted answer is good enough But my solution is more simple I just test in Linux  use gcc  Ubuntu 7 2 0-8ubuntu3 2  7 2 0  Alse use gettimeofday  the tv sec is the part of second  and the tv usec is microseconds  not milliseconds  long currentTimeMillis       struct timeval time    gettimeofday  amp time  NULL      return time tv sec   1000   time tv usec   1000     int main       printf  quot  ld n quot   currentTimeMillis          wait 1 second   sleep 1     printf  quot  ld n quot   currentTimeMillis       return 0      It print  1522139691342 1522139692342  exactly a second

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I always use the clock gettime   function  returning time from the CLOCK MONOTONIC clock  The time returned is the amount of time  in seconds and nanoseconds  since some unspecified point in the past  such as system startup of the epoch    include  lt stdio h gt   include  lt stdint h gt   include  lt time h gt   int64 t timespecDiff struct timespec  timeA p  struct timespec  timeB p      return   timeA p- gt tv sec   1000000000    timeA p- gt tv nsec  -              timeB p- gt tv sec   1000000000    timeB p- gt tv nsec      int main int argc  char   argv      struct timespec start  end    clock gettime CLOCK MONOTONIC   amp start         Some code I am interested in measuring     clock gettime CLOCK MONOTONIC   amp end      uint64 t timeElapsed   timespecDiff  amp end   amp start

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I always use the clock gettime   function  returning time from the CLOCK MONOTONIC clock  The time returned is the amount of time  in seconds and nanoseconds  since some unspecified point in the past  such as system startup of the epoch    include  lt stdio h gt   include  lt stdint h gt   include  lt time h gt   int64 t timespecDiff struct timespec  timeA p  struct timespec  timeB p      return   timeA p- gt tv sec   1000000000    timeA p- gt tv nsec  -              timeB p- gt tv sec   1000000000    timeB p- gt tv nsec      int main int argc  char   argv      struct timespec start  end    clock gettime CLOCK MONOTONIC   amp start         Some code I am interested in measuring     clock gettime CLOCK MONOTONIC   amp end      uint64 t timeElapsed   timespecDiff  amp end   amp start

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Implementing a portable solution  As it was already mentioned here that there is no proper ANSI solution with sufficient precision for the time measurement problem  I want to write about the ways how to get a portable and  if possible  a high-resolution time measurement solution   Monotonic clock vs  time stamps  Generally speaking there are two ways of time measurement    monotonic clock  current  date time stamp    The first one uses a monotonic clock counter  sometimes it is called a tick counter  which counts ticks with a predefined frequency  so if you have a ticks value and the frequency is known  you can easily convert ticks to elapsed time  It is actually not guaranteed that a monotonic clock reflects the current system time in any way  it may also count ticks since a system startup  But it guarantees that a clock is always run up in an increasing fashion regardless of the system state  Usually the frequency is bound to a hardware high-resolution source  that s why it provides a high accuracy  depends on hardware  but most of the modern hardware has no problems with high-resolution clock sources    The second way provides a  date time value based on the current system clock value  It may also have a high resolution  but it has one major drawback  this kind of time value can be affected by different system time adjustments  i e  time zone change  daylight saving time  DST  change  NTP server update  system hibernation and so on  In some circumstances you can get a negative elapsed time value which can lead to an undefined behavior  Actually this kind of time source is less reliable than the first one   So the first rule in time interval measuring is to use a monotonic clock if possible  It usually has a high precision  and it is reliable by design   Fallback strategy  When implementing a portable solution it is worth to consider a fallback strategy  use a monotonic clock if available and fallback to time stamps approach if there is no monotonic clock in the system   Windows  There is a great article called Acquiring high-resolution time stamps on MSDN about time measurement on Windows which describes all the details you may need to know about software and hardware support  To acquire a high precision time stamp on Windows you should    query a timer frequency  ticks per second  with QueryPerformanceFrequency   LARGE INTEGER tcounter  LARGE INTEGER freq       if  QueryPerformanceFrequency   amp tcounter     0      freq   tcounter QuadPart    The timer frequency is fixed on the system boot so you need to get it only once  query the current ticks value with QueryPerformanceCounter   LARGE INTEGER tcounter  LARGE INTEGER tick value   if  QueryPerformanceCounter   amp tcounter     0      tick value   tcounter QuadPart   scale the ticks to elapsed time  i e  to microseconds   LARGE INTEGER usecs    tick value - prev tick value     freq   1000000      According to Microsoft you should not have any problems with this approach on Windows XP and later versions in most cases  But you can also use two fallback solutions on Windows    GetTickCount provides the number of milliseconds that have elapsed since the system was started  It wraps every 49 7 days  so be careful in measuring longer intervals  GetTickCount64 is a 64-bit version of GetTickCount  but it is available starting from Windows Vista and above    OS X  macOS   OS X  macOS  has its own Mach absolute time units which represent a monotonic clock  The best way to start is the Apple s article Technical Q amp A QA1398  Mach Absolute Time Units which describes  with the code examples  how to use Mach-specific API to get monotonic ticks  There is also a local question about it called clock gettime alternative in Mac OS X which at the end may leave you a bit confused what to do with the possible value overflow because the counter frequency is used in the form of numerator and denominator  So  a short example how to get elapsed time    get the clock frequency numerator and denominator    include  lt mach mach time h gt   include  lt stdint h gt   static uint64 t freq num     0  static uint64 t freq denom   0   void init clock frequency          mach timebase info data t tb       if  mach timebase info   amp tb     KERN SUCCESS  amp  amp  tb denom    0            freq num      uint64 t  tb numer          freq denom    uint64 t  tb denom            You need to do that only once  query the current tick value with mach absolute time   uint64 t tick value   mach absolute time      scale the ticks to elapsed time  i e  to microseconds  using previously queried numerator and denominator   uint64 t value diff   tick value - prev tick value      To prevent overflow    value diff    1000   value diff    freq num  value diff    freq denom    The main idea to prevent an overflow is to scale down the ticks to desired accuracy before using the numerator and denominator  As the initial timer resolution is in nanoseconds  we divide it by 1000 to get microseconds  You can find the same approach used in Chromium s time mac c  If you really need a nanosecond accuracy consider reading the How can I use mach absolute time without overflowing     Linux and UNIX  The clock gettime call is your best way on any POSIX-friendly system  It can query time from different clock sources  and the one we need is CLOCK MONOTONIC  Not all systems which have clock gettime support CLOCK MONOTONIC  so the first thing you need to do is to check its availability    if  POSIX MONOTONIC CLOCK is defined to a value  gt   0 it means that CLOCK MONOTONIC is avaiable  if  POSIX MONOTONIC CLOCK is defined to 0 it means that you should additionally check if it works at runtime  I suggest to use sysconf    include  lt unistd h gt    ifdef  SC MONOTONIC CLOCK if  sysconf   SC MONOTONIC CLOCK   gt  0           A monotonic clock presents       endif  otherwise a monotonic clock is not supported and you should use a fallback strategy  see below     Usage of clock gettime is pretty straight forward    get the time value    include  lt time h gt   include  lt sys time h gt   include  lt stdint h gt   uint64 t get posix clock time          struct timespec ts       if  clock gettime  CLOCK MONOTONIC   amp ts     0          return  uint64 t   ts tv sec   1000000   ts tv nsec   1000       else         return 0      I ve scaled down the time to microseconds here  calculate the difference with the previous time value received the same way   uint64 t prev time value  time value  uint64 t time diff      Initial time    prev time value   get posix clock time         Do some work here        Final time    time value   get posix clock time         Time difference    time diff   time value - prev time value     The best fallback strategy is to use the gettimeofday call  it is not a monotonic  but it provides quite a good resolution  The idea is the same as with clock gettime  but to get a time value you should    include  lt time h gt   include  lt sys time h gt   include  lt stdint h gt   uint64 t get gtod clock time          struct timeval tv       if  gettimeofday   amp tv  NULL     0          return  uint64 t   tv tv sec   1000000   tv tv usec       else         return 0      Again  the time value is scaled down to microseconds   SGI IRIX  IRIX has the clock gettime call  but it lacks CLOCK MONOTONIC  Instead it has its own monotonic clock source defined as CLOCK SGI CYCLE which you should use instead of CLOCK MONOTONIC with clock gettime   Solaris and HP-UX  Solaris has its own high-resolution timer interface gethrtime which returns the current timer value in nanoseconds  Though the newer versions of Solaris may have clock gettime  you can stick to gethrtime if you need to support old Solaris versions   Usage is simple    include  lt sys time h gt   void time measure example          hrtime t prev time value  time value      hrtime t time diff          Initial time        prev time value   gethrtime             Do some work here            Final time        time value   gethrtime             Time difference        time diff   time value - prev time value      HP-UX lacks clock gettime  but it supports gethrtime which you should use in the same way as on Solaris   BeOS  BeOS also has its own high-resolution timer interface system time which returns the number of microseconds have elapsed since the computer was booted   Example usage    include  lt kernel OS h gt   void time measure example          bigtime t prev time value  time value      bigtime t time diff          Initial time        prev time value   system time             Do some work here            Final time        time value   system time             Time difference        time diff   time value - prev time value      OS 2  OS 2 has its own API to retrieve high-precision time stamps    query a timer frequency  ticks per unit  with DosTmrQueryFreq  for GCC compiler     define INCL DOSPROFILE  define INCL DOSERRORS  include  lt os2 h gt   include  lt stdint h gt   ULONG freq   DosTmrQueryFreq   amp freq    query the current ticks value with DosTmrQueryTime   QWORD    tcounter  unit64 t time low  unit64 t time high  unit64 t timestamp   if  DosTmrQueryTime   amp tcounter     NO ERROR        time low     unit64 t  tcounter ulLo      time high    unit64 t  tcounter ulHi       timestamp    time high  lt  lt  32    time low     scale the ticks to elapsed time  i e  to microseconds   uint64 t usecs    prev timestamp - timestamp     freq   1000000      Example implementation  You can take a look at the plibsys library which implements all the described above strategies  see ptimeprofiler  c for details

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The best precision you can possibly get is through the use of the x86-only  rdtsc  instruction  which can provide clock-level resolution  ne must of course take into account the cost of the rdtsc call itself  which can be measured easily on application startup    The main catch here is measuring the number of clocks per second  which shouldn t be too hard

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The best precision you can possibly get is through the use of the x86-only  rdtsc  instruction  which can provide clock-level resolution  ne must of course take into account the cost of the rdtsc call itself  which can be measured easily on application startup    The main catch here is measuring the number of clocks per second  which shouldn t be too hard

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There is no ANSI C function that provides better than 1 second time resolution but the POSIX function gettimeofday provides microsecond resolution   The clock function only measures the amount of time that a process has spent executing and is not accurate on many systems   You can use this function like this   struct timeval tval before  tval after  tval result   gettimeofday  amp tval before  NULL       Some code you want to time  for example  sleep 1    gettimeofday  amp tval after  NULL    timersub  amp tval after   amp tval before   amp tval result    printf  Time elapsed   ld  06ld n    long int tval result tv sec   long int tval result tv usec     This returns Time elapsed  1 000870 on my machine

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include  lt time h gt  clock t uptime   clock      CLOCKS PER SEC   1000

User · Answer

The best precision you can possibly get is through the use of the x86-only  rdtsc  instruction  which can provide clock-level resolution  ne must of course take into account the cost of the rdtsc call itself  which can be measured easily on application startup    The main catch here is measuring the number of clocks per second  which shouldn t be too hard

User · Answer

The accepted answer is good enough But my solution is more simple I just test in Linux  use gcc  Ubuntu 7 2 0-8ubuntu3 2  7 2 0  Alse use gettimeofday  the tv sec is the part of second  and the tv usec is microseconds  not milliseconds  long currentTimeMillis       struct timeval time    gettimeofday  amp time  NULL      return time tv sec   1000   time tv usec   1000     int main       printf  quot  ld n quot   currentTimeMillis          wait 1 second   sleep 1     printf  quot  ld n quot   currentTimeMillis       return 0      It print  1522139691342 1522139692342  exactly a second

User · Answer

There is no ANSI C function that provides better than 1 second time resolution but the POSIX function gettimeofday provides microsecond resolution   The clock function only measures the amount of time that a process has spent executing and is not accurate on many systems   You can use this function like this   struct timeval tval before  tval after  tval result   gettimeofday  amp tval before  NULL       Some code you want to time  for example  sleep 1    gettimeofday  amp tval after  NULL    timersub  amp tval after   amp tval before   amp tval result    printf  Time elapsed   ld  06ld n    long int tval result tv sec   long int tval result tv usec     This returns Time elapsed  1 000870 on my machine

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I always use the clock gettime   function  returning time from the CLOCK MONOTONIC clock  The time returned is the amount of time  in seconds and nanoseconds  since some unspecified point in the past  such as system startup of the epoch    include  lt stdio h gt   include  lt stdint h gt   include  lt time h gt   int64 t timespecDiff struct timespec  timeA p  struct timespec  timeB p      return   timeA p- gt tv sec   1000000000    timeA p- gt tv nsec  -              timeB p- gt tv sec   1000000000    timeB p- gt tv nsec      int main int argc  char   argv      struct timespec start  end    clock gettime CLOCK MONOTONIC   amp start         Some code I am interested in measuring     clock gettime CLOCK MONOTONIC   amp end      uint64 t timeElapsed   timespecDiff  amp end   amp start

User · Answer

Implementing a portable solution  As it was already mentioned here that there is no proper ANSI solution with sufficient precision for the time measurement problem  I want to write about the ways how to get a portable and  if possible  a high-resolution time measurement solution   Monotonic clock vs  time stamps  Generally speaking there are two ways of time measurement    monotonic clock  current  date time stamp    The first one uses a monotonic clock counter  sometimes it is called a tick counter  which counts ticks with a predefined frequency  so if you have a ticks value and the frequency is known  you can easily convert ticks to elapsed time  It is actually not guaranteed that a monotonic clock reflects the current system time in any way  it may also count ticks since a system startup  But it guarantees that a clock is always run up in an increasing fashion regardless of the system state  Usually the frequency is bound to a hardware high-resolution source  that s why it provides a high accuracy  depends on hardware  but most of the modern hardware has no problems with high-resolution clock sources    The second way provides a  date time value based on the current system clock value  It may also have a high resolution  but it has one major drawback  this kind of time value can be affected by different system time adjustments  i e  time zone change  daylight saving time  DST  change  NTP server update  system hibernation and so on  In some circumstances you can get a negative elapsed time value which can lead to an undefined behavior  Actually this kind of time source is less reliable than the first one   So the first rule in time interval measuring is to use a monotonic clock if possible  It usually has a high precision  and it is reliable by design   Fallback strategy  When implementing a portable solution it is worth to consider a fallback strategy  use a monotonic clock if available and fallback to time stamps approach if there is no monotonic clock in the system   Windows  There is a great article called Acquiring high-resolution time stamps on MSDN about time measurement on Windows which describes all the details you may need to know about software and hardware support  To acquire a high precision time stamp on Windows you should    query a timer frequency  ticks per second  with QueryPerformanceFrequency   LARGE INTEGER tcounter  LARGE INTEGER freq       if  QueryPerformanceFrequency   amp tcounter     0      freq   tcounter QuadPart    The timer frequency is fixed on the system boot so you need to get it only once  query the current ticks value with QueryPerformanceCounter   LARGE INTEGER tcounter  LARGE INTEGER tick value   if  QueryPerformanceCounter   amp tcounter     0      tick value   tcounter QuadPart   scale the ticks to elapsed time  i e  to microseconds   LARGE INTEGER usecs    tick value - prev tick value     freq   1000000      According to Microsoft you should not have any problems with this approach on Windows XP and later versions in most cases  But you can also use two fallback solutions on Windows    GetTickCount provides the number of milliseconds that have elapsed since the system was started  It wraps every 49 7 days  so be careful in measuring longer intervals  GetTickCount64 is a 64-bit version of GetTickCount  but it is available starting from Windows Vista and above    OS X  macOS   OS X  macOS  has its own Mach absolute time units which represent a monotonic clock  The best way to start is the Apple s article Technical Q amp A QA1398  Mach Absolute Time Units which describes  with the code examples  how to use Mach-specific API to get monotonic ticks  There is also a local question about it called clock gettime alternative in Mac OS X which at the end may leave you a bit confused what to do with the possible value overflow because the counter frequency is used in the form of numerator and denominator  So  a short example how to get elapsed time    get the clock frequency numerator and denominator    include  lt mach mach time h gt   include  lt stdint h gt   static uint64 t freq num     0  static uint64 t freq denom   0   void init clock frequency          mach timebase info data t tb       if  mach timebase info   amp tb     KERN SUCCESS  amp  amp  tb denom    0            freq num      uint64 t  tb numer          freq denom    uint64 t  tb denom            You need to do that only once  query the current tick value with mach absolute time   uint64 t tick value   mach absolute time      scale the ticks to elapsed time  i e  to microseconds  using previously queried numerator and denominator   uint64 t value diff   tick value - prev tick value      To prevent overflow    value diff    1000   value diff    freq num  value diff    freq denom    The main idea to prevent an overflow is to scale down the ticks to desired accuracy before using the numerator and denominator  As the initial timer resolution is in nanoseconds  we divide it by 1000 to get microseconds  You can find the same approach used in Chromium s time mac c  If you really need a nanosecond accuracy consider reading the How can I use mach absolute time without overflowing     Linux and UNIX  The clock gettime call is your best way on any POSIX-friendly system  It can query time from different clock sources  and the one we need is CLOCK MONOTONIC  Not all systems which have clock gettime support CLOCK MONOTONIC  so the first thing you need to do is to check its availability    if  POSIX MONOTONIC CLOCK is defined to a value  gt   0 it means that CLOCK MONOTONIC is avaiable  if  POSIX MONOTONIC CLOCK is defined to 0 it means that you should additionally check if it works at runtime  I suggest to use sysconf    include  lt unistd h gt    ifdef  SC MONOTONIC CLOCK if  sysconf   SC MONOTONIC CLOCK   gt  0           A monotonic clock presents       endif  otherwise a monotonic clock is not supported and you should use a fallback strategy  see below     Usage of clock gettime is pretty straight forward    get the time value    include  lt time h gt   include  lt sys time h gt   include  lt stdint h gt   uint64 t get posix clock time          struct timespec ts       if  clock gettime  CLOCK MONOTONIC   amp ts     0          return  uint64 t   ts tv sec   1000000   ts tv nsec   1000       else         return 0      I ve scaled down the time to microseconds here  calculate the difference with the previous time value received the same way   uint64 t prev time value  time value  uint64 t time diff      Initial time    prev time value   get posix clock time         Do some work here        Final time    time value   get posix clock time         Time difference    time diff   time value - prev time value     The best fallback strategy is to use the gettimeofday call  it is not a monotonic  but it provides quite a good resolution  The idea is the same as with clock gettime  but to get a time value you should    include  lt time h gt   include  lt sys time h gt   include  lt stdint h gt   uint64 t get gtod clock time          struct timeval tv       if  gettimeofday   amp tv  NULL     0          return  uint64 t   tv tv sec   1000000   tv tv usec       else         return 0      Again  the time value is scaled down to microseconds   SGI IRIX  IRIX has the clock gettime call  but it lacks CLOCK MONOTONIC  Instead it has its own monotonic clock source defined as CLOCK SGI CYCLE which you should use instead of CLOCK MONOTONIC with clock gettime   Solaris and HP-UX  Solaris has its own high-resolution timer interface gethrtime which returns the current timer value in nanoseconds  Though the newer versions of Solaris may have clock gettime  you can stick to gethrtime if you need to support old Solaris versions   Usage is simple    include  lt sys time h gt   void time measure example          hrtime t prev time value  time value      hrtime t time diff          Initial time        prev time value   gethrtime             Do some work here            Final time        time value   gethrtime             Time difference        time diff   time value - prev time value      HP-UX lacks clock gettime  but it supports gethrtime which you should use in the same way as on Solaris   BeOS  BeOS also has its own high-resolution timer interface system time which returns the number of microseconds have elapsed since the computer was booted   Example usage    include  lt kernel OS h gt   void time measure example          bigtime t prev time value  time value      bigtime t time diff          Initial time        prev time value   system time             Do some work here            Final time        time value   system time             Time difference        time diff   time value - prev time value      OS 2  OS 2 has its own API to retrieve high-precision time stamps    query a timer frequency  ticks per unit  with DosTmrQueryFreq  for GCC compiler     define INCL DOSPROFILE  define INCL DOSERRORS  include  lt os2 h gt   include  lt stdint h gt   ULONG freq   DosTmrQueryFreq   amp freq    query the current ticks value with DosTmrQueryTime   QWORD    tcounter  unit64 t time low  unit64 t time high  unit64 t timestamp   if  DosTmrQueryTime   amp tcounter     NO ERROR        time low     unit64 t  tcounter ulLo      time high    unit64 t  tcounter ulHi       timestamp    time high  lt  lt  32    time low     scale the ticks to elapsed time  i e  to microseconds   uint64 t usecs    prev timestamp - timestamp     freq   1000000      Example implementation  You can take a look at the plibsys library which implements all the described above strategies  see ptimeprofiler  c for details

User · Answer

The best precision you can possibly get is through the use of the x86-only  rdtsc  instruction  which can provide clock-level resolution  ne must of course take into account the cost of the rdtsc call itself  which can be measured easily on application startup    The main catch here is measuring the number of clocks per second  which shouldn t be too hard

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include  lt time h gt  clock t uptime   clock      CLOCKS PER SEC   1000

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timespec get from C11  Returns up to nanoseconds  rounded to the resolution of the implementation   Looks like an ANSI ripoff from POSIX  clock gettime   Example  a printf is done every 100ms on Ubuntu 15 10    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   static long get nanos void        struct timespec ts      timespec get  amp ts  TIME UTC       return  long ts tv sec   1000000000L   ts tv nsec     int main void        long nanos      long last nanos      long start      nanos   get nanos        last nanos   nanos      start   nanos      while  1            nanos   get nanos            if  nanos - last nanos  gt  100000000L                printf  current nanos   ld n   nanos - start               last nanos   nanos                      return EXIT SUCCESS      The C11 N1570 standard draft 7 27 2 5  The timespec get function says       If base is TIME UTC  the tv sec member is set to the number of seconds since an   implementation defined epoch  truncated to a whole value and the tv nsec member is   set to the integral number of nanoseconds  rounded to the resolution of the system clock   321       321  Although a struct timespec object describes times with nanosecond resolution  the available   resolution is system dependent and may even be greater than 1 second    C  11 also got std  chrono  high resolution clock  C   Cross-Platform High-Resolution Timer  glibc 2 21 implementation  Can be found under sysdeps posix timespec get c as   int timespec get  struct timespec  ts  int base      switch  base            case TIME UTC        if    clock gettime  CLOCK REALTIME  ts   lt  0          return 0        break       default        return 0           return base      so clearly    only TIME UTC is currently supported it forwards to   clock gettime  CLOCK REALTIME  ts   which is a POSIX API  http   pubs opengroup org onlinepubs 9699919799 functions clock getres html  Linux x86-64 has a clock gettime system call   Note that this is not a fail-proof micro-benchmarking method because    man clock gettime says that this measure may have discontinuities if you change some system time setting while your program runs  This should be a rare event of course  and you might be able to ignore it  this measures wall time  so if the scheduler decides to forget about your task  it will appear to run for longer    For those reasons getrusage   might be a better better POSIX benchmarking tool  despite it s lower microsecond maximum precision   More information at  Measure time in Linux - time vs clock vs getrusage vs clock gettime vs gettimeofday vs timespec get

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Under windows   SYSTEMTIME t  GetLocalTime  amp t   swprintf s buff  L   02d  02d  02d  d  t   t wHour  t wMinute  t wSecond  t wMilliseconds

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I always use the clock gettime   function  returning time from the CLOCK MONOTONIC clock  The time returned is the amount of time  in seconds and nanoseconds  since some unspecified point in the past  such as system startup of the epoch    include  lt stdio h gt   include  lt stdint h gt   include  lt time h gt   int64 t timespecDiff struct timespec  timeA p  struct timespec  timeB p      return   timeA p- gt tv sec   1000000000    timeA p- gt tv nsec  -              timeB p- gt tv sec   1000000000    timeB p- gt tv nsec      int main int argc  char   argv      struct timespec start  end    clock gettime CLOCK MONOTONIC   amp start         Some code I am interested in measuring     clock gettime CLOCK MONOTONIC   amp end      uint64 t timeElapsed   timespecDiff  amp end   amp start

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timespec get from C11  Returns up to nanoseconds  rounded to the resolution of the implementation   Looks like an ANSI ripoff from POSIX  clock gettime   Example  a printf is done every 100ms on Ubuntu 15 10    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   static long get nanos void        struct timespec ts      timespec get  amp ts  TIME UTC       return  long ts tv sec   1000000000L   ts tv nsec     int main void        long nanos      long last nanos      long start      nanos   get nanos        last nanos   nanos      start   nanos      while  1            nanos   get nanos            if  nanos - last nanos  gt  100000000L                printf  current nanos   ld n   nanos - start               last nanos   nanos                      return EXIT SUCCESS      The C11 N1570 standard draft 7 27 2 5  The timespec get function says       If base is TIME UTC  the tv sec member is set to the number of seconds since an   implementation defined epoch  truncated to a whole value and the tv nsec member is   set to the integral number of nanoseconds  rounded to the resolution of the system clock   321       321  Although a struct timespec object describes times with nanosecond resolution  the available   resolution is system dependent and may even be greater than 1 second    C  11 also got std  chrono  high resolution clock  C   Cross-Platform High-Resolution Timer  glibc 2 21 implementation  Can be found under sysdeps posix timespec get c as   int timespec get  struct timespec  ts  int base      switch  base            case TIME UTC        if    clock gettime  CLOCK REALTIME  ts   lt  0          return 0        break       default        return 0           return base      so clearly    only TIME UTC is currently supported it forwards to   clock gettime  CLOCK REALTIME  ts   which is a POSIX API  http   pubs opengroup org onlinepubs 9699919799 functions clock getres html  Linux x86-64 has a clock gettime system call   Note that this is not a fail-proof micro-benchmarking method because    man clock gettime says that this measure may have discontinuities if you change some system time setting while your program runs  This should be a rare event of course  and you might be able to ignore it  this measures wall time  so if the scheduler decides to forget about your task  it will appear to run for longer    For those reasons getrusage   might be a better better POSIX benchmarking tool  despite it s lower microsecond maximum precision   More information at  Measure time in Linux - time vs clock vs getrusage vs clock gettime vs gettimeofday vs timespec get

User · Answer

There is no ANSI C function that provides better than 1 second time resolution but the POSIX function gettimeofday provides microsecond resolution   The clock function only measures the amount of time that a process has spent executing and is not accurate on many systems   You can use this function like this   struct timeval tval before  tval after  tval result   gettimeofday  amp tval before  NULL       Some code you want to time  for example  sleep 1    gettimeofday  amp tval after  NULL    timersub  amp tval after   amp tval before   amp tval result    printf  Time elapsed   ld  06ld n    long int tval result tv sec   long int tval result tv usec     This returns Time elapsed  1 000870 on my machine

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include  lt time h gt  clock t uptime   clock      CLOCKS PER SEC   1000

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Under windows   SYSTEMTIME t  GetLocalTime  amp t   swprintf s buff  L   02d  02d  02d  d  t   t wHour  t wMinute  t wSecond  t wMilliseconds

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There is no ANSI C function that provides better than 1 second time resolution but the POSIX function gettimeofday provides microsecond resolution   The clock function only measures the amount of time that a process has spent executing and is not accurate on many systems   You can use this function like this   struct timeval tval before  tval after  tval result   gettimeofday  amp tval before  NULL       Some code you want to time  for example  sleep 1    gettimeofday  amp tval after  NULL    timersub  amp tval after   amp tval before   amp tval result    printf  Time elapsed   ld  06ld n    long int tval result tv sec   long int tval result tv usec     This returns Time elapsed  1 000870 on my machine

[c] How to measure time in milliseconds using ANSI C?

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