Get a timestamp in C in microseconds

Question

How do I get a microseconds timestamp in C   I m trying to do   struct timeval tv  gettimeofday  amp tv NULL   return tv tv usec    But this returns some nonsense value that if I get two timestamps  the second one can be smaller or bigger than the first  second one should always be bigger   Would it be possible to convert the magic integer returned by gettimeofday to a normal number which can actually be worked with

User · Answer

But this returns some nonsense value that if I get two timestamps, the second one can be smaller or bigger than the first (second one should always be bigger).

What makes you think that? The value is probably OK. It’s the same situation as with seconds and minutes – when you measure time in minutes and seconds, the number of seconds rolls over to zero when it gets to sixty.

To convert the returned value into a “linear” number you could multiply the number of seconds and add the microseconds. But if I count correctly, one year is about 1e6*60*60*24*360 µsec and that means you’ll need more than 32 bits to store the result:

$ perl -E '$_=1e6*60*60*24*360; say int log($_)/log(2)'
44

That’s probably one of the reasons to split the original returned value into two pieces.

User · Answer

You have two choices for getting a microsecond timestamp  The first  and best  choice  is to use the timeval type directly   struct timeval GetTimeStamp         struct timeval tv      gettimeofday  amp tv NULL       return tv      The second  and for me less desirable  choice is to build a uint64 t out of a timeval   uint64 t GetTimeStamp         struct timeval tv      gettimeofday  amp tv NULL       return tv tv sec  uint64 t 1000000 tv tv usec

User · Answer

First we need to know on the range of microseconds i e  000 000 to 999 999  1000000 microseconds is equal to 1second   tv tv usec will return value from 0 to 999999 not 000000 to 999999 so when using it with seconds we might get 2 1seconds instead of 2 000001 seconds because when only talking about tv usec 000001 is essentially 1   Its better if you insert   if tv tv usec lt 10     printf  00000       else if tv tv usec lt 100 amp  amp tv tv usec gt 9    i e  2digits    printf  0000        and so on

User · Answer

struct timeval contains two components  the second and the microsecond  A timestamp with microsecond precision is represented as seconds since the epoch stored in the tv sec field and the fractional microseconds in tv usec  Thus you cannot just ignore tv sec and expect sensible results   If you use Linux or  BSD  you can use timersub   to subtract two struct timeval values  which might be what you want

User · Answer

use an unsigned long long  i e  a 64 bit unit  to represent the system time   typedef unsigned long long u64   u64 u64useconds  struct timeval tv   gettimeofday  amp tv NULL   u64useconds    1000000 tv tv sec    tv tv usec

User · Answer

timespec get from C11 returns up to nanoseconds  rounded to the resolution of the implementation    include  lt time h gt  struct timespec ts  timespec get  amp ts  TIME UTC   struct timespec       time t   tv sec            seconds        long     tv nsec           nanoseconds         More details here  https   stackoverflow com a 36095407 895245

User · Answer

You need to add in the seconds  too   unsigned long time in micros   1000000   tv tv sec   tv tv usec    Note that this will only last for about 232 106    4295 seconds  or roughly 71 minutes though  on a typical 32-bit system

[c] Get a timestamp in C in microseconds?

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