C - gettimeofday for computing time

Question

do you know how to use gettimeofday for measuring computing time  I can measure one time by this code     char buffer 30     struct timeval tv     time t curtime      gettimeofday  amp tv  NULL     curtime tv tv sec    strftime buffer 30   m- d- Y   T   localtime  amp curtime     printf   s ld n  buffer tv tv usec     This one is made before computing  second one after  But do you know how to subtracts it   I need result in miliseconds

User · Answer

No  gettimeofday should NEVER be used to measure time   This is causing bugs all over the place  Please don t add more bugs

User · Answer

Your curtime variable holds the number of seconds since the epoch   If you get one before and one after  the later one minus the earlier one is the elapsed time in seconds   You can subtract time t values just fine

User · Answer

The answer offered by  Daniel Kamil Kozar is the correct answer - gettimeofday actually should not be used to measure the elapsed time  Use clock gettime CLOCK MONOTONIC  instead     Man Pages say - The time returned by gettimeofday   is affected by discontinuous jumps in the system time  e g   if the system administrator manually changes the system time    If you need a monotonically increasing clock  see clock gettime 2    The Opengroup says - Applications should use the clock gettime   function instead of the obsolescent gettimeofday   function   Everyone seems to love gettimeofday until they run into a case where it does not work or is not there  VxWorks      clock gettime is fantastically awesome and portable    lt  lt

User · Answer

To subtract timevals   gettimeofday  amp t0  0             gettimeofday  amp t1  0   long elapsed    t1 tv sec-t0 tv sec  1000000   t1 tv usec-t0 tv usec    This is assuming you ll be working with intervals shorter than  2000 seconds  at which point the arithmetic may overflow depending on the types used  If you need to work with longer intervals just change the last line to   long long elapsed    t1 tv sec-t0 tv sec  1000000LL   t1 tv usec-t0 tv usec

User · Answer

If you want to measure code efficiency  or in any other way measure time intervals  the following will be easier    include  lt time h gt   int main        clock t start   clock             do work here    clock t end   clock       double time elapsed in seconds    end - start   double CLOCKS PER SEC     return 0      hth

[c] C - gettimeofday for computing time?

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