[c] C - gettimeofday for computing time?

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(&tv, NULL); 
 curtime=tv.tv_sec;

 strftime(buffer,30,"%m-%d-%Y  %T.",localtime(&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

No. gettimeofday should NEVER be used to measure time.

This is causing bugs all over the place. Please don't add more bugs.

To subtract timevals:

gettimeofday(&t0, 0);
/* ... */
gettimeofday(&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;

If you want to measure code efficiency, or in any other way measure time intervals, the following will be easier:

#include <time.h>

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

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.

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