This is a handy example that shows how to use the gettimeofday and localtime functions to calculate elapsed time in milliseconds.

We will be use the gettimeofday and localtime function to store a time-stamp in milliseconds at the beginning of the code block then another time-stamp at the end. The elapsed time in milliseconds is the difference between the two time-stamps.

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
 
uint32_t         stampstart();
uint32_t         stampstop(uint32_t start);
 
int
main()
{
 
	uint32_t         start, stop;
 
	start = stampstart();
 
	/* Your code goes here */
 
	stop = stampstop(start);
 
	return 0;
 
}
 
uint32_t
stampstart()
{
	struct timeval  tv;
	struct timezone tz;
	struct tm      *tm;
	uint32_t         start;
 
	gettimeofday(&tv, &tz);
	tm = localtime(&tv.tv_sec);
 
	printf("TIMESTAMP-START\t  %d:%02d:%02d:%d (~%d ms)\n", tm->tm_hour,
	       tm->tm_min, tm->tm_sec, tv.tv_usec,
	       tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
	       tm->tm_sec * 1000 + tv.tv_usec / 1000);
 
	start = tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
		tm->tm_sec * 1000 + tv.tv_usec / 1000;
 
	return (start);
 
}
 
uint32_t
stampstop(uint32_t start)
{
 
	struct timeval  tv;
	struct timezone tz;
	struct tm      *tm;
	uint32_t         stop;
 
	gettimeofday(&tv, &tz);
	tm = localtime(&tv.tv_sec);
 
	stop = tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
		tm->tm_sec * 1000 + tv.tv_usec / 1000;
 
	printf("TIMESTAMP-END\t  %d:%02d:%02d:%d (~%d ms) \n", tm->tm_hour,
	       tm->tm_min, tm->tm_sec, tv.tv_usec,
	       tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
	       tm->tm_sec * 1000 + tv.tv_usec / 1000);
 
	printf("ELAPSED\t  %d ms\n", stop - start);
 
	return (stop);
 
}

SYNOPSIS

       #include <sys/time.h>

       int gettimeofday(struct timeval *restrict tp, void *restrict tzp);

DESCRIPTION

       The  gettimeofday() function shall obtain the current time, expressed as seconds and microseconds since the Epoch, and store it in the timeval structure pointed to by tp. The reso-
       lution of the system clock is unspecified.

       struct timeval {
             long    tv_sec;         /* seconds since Jan. 1, 1970 */
             long    tv_usec;        /* and microseconds */
       };

SYNOPSIS

       #include <time.h>

       struct tm *        localtime(const time_t *clock);

DESCRIPTION

       The localtime() function shall convert the time in seconds since the Epoch pointed to by timer into a broken-down time, expressed as a local time. The  function  corrects  for  the
       timezone and any seasonal time adjustments.

  The structure (of type) struct tm includes the following fields:

           int tm_sec;     /* seconds after the minute [0,61] */
           int tm_min;     /* minutes after the hour [0,59] */
           int tm_hour;    /* hours since midnight [0,23] */
           int tm_mday;    /* day of the month [1,31] */
           int tm_mon;     /* months since January [0,11] */
           int tm_year;    /* years since 1900 */
           int tm_wday;    /* day of week [0,6] (Sunday = 0) */
           int tm_yday;    /* day of year [0,365] (Jan 1 = 0) */
           int tm_isdst;   /* daylight savings flag */
           long tm_gmtoff; /* offset from UTC in seconds */
           char *tm_zone;  /* abbreviation of timezone name */

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