measure the elapsed time on linux

e_time.c

#include <time.h>
#include <sys/time.h>
#include <sys/resource.h>

// How to use?
//
// double st = e_time();
//    ... where computation going on ...
// double en = e_time();
//
// the elapsed time is en - st in seconds

double e_time(void)
{
  static struct timeval now;
  gettimeofday(&now, NULL);
  return (double)(now.tv_sec  + now.tv_usec/1000000.0);
}