#include <stdio.h>

struct timeval
{
  long tv_sec;
  long tv_usec;
};

int timeval_subtract(struct timeval *result, struct timeval *x, struct timeval *y)
{  
  // preserve *y
  struct timeval yy = *y;
  y = &yy;

  /* Perform the carry for the later subtraction by updating y. */  
  if (x->tv_usec < y->tv_usec) {  
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;  
    y->tv_usec -= 1000000 * nsec;  
    y->tv_sec += nsec;  
  }  
  if (x->tv_usec - y->tv_usec > 1000000) {  
    int nsec = (y->tv_usec - x->tv_usec) / 1000000;  
    y->tv_usec += 1000000 * nsec;  
    y->tv_sec -= nsec;  
  }  

  /* Compute the time remaining to wait.
     tv_usec is certainly positive. */  
  result->tv_sec = x->tv_sec - y->tv_sec;  
  result->tv_usec = x->tv_usec - y->tv_usec;  

  /* Return 1 if result is negative. */  
  return x->tv_sec < y->tv_sec;  
}

struct timeval testData00 = { 0, 0 };
struct timeval testData01 = { 0, 1 };

int main(void)
{
  struct timeval diff;
  int res;

  res = timeval_subtract(&diff, &testData00, &testData00);
  printf("%d %ld:%ld\n", res, diff.tv_sec, diff.tv_usec);

  res = timeval_subtract(&diff, &testData01, &testData01);
  printf("%d %ld:%ld\n", res, diff.tv_sec, diff.tv_usec);

  res = timeval_subtract(&diff, &testData01, &testData00);
  printf("%d %ld:%ld\n", res, diff.tv_sec, diff.tv_usec);

  res = timeval_subtract(&diff, &testData00, &testData01);
  printf("%d %ld:%ld\n", res, diff.tv_sec, diff.tv_usec);

  return 0;
}