gettimeofday timeval 빼고 더하기

sys/time.h에 있는 매크로 이용

# define timeradd(a, b, result)                           \
  do {                                        \
    (result)->tv_sec = (a)->tv_sec + (b)->tv_sec;                 \
    (result)->tv_usec = (a)->tv_usec + (b)->tv_usec;                  \
    if ((result)->tv_usec >= 1000000)                         \
      {                                       \
    ++(result)->tv_sec;                           \
    (result)->tv_usec -= 1000000;                         \
      }                                       \
  } while (0)

# define timersub(a, b, result)                           \
  do {                                        \
    (result)->tv_sec = (a)->tv_sec - (b)->tv_sec;                 \
    (result)->tv_usec = (a)->tv_usec - (b)->tv_usec;                  \
    if ((result)->tv_usec < 0) {                          \
      --(result)->tv_sec;                             \
      (result)->tv_usec += 1000000;                       \
    }                                         \
  } while (0)