How do I compare two timestamps in C?

c sockets timestamp timeval
Vishal Kotcherlakota picture Vishal Kotcherlakota · Dec 7, 2009 · Viewed 22.8k times · Source

I'm writing a socket program that maintains FIFO queues for two input sockets. When deciding which queue to service, the program pulls the most recent time-stamp from each queue.

I need a reliable method for comparing two timeval structs. I tried using timercmp(), but my version of gcc doesn't support it, and documentation states that the function is not POSIX compliant.

What should I do?

Answer

Gonzalo picture Gonzalo · Dec 7, 2009

timercmp() is just a macro in libc (sys/time.h):

# define timercmp(a, b, CMP)                                                  \
  (((a)->tv_sec == (b)->tv_sec) ?                                             \
   ((a)->tv_usec CMP (b)->tv_usec) :                                          \
   ((a)->tv_sec CMP (b)->tv_sec))

If you need timersub():

# 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)

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