c timeval vs timespec

Question

Apart from the difference of precision, what are the differences between struct timeval and struct timespec? If I need less precision than µs (say, milliseconds), why would I use one over the other?

On my compiler (gcc for ARM):

/* POSIX.1b structure for a time value.  This is like a `struct timeval' but
   has nanoseconds instead of microseconds.  */
struct timespec
  {
    __time_t tv_sec;        /* Seconds.  */
    __syscall_slong_t tv_nsec;  /* Nanoseconds.  */
  };

/* A time value that is accurate to the nearest
   microsecond but also has a range of years.  */
struct timeval
  {
    __time_t tv_sec;        /* Seconds.  */
    __suseconds_t tv_usec;  /* Microseconds.  */
  };

With both __syscall_slong_t and __suseconds_t defined as a "long word".

Answer 1

I think it's really just a matter of API [in]compatibility. POSIX-y calls like pselect() and clock_gettime() use struct timespec. Various filesystem calls like utimes(), and some assorted Linux calls like gettimeofday() and select(), use struct timeval. Broadly generalizing from a few man pages, I suspect that struct timeval has a BSD legacy whereas struct timespec is POSIX.

If you're doing interval measurements, there's no reason not to leverage the extra precision from clock_gettime() — though beware that it's usually hardware, not the header file, that limits your measuring precision. Dividing by a million for display purposes is hardly better or worse than dividing by a thousand. Also, Mac OS X does not support clock_gettime().

But if you're doing lots of file time manipulation, it might make more sense to use the struct timeval used in APIs like utimes(). struct timeval also has some comparison functions on Linux, BSD and Mac OS X, e.g. timercmp(), timersub() (again, see man pages).

I'd make the decision based on the APIs you intend to use, rather than on the structures themselves. (Or write a wrapper class with conversion methods if necessary.)