(libc.info.gz) Elapsed Time

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 21.2 Elapsed Time
 =================
 
 One way to represent an elapsed time is with a simple arithmetic data
 type, as with the following function to compute the elapsed time between
 two calendar times.  This function is declared in `time.h'.
 
  -- Function: double difftime (time_t TIME1, time_t TIME0)
      The `difftime' function returns the number of seconds of elapsed
      time between calendar time TIME1 and calendar time TIME0, as a
      value of type `double'.  The difference ignores leap seconds
      unless leap second support is enabled.
 
      In the GNU system, you can simply subtract `time_t' values.  But on
      other systems, the `time_t' data type might use some other encoding
      where subtraction doesn't work directly.
 
    The GNU C library provides two data types specifically for
 representing an elapsed time.  They are used by various GNU C library
 functions, and you can use them for your own purposes too.  They're
 exactly the same except that one has a resolution in microseconds, and
 the other, newer one, is in nanoseconds.
 
  -- Data Type: struct timeval
      The `struct timeval' structure represents an elapsed time.  It is
      declared in `sys/time.h' and has the following members:
 
     `long int tv_sec'
           This represents the number of whole seconds of elapsed time.
 
     `long int tv_usec'
           This is the rest of the elapsed time (a fraction of a second),
           represented as the number of microseconds.  It is always less
           than one million.
 
 
  -- Data Type: struct timespec
      The `struct timespec' structure represents an elapsed time.  It is
      declared in `time.h' and has the following members:
 
     `long int tv_sec'
           This represents the number of whole seconds of elapsed time.
 
     `long int tv_nsec'
           This is the rest of the elapsed time (a fraction of a second),
           represented as the number of nanoseconds.  It is always less
           than one billion.
 
 
    It is often necessary to subtract two values of type
 `struct timeval' or `struct timespec'.  Here is the best way to do
 this.  It works even on some peculiar operating systems where the
 `tv_sec' member has an unsigned type.
 
      /* Subtract the `struct timeval' values X and Y,
         storing the result in RESULT.
         Return 1 if the difference is negative, otherwise 0.  */
 
      int
      timeval_subtract (result, x, y)
           struct timeval *result, *x, *y;
      {
        /* 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 = (x->tv_usec - y->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;
      }
 
    Common functions that use `struct timeval' are `gettimeofday' and
 `settimeofday'.
 
    There are no GNU C library functions specifically oriented toward
 dealing with elapsed times, but the calendar time, processor time, and
 alarm and sleeping functions have a lot to do with them.
 

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