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(libc.info)Elapsed Time

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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.