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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.