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### 21.3 Calculating Elapsed Time

Often, one wishes to calculate an elapsed time as the difference between two simple calendar times. The GNU C Library provides only one function for this purpose.

Function: double difftime (time_t end, time_t begin)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The `difftime` function returns the number of seconds of elapsed time from calendar time begin to calendar time end, as a value of type `double`.

On POSIX-conformant systems, the advantage of using ‘difftime (end, begin)’ over ‘end - begin’ is that it will produce the mathematically correct result even if end and begin are so far apart that a simple subtraction would overflow. However, if they are so far apart that a `double` cannot exactly represent the difference, the result will be inexact.

On other systems, `time_t` values might be encoded in a way that prevents subtraction from working directly, and then `difftime` would be the only way to compute their difference.

The GNU C Library does not provide any functions for computing the difference between two values of type `struct timeval` or `struct timespec`. Here is the recommended way to do this calculation by hand. 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 (struct timeval *result, struct timeval *x, struct timeval *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;
}
```

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