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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 timetime1and calendar timetime0, 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 insys/time.hand 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 intime.hand 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 updatingy. */ 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.