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.