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'.
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.
struct timeval
structure represents an elapsed time. It is
declared in `sys/time.h' and has the following members:
long int tv_sec
long int tv_usec
struct timespec
structure represents an elapsed time. It is
declared in `time.h' and has the following members:
long int tv_sec
long int tv_nsec
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.
Go to the first, previous, next, last section, table of contents.