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.
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;
}