difftime - calculate time difference
The difftime() function returns the number of
seconds elapsed between time
time1 and time
represented as a
double. Each of the times is specified in
calendar time, which means its value is a measurement (in seconds)
relative to the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
On a POSIX system,
time_t is an arithmetic type, and one
could just define
#define difftime(t1,t0) (double)(t1 - t0)
when the possible overflow in the subtraction is not a concern.
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