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time(2) System Calls Manual time(2)
NAME
time - get time in seconds
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <time.h>
time_t time(time_t *_Nullable tloc);
DESCRIPTION
time() returns the time as the number of seconds since the Epoch,
1970-01-01 00:00:00 +0000 (UTC).
If tloc is non-NULL, the return value is also stored in the memory
pointed to by tloc.
RETURN VALUE
On success, the value of time in seconds since the Epoch is returned.
On error, ((time_t) -1) is returned, and errno is set to indicate the
error.
ERRORS
EFAULT tloc points outside your accessible address space (but see
BUGS).
On systems where the C library time() wrapper function invokes
an implementation provided by the vdso(7) (so that there is no
trap into the kernel), an invalid address may instead trigger a
SIGSEGV signal.
VERSIONS
POSIX.1 defines seconds since the Epoch using a formula that approxi-
mates the number of seconds between a specified time and the Epoch.
This formula takes account of the facts that all years that are evenly
divisible by 4 are leap years, but years that are evenly divisible by
100 are not leap years unless they are also evenly divisible by 400, in
which case they are leap years. This value is not the same as the ac-
tual number of seconds between the time and the Epoch, because of leap
seconds and because system clocks are not required to be synchronized
to a standard reference. The intention is that the interpretation of
seconds since the Epoch values be consistent; see POSIX.1-2008 Ratio-
nale A.4.15 for further rationale.
On Linux, a call to time() with tloc specified as NULL cannot fail with
the error EOVERFLOW, even on ABIs where time_t is a signed 32-bit inte-
ger and the clock reaches or exceeds 2**31 seconds (2038-01-19 03:14:08
UTC, ignoring leap seconds). (POSIX.1 permits, but does not require,
the EOVERFLOW error in the case where the seconds since the Epoch will
not fit in time_t.) Instead, the behavior on Linux is undefined when
the system time is out of the time_t range. Applications intended to
run after 2038 should use ABIs with time_t wider than 32 bits.
C library/kernel differences
On some architectures, an implementation of time() is provided in the
vdso(7).
STANDARDS
C11, POSIX.1-2008.
HISTORY
SVr4, 4.3BSD, C89, POSIX.1-2001.
BUGS
Error returns from this system call are indistinguishable from success-
ful reports that the time is a few seconds before the Epoch, so the C
library wrapper function never sets errno as a result of this call.
The tloc argument is obsolescent and should always be NULL in new code.
When tloc is NULL, the call cannot fail.
SEE ALSO
date(1), gettimeofday(2), ctime(3), ftime(3), time(7), vdso(7)
Linux man-pages 6.04 2023-03-30 time(2)
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