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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | BUGS | SEE ALSO |
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time(2) System Calls Manual time(2)
time - get time in seconds
Standard C library (libc, -lc)
#include <time.h>
time_t time(time_t *_Nullable tloc);
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.
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.
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.
POSIX.1 defines seconds since the Epoch using a formula that
approximates 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 actual 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
Rationale 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 integer 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).
C11, POSIX.1-2008.
SVr4, 4.3BSD, C89, POSIX.1-2001.
Error returns from this system call are indistinguishable from
successful 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.
date(1), gettimeofday(2), ctime(3), ftime(3), time(7), vdso(7)
Linux man-pages 6.04 2023-03-30 time(2)
Pages that refer to this page: clock_getres(2), gettimeofday(2), seccomp(2), syscalls(2), ctime(3), difftime(3), ftime(3), getdate(3), misc_conv(3), pmtimeval(3), __ppc_get_timebase(3), pthread_tryjoin_np(3), strftime(3), strptime(3), time_t(3type), tzset(3), uuid_time(3), rtc(4), tzfile(5), utmp(5), signal-safety(7), time(7), lsof(8)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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