man2/time.2

.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified Sat Jul 24 14:13:40 1993 by Rik Faith <faith@cs.unc.edu>
.\" Additions by Joseph S. Myers <jsm28@cam.ac.uk>, 970909
.\"
.TH TIME 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
time \- get time in seconds
.SH SYNOPSIS
.B #include <time.h>
.PP
.BI "time_t time(time_t *" tloc );
.SH DESCRIPTION
.BR time ()
returns the time as the number of seconds since the
Epoch, 1970-01-01 00:00:00 +0000 (UTC).
.PP
If
.I tloc
is non-NULL,
the return value is also stored in the memory pointed to by
.IR tloc .
.SH RETURN VALUE
On success, the value of time in seconds since the Epoch is returned.
On error, \fI((time_t)\ \-1)\fP is returned, and \fIerrno\fP is set
appropriately.
.SH ERRORS
.TP
.B EFAULT
.I tloc
points outside your accessible address space (but see BUGS).
.IP
On systems where the C library
.BR time ()
wrapper function invokes an implementation provided by the
.BR vdso (7)
(so that there is no trap into the kernel),
an invalid address may instead trigger a
.B SIGSEGV
signal.
.SH CONFORMING TO
SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
.\" Under 4.3BSD, this call is obsoleted by
.\" .BR gettimeofday (2).
POSIX does not specify any error conditions.
.SH NOTES
POSIX.1 defines
.I 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.
.PP
On Linux, a call to
.BR time ()
with
.I tloc
specified as NULL cannot fail with the error
.BR EOVERFLOW ,
even on ABIs where
.I time_t
is a signed 32-bit integer and the clock ticks past the time 2**31
(2038-01-19 03:14:08 UTC, ignoring leap seconds).
(POSIX.1 permits, but does not require, the
.B EOVERFLOW
error in the case where the seconds since the Epoch will not fit in
.IR time_t .)
Instead, the behavior on Linux is undefined when the system time is out of the
.I time_t
range.
Applications intended to run after 2038 should use ABIs with
.I time_t
wider than 32 bits.
.SH BUGS
Error returns from this system call are indistinguishable from
successful reports that the time is a few seconds
.I before
the Epoch, so the C library wrapper function never sets
.I errno
as a result of this call.
.PP
The
.I tloc
argument is obsolescent and should always be NULL in new code.
When
.I tloc
is NULL, the call cannot fail.
.\"
.SS C library/kernel differences
On some architectures, an implementation of
.BR time ()
is provided in the
.BR vdso (7).
.SH SEE ALSO
.BR date (1),
.BR gettimeofday (2),
.BR ctime (3),
.BR ftime (3),
.BR time (7),
.BR vdso (7)