-.\" Hey Emacs! This file is -*- nroff -*- source.
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.\" 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 1997-09-09 "Linux 2.0.30" "Linux Programmer's Manual"
+.TH TIME 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
time \- get time in seconds
.SH SYNOPSIS
.B #include <time.h>
-.sp
-.BI "time_t time(time_t *" t );
+.PP
+.BI "time_t time(time_t *" tloc );
.SH DESCRIPTION
-\fBtime\fP() returns the time since the Epoch
-(00:00:00 UTC, January 1, 1970), measured in seconds.
-
+.BR time ()
+returns the time as the number of seconds since the
+Epoch, 1970-01-01 00:00:00 +0000 (UTC).
+.PP
If
-.I t
+.I tloc
is non-NULL,
the return value is also stored in the memory pointed to by
-.IR t .
-.SH "RETURN VALUE"
+.IR tloc .
+.SH RETURN VALUE
On success, the value of time in seconds since the Epoch is returned.
-On error, ((time_t)\-1) is returned, and \fIerrno\fP is set
+On error, \fI((time_t)\ \-1)\fP is returned, and \fIerrno\fP is set
appropriately.
.SH ERRORS
.TP
.B EFAULT
-.I t
-points outside your accessible address space.
+.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
-as a value to be interpreted as the number of seconds between a
-specified time and the Epoch, according to a formula for conversion
-from UTC equivalent to conversion on the naive basis that leap
-seconds are ignored and all years divisible by 4 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 clocks are not
-required to be synchronised to a standard reference. The intention is
-that the interpretation of seconds since the Epoch values be
-consistent; see POSIX.1 Annex B 2.2.2 for further rationale.
-.SH "CONFORMING TO"
-SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
-.br
-Under 4.3BSD, this call is obsoleted by
-.BR gettimeofday (2).
-POSIX does not specify any error conditions.
-.SH "SEE ALSO"
+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 time (7),
+.BR vdso (7)
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