mirror of https://github.com/mkerrisk/man-pages
132 lines
4.2 KiB
Groff
132 lines
4.2 KiB
Groff
.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Modified by Michael Haardt <michael@moria.de>
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.\" Modified Sat Jul 24 14:13:40 1993 by Rik Faith <faith@cs.unc.edu>
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.\" Additions by Joseph S. Myers <jsm28@cam.ac.uk>, 970909
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.\"
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.TH TIME 2 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
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time \- get time in seconds
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.SH SYNOPSIS
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.B #include <time.h>
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.PP
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.BI "time_t time(time_t *" tloc );
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.SH DESCRIPTION
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.BR time ()
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returns the time as the number of seconds since the
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Epoch, 1970-01-01 00:00:00 +0000 (UTC).
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.PP
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If
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.I tloc
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is non-NULL,
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the return value is also stored in the memory pointed to by
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.IR tloc .
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.SH RETURN VALUE
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On success, the value of time in seconds since the Epoch is returned.
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On error, \fI((time_t)\ \-1)\fP is returned, and \fIerrno\fP is set
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appropriately.
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.SH ERRORS
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.TP
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.B EFAULT
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.I tloc
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points outside your accessible address space (but see BUGS).
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.IP
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On systems where the C library
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.BR time ()
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wrapper function invokes an implementation provided by the
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.BR vdso (7)
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(so that there is no trap into the kernel),
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an invalid address may instead trigger a
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.B SIGSEGV
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signal.
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.SH CONFORMING TO
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SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
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.\" Under 4.3BSD, this call is obsoleted by
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.\" .BR gettimeofday (2).
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POSIX does not specify any error conditions.
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.SH NOTES
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POSIX.1 defines
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.I seconds since the Epoch
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using a formula that approximates the number of seconds between a
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specified time and the Epoch.
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This formula takes account of the facts that
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all years that are evenly divisible by 4 are leap years,
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but years that are evenly divisible by 100 are not leap years
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unless they are also evenly divisible by 400,
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in which case they are leap years.
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This value is not the same as the actual number of seconds between the time
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and the Epoch, because of leap seconds and because system clocks are not
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required to be synchronized to a standard reference.
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The intention is that the interpretation of seconds since the Epoch values be
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consistent; see POSIX.1-2008 Rationale A.4.15 for further rationale.
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.PP
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On Linux, a call to
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.BR time ()
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with
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.I tloc
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specified as NULL cannot fail with the error
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.BR EOVERFLOW ,
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even on ABIs where
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.I time_t
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is a signed 32-bit integer and the clock ticks past the time 2**31
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(2038-01-19 03:14:08 UTC, ignoring leap seconds).
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(POSIX.1 permits, but does not require, the
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.B EOVERFLOW
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error in the case where the seconds since the Epoch will not fit in
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.IR time_t .)
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Instead, the behavior on Linux is undefined when the system time is out of the
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.I time_t
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range.
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Applications intended to run after 2038 should use ABIs with
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.I time_t
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wider than 32 bits.
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.SH BUGS
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Error returns from this system call are indistinguishable from
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successful reports that the time is a few seconds
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.I before
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the Epoch, so the C library wrapper function never sets
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.I errno
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as a result of this call.
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.PP
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The
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.I tloc
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argument is obsolescent and should always be NULL in new code.
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When
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.I tloc
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is NULL, the call cannot fail.
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.\"
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.SS C library/kernel differences
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On some architectures, an implementation of
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.BR time ()
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is provided in the
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.BR vdso (7).
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.SH SEE ALSO
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.BR date (1),
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.BR gettimeofday (2),
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.BR ctime (3),
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.BR ftime (3),
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.BR time (7),
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.BR vdso (7)
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