man2/time.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one.
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .\" Modified by Michael Haardt <michael@moria.de>
  26 .\" Modified Sat Jul 24 14:13:40 1993 by Rik Faith <faith@cs.unc.edu>
  27 .\" Additions by Joseph S. Myers <jsm28@cam.ac.uk>, 970909
  28 .\"
  29 .TH TIME 2 2011-09-09 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 time \- get time in seconds
  32 .SH SYNOPSIS
  33 .B #include <time.h>
  34 .sp
  35 .BI "time_t time(time_t *" tloc );
  36 .SH DESCRIPTION
  37 .BR time ()
  38 returns the time as the number of seconds since the
  39 Epoch, 1970-01-01 00:00:00 +0000 (UTC).
  40
  41 If
  42 .I tloc
  43 is non-NULL,
  44 the return value is also stored in the memory pointed to by
  45 .IR tloc .
  46 .SH RETURN VALUE
  47 On success, the value of time in seconds since the Epoch is returned.
  48 On error, \fI((time_t)\ \-1)\fP is returned, and \fIerrno\fP is set
  49 appropriately.
  50 .SH ERRORS
  51 .TP
  52 .B EFAULT
  53 .I tloc
  54 points outside your accessible address space (but see BUGS).
  55
  56 On systems where the C library
  57 .BR time ()
  58 wrapper function invokes an implementation provided by the
  59 .BR vdso (7)
  60 (so that there is no trap into the kernel),
  61 an invalid address may instead trigger a
  62 .B SIGSEGV
  63 signal.
  64 .SH CONFORMING TO
  65 SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
  66 .\" .br
  67 .\" Under 4.3BSD, this call is obsoleted by
  68 .\" .BR gettimeofday (2).
  69 POSIX does not specify any error conditions.
  70 .SH NOTES
  71 POSIX.1 defines
  72 .I seconds since the Epoch
  73 using a formula that approximates the number of seconds between a
  74 specified time and the Epoch.
  75 This formula takes account of the facts that
  76 all years that are evenly divisible by 4 are leap years,
  77 but years that are evenly divisible by 100 are not leap years
  78 unless they are also evenly divisible by 400,
  79 in which case they are leap years.
  80 This value is not the same as the actual number of seconds between the time
  81 and the Epoch, because of leap seconds and because system clocks are not
  82 required to be synchronized to a standard reference.
  83 The intention is that the interpretation of seconds since the Epoch values be
  84 consistent; see POSIX.1-2008 Rationale A.4.15 for further rationale.
  85
  86 On Linux, a call to
  87 .BR time ()
  88 with
  89 .I tloc
  90 specified as NULL cannot fail with the error
  91 .BR EOVERFLOW ,
  92 even on ABIs where
  93 .I time_t
  94 is a signed 32-bit integer and the clock ticks past the time 2**31
  95 (2038-01-19 03:14:08 UTC, ignoring leap seconds).
  96 (POSIX.1 permits, but does not require, the
  97 .B EOVERFLOW
  98 error in the case where the seconds since the Epoch will not fit in
  99 .IR time_t .)
 100 Instead, the behavior on Linux is undefined when the system time is out of the
 101 .I time_t
 102 range.
 103 Applications intended to run after 2038 should use ABIs with
 104 .I time_t
 105 wider than 32 bits.
 106 .SH BUGS
 107 Error returns from this system call are indistinguishable from
 108 successful reports that the time is a few seconds
 109 .I before
 110 the Epoch, so the C library wrapper function never sets
 111 .I errno
 112 as a result of this call.
 113
 114 The
 115 .I tloc
 116 argument is obsolescent and should always be NULL in new code.
 117 When
 118 .I tloc
 119 is NULL, the call cannot fail.
 120 .SH SEE ALSO
 121 .BR date (1),
 122 .BR gettimeofday (2),
 123 .BR ctime (3),
 124 .BR ftime (3),
 125 .BR time (7),
 126 .BR vdso (7)