man2/time.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  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 2017-09-15 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 time \- get time in seconds
  32 .SH SYNOPSIS
  33 .B #include <time.h>
  34 .PP
  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 .PP
  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 .IP
  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 .\" Under 4.3BSD, this call is obsoleted by
  67 .\" .BR gettimeofday (2).
  68 POSIX does not specify any error conditions.
  69 .SH NOTES
  70 POSIX.1 defines
  71 .I seconds since the Epoch
  72 using a formula that approximates the number of seconds between a
  73 specified time and the Epoch.
  74 This formula takes account of the facts that
  75 all years that are evenly divisible by 4 are leap years,
  76 but years that are evenly divisible by 100 are not leap years
  77 unless they are also evenly divisible by 400,
  78 in which case they are leap years.
  79 This value is not the same as the actual number of seconds between the time
  80 and the Epoch, because of leap seconds and because system clocks are not
  81 required to be synchronized to a standard reference.
  82 The intention is that the interpretation of seconds since the Epoch values be
  83 consistent; see POSIX.1-2008 Rationale A.4.15 for further rationale.
  84 .PP
  85 On Linux, a call to
  86 .BR time ()
  87 with
  88 .I tloc
  89 specified as NULL cannot fail with the error
  90 .BR EOVERFLOW ,
  91 even on ABIs where
  92 .I time_t
  93 is a signed 32-bit integer and the clock ticks past the time 2**31
  94 (2038-01-19 03:14:08 UTC, ignoring leap seconds).
  95 (POSIX.1 permits, but does not require, the
  96 .B EOVERFLOW
  97 error in the case where the seconds since the Epoch will not fit in
  98 .IR time_t .)
  99 Instead, the behavior on Linux is undefined when the system time is out of the
 100 .I time_t
 101 range.
 102 Applications intended to run after 2038 should use ABIs with
 103 .I time_t
 104 wider than 32 bits.
 105 .SH BUGS
 106 Error returns from this system call are indistinguishable from
 107 successful reports that the time is a few seconds
 108 .I before
 109 the Epoch, so the C library wrapper function never sets
 110 .I errno
 111 as a result of this call.
 112 .PP
 113 The
 114 .I tloc
 115 argument is obsolescent and should always be NULL in new code.
 116 When
 117 .I tloc
 118 is NULL, the call cannot fail.
 119 .\"
 120 .SS C library/kernel differences
 121 On some architectures, an implementation of
 122 .BR time ()
 123 is provided in the
 124 .BR vdso (7).
 125 .SH SEE ALSO
 126 .BR date (1),
 127 .BR gettimeofday (2),
 128 .BR ctime (3),
 129 .BR ftime (3),
 130 .BR time (7),
 131 .BR vdso (7)