man2/gettimeofday.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  24 .\"
  25 .\" Modified by Michael Haardt (michael@moria.de)
  26 .\" Modified 1993-07-23 by Rik Faith (faith@cs.unc.edu)
  27 .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
  28 .\"   Fixed necessary '#include' lines.
  29 .\" Modified 1995-04-15 by Michael Chastain (mec@shell.portal.com):
  30 .\"   Added reference to adjtimex.
  31 .\" Removed some nonsense lines pointed out by Urs Thuermann,
  32 .\"   (urs@isnogud.escape.de), aeb, 950722.
  33 .\" Modified 1997-01-14 by Austin Donnelly (and1000@debian.org):
  34 .\"   Added return values section, and bit on EFAULT
  35 .\" Added clarification on timezone, aeb, 971210.
  36 .\" Removed "#include <unistd.h>", aeb, 010316.
  37 .\" Modified, 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
  38 .\"   Added notes on capability requirement.
  39 .\"
  40 .TH GETTIMEOFDAY 2 2016-03-15 "Linux" "Linux Programmer's Manual"
  41 .SH NAME
  42 gettimeofday, settimeofday \- get / set time
  43 .SH SYNOPSIS
  44 .nf
  45 .B #include <sys/time.h>
  46
  47 .BI "int gettimeofday(struct timeval *" tv ", struct timezone *" tz );
  48
  49 .BI "int settimeofday(const struct timeval *" tv \
  50 ", const struct timezone *" tz );
  51
  52 .fi
  53 .in -4n
  54 Feature Test Macro Requirements for glibc (see
  55 .BR feature_test_macros (7)):
  56 .in
  57 .sp
  58 .BR settimeofday ():
  59     Since glibc 2.19:
  60         _DEFAULT_SOURCE
  61     Glibc 2.19 and earlier:
  62         _BSD_SOURCE
  63 .SH DESCRIPTION
  64 The functions
  65 .BR gettimeofday ()
  66 and
  67 .BR settimeofday ()
  68 can get and set the time as well as a timezone.
  69 The
  70 .I tv
  71 argument is a
  72 .I struct timeval
  73 (as specified in
  74 .IR <sys/time.h> ):
  75 .sp
  76 .in +4n
  77 .nf
  78 struct timeval {
  79     time_t      tv_sec;     /* seconds */
  80     suseconds_t tv_usec;    /* microseconds */
  81 };
  82 .fi
  83 .in
  84 .sp
  85 and gives the number of seconds and microseconds since the Epoch (see
  86 .BR time (2)).
  87 The
  88 .I tz
  89 argument is a
  90 .IR "struct timezone" :
  91 .sp
  92 .in +4n
  93 .nf
  94 struct timezone {
  95     int tz_minuteswest;     /* minutes west of Greenwich */
  96     int tz_dsttime;         /* type of DST correction */
  97 };
  98 .fi
  99 .in
 100 .PP
 101 If either
 102 .I tv
 103 or
 104 .I tz
 105 is NULL, the corresponding structure is not set or returned.
 106 .\" FIXME . The compilation warning looks to be going away in 2.17
 107 .\" see glibc commit 4b7634a5e03b0da6f8875de9d3f74c1cf6f2a6e8
 108 (However, compilation warnings will result if
 109 .I tv
 110 is NULL.)
 111 .\" The following is covered under EPERM below:
 112 .\" .PP
 113 .\" Only the superuser may use
 114 .\" .BR settimeofday ().
 115 .PP
 116 The use of the
 117 .I timezone
 118 structure is obsolete; the
 119 .I tz
 120 argument should normally be specified as NULL.
 121 (See NOTES below.)
 122
 123 Under Linux, there are some peculiar "warp clock" semantics associated
 124 with the
 125 .BR settimeofday ()
 126 system call if on the very first call (after booting)
 127 that has a non-NULL
 128 .I tz
 129 argument, the
 130 .I tv
 131 argument is NULL and the
 132 .I tz_minuteswest
 133 field is nonzero.
 134 (The
 135 .I tz_dsttime
 136 field should be zero for this case.)
 137 In such a case it is assumed that the CMOS clock
 138 is on local time, and that it has to be incremented by this amount
 139 to get UTC system time.
 140 No doubt it is a bad idea to use this feature.
 141 .SH RETURN VALUE
 142 .BR gettimeofday ()
 143 and
 144 .BR settimeofday ()
 145 return 0 for success, or \-1 for failure (in which case
 146 .I errno
 147 is set appropriately).
 148 .SH ERRORS
 149 .TP
 150 .B EFAULT
 151 One of
 152 .I tv
 153 or
 154 .I tz
 155 pointed outside the accessible address space.
 156 .TP
 157 .B EINVAL
 158 Timezone (or something else) is invalid.
 159 .TP
 160 .B EPERM
 161 The calling process has insufficient privilege to call
 162 .BR settimeofday ();
 163 under Linux the
 164 .B CAP_SYS_TIME
 165 capability is required.
 166 .SH CONFORMING TO
 167 SVr4, 4.3BSD.
 168 POSIX.1-2001 describes
 169 .BR gettimeofday ()
 170 but not
 171 .BR settimeofday ().
 172 POSIX.1-2008 marks
 173 .BR gettimeofday ()
 174 as obsolete, recommending the use of
 175 .BR clock_gettime (2)
 176 instead.
 177 .SH NOTES
 178 The time returned by
 179 .BR gettimeofday ()
 180 .I is
 181 affected by discontinuous jumps in the system time
 182 (e.g., if the system administrator manually changes the system time).
 183 If you need a monotonically increasing clock, see
 184 .BR clock_gettime (2).
 185
 186 Macros for operating on
 187 .I timeval
 188 structures are described in
 189 .BR timeradd (3).
 190
 191 Traditionally, the fields of
 192 .I struct timeval
 193 were of type
 194 .IR long .
 195 .\"
 196 .SS The tz_dsttime field
 197 On a non-Linux kernel, with glibc, the
 198 .I tz_dsttime
 199 field of
 200 .I struct timezone
 201 will be set to a nonzero value by
 202 .BR gettimeofday ()
 203 if the current timezone has ever had or will have a daylight saving
 204 rule applied.
 205 In this sense it exactly mirrors the meaning of
 206 .BR daylight (3)
 207 for the current zone.
 208 On Linux, with glibc, the setting of the
 209 .I tz_dsttime
 210 field of
 211 .I struct timezone
 212 has never been used by
 213 .BR settimeofday ()
 214 or
 215 .BR gettimeofday ().
 216 .\" it has not
 217 .\" been and will not be supported by libc or glibc.
 218 .\" Each and every occurrence of this field in the kernel source
 219 .\" (other than the declaration) is a bug.
 220 Thus, the following is purely of historical interest.
 221
 222 On old systems, the field
 223 .I tz_dsttime
 224 contains a symbolic constant (values are given below)
 225 that indicates in which part of the year Daylight Saving Time
 226 is in force.
 227 (Note: this value is constant throughout the year:
 228 it does not indicate that DST is in force, it just selects an
 229 algorithm.)
 230 The daylight saving time algorithms defined are as follows:
 231 .in +4n
 232 .nf
 233
 234 \fBDST_NONE\fP     /* not on DST */
 235 .br
 236 \fBDST_USA\fP      /* USA style DST */
 237 .br
 238 \fBDST_AUST\fP     /* Australian style DST */
 239 .br
 240 \fBDST_WET\fP      /* Western European DST */
 241 .br
 242 \fBDST_MET\fP      /* Middle European DST */
 243 .br
 244 \fBDST_EET\fP      /* Eastern European DST */
 245 .br
 246 \fBDST_CAN\fP      /* Canada */
 247 .br
 248 \fBDST_GB\fP       /* Great Britain and Eire */
 249 .br
 250 \fBDST_RUM\fP      /* Romania */
 251 .br
 252 \fBDST_TUR\fP      /* Turkey */
 253 .br
 254 \fBDST_AUSTALT\fP  /* Australian style with shift in 1986 */
 255 .fi
 256 .in
 257 .PP
 258 Of course it turned out that the period in which
 259 Daylight Saving Time is in force cannot be given
 260 by a simple algorithm, one per country; indeed,
 261 this period is determined by unpredictable political
 262 decisions.
 263 So this method of representing timezones
 264 has been abandoned.
 265 .SH SEE ALSO
 266 .BR date (1),
 267 .BR adjtimex (2),
 268 .BR clock_gettime (2),
 269 .BR time (2),
 270 .BR ctime (3),
 271 .BR ftime (3),
 272 .BR timeradd (3),
 273 .BR capabilities (7),
 274 .BR time (7),
 275 .BR vdso (7),
 276 .BR hwclock (8)