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 2019-03-06 "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 .PP
  47 .BI "int gettimeofday(struct timeval *" tv ", struct timezone *" tz );
  48 .PP
  49 .BI "int settimeofday(const struct timeval *" tv \
  50 ", const struct timezone *" tz );
  51 .fi
  52 .PP
  53 .in -4n
  54 Feature Test Macro Requirements for glibc (see
  55 .BR feature_test_macros (7)):
  56 .in
  57 .PP
  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 .PP
  76 .in +4n
  77 .EX
  78 struct timeval {
  79     time_t      tv_sec;     /* seconds */
  80     suseconds_t tv_usec;    /* microseconds */
  81 };
  82 .EE
  83 .in
  84 .PP
  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 .PP
  92 .in +4n
  93 .EX
  94 struct timezone {
  95     int tz_minuteswest;     /* minutes west of Greenwich */
  96     int tz_dsttime;         /* type of DST correction */
  97 };
  98 .EE
  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 .PP
 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 .RB ( settimeofday ()):
 159 .I timezone
 160 is invalid.
 161 .TP
 162 .B EINVAL
 163 .RB ( settimeofday ()):
 164 .I tv.tv_sec
 165 is negative or
 166 .I tv.tv_usec
 167 is outside the range [0..999,999].
 168 .TP
 169 .BR EINVAL " (since Linux 4.3)"
 170 .\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
 171 .RB ( settimeofday ()):
 172 An attempt was made to set the time to a value less than
 173 the current value of the
 174 .B CLOCK_MONOTONIC
 175 clock (see
 176 .BR clock_gettime (2)).
 177 .TP
 178 .B EPERM
 179 The calling process has insufficient privilege to call
 180 .BR settimeofday ();
 181 under Linux the
 182 .B CAP_SYS_TIME
 183 capability is required.
 184 .SH CONFORMING TO
 185 SVr4, 4.3BSD.
 186 POSIX.1-2001 describes
 187 .BR gettimeofday ()
 188 but not
 189 .BR settimeofday ().
 190 POSIX.1-2008 marks
 191 .BR gettimeofday ()
 192 as obsolete, recommending the use of
 193 .BR clock_gettime (2)
 194 instead.
 195 .SH NOTES
 196 The time returned by
 197 .BR gettimeofday ()
 198 .I is
 199 affected by discontinuous jumps in the system time
 200 (e.g., if the system administrator manually changes the system time).
 201 If you need a monotonically increasing clock, see
 202 .BR clock_gettime (2).
 203 .PP
 204 Macros for operating on
 205 .I timeval
 206 structures are described in
 207 .BR timeradd (3).
 208 .PP
 209 Traditionally, the fields of
 210 .I struct timeval
 211 were of type
 212 .IR long .
 213 .\"
 214 .SS C library/kernel differences
 215 On some architectures, an implementation of
 216 .BR gettimeofday ()
 217 is provided in the
 218 .BR vdso (7).
 219 .\"
 220 .SS The tz_dsttime field
 221 On a non-Linux kernel, with glibc, the
 222 .I tz_dsttime
 223 field of
 224 .I struct timezone
 225 will be set to a nonzero value by
 226 .BR gettimeofday ()
 227 if the current timezone has ever had or will have a daylight saving
 228 rule applied.
 229 In this sense it exactly mirrors the meaning of
 230 .BR daylight (3)
 231 for the current zone.
 232 On Linux, with glibc, the setting of the
 233 .I tz_dsttime
 234 field of
 235 .I struct timezone
 236 has never been used by
 237 .BR settimeofday ()
 238 or
 239 .BR gettimeofday ().
 240 .\" it has not
 241 .\" been and will not be supported by libc or glibc.
 242 .\" Each and every occurrence of this field in the kernel source
 243 .\" (other than the declaration) is a bug.
 244 Thus, the following is purely of historical interest.
 245 .PP
 246 On old systems, the field
 247 .I tz_dsttime
 248 contains a symbolic constant (values are given below)
 249 that indicates in which part of the year Daylight Saving Time
 250 is in force.
 251 (Note: this value is constant throughout the year:
 252 it does not indicate that DST is in force, it just selects an
 253 algorithm.)
 254 The daylight saving time algorithms defined are as follows:
 255 .PP
 256 .in +4n
 257 .EX
 258 \fBDST_NONE\fP     /* not on DST */
 259 \fBDST_USA\fP      /* USA style DST */
 260 \fBDST_AUST\fP     /* Australian style DST */
 261 \fBDST_WET\fP      /* Western European DST */
 262 \fBDST_MET\fP      /* Middle European DST */
 263 \fBDST_EET\fP      /* Eastern European DST */
 264 \fBDST_CAN\fP      /* Canada */
 265 \fBDST_GB\fP       /* Great Britain and Eire */
 266 \fBDST_RUM\fP      /* Romania */
 267 \fBDST_TUR\fP      /* Turkey */
 268 \fBDST_AUSTALT\fP  /* Australian style with shift in 1986 */
 269 .EE
 270 .in
 271 .PP
 272 Of course it turned out that the period in which
 273 Daylight Saving Time is in force cannot be given
 274 by a simple algorithm, one per country; indeed,
 275 this period is determined by unpredictable political
 276 decisions.
 277 So this method of representing timezones
 278 has been abandoned.
 279 .SH SEE ALSO
 280 .BR date (1),
 281 .BR adjtimex (2),
 282 .BR clock_gettime (2),
 283 .BR time (2),
 284 .BR ctime (3),
 285 .BR ftime (3),
 286 .BR timeradd (3),
 287 .BR capabilities (7),
 288 .BR time (7),
 289 .BR vdso (7),
 290 .BR hwclock (8)