stdtime/FreeBSD/ctime.3

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  34 .\"     From: @(#)ctime.3       8.1 (Berkeley) 6/4/93
  35 .\" $FreeBSD: src/lib/libc/stdtime/ctime.3,v 1.20 2002/12/19 09:33:34 ru Exp $
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  37 .Dd January 2, 1999
  38 .Dt CTIME 3
  39 .Os
  40 .Sh NAME
  41 .Nm asctime ,
  42 .Nm asctime_r ,
  43 .Nm ctime ,
  44 .Nm ctime_r ,
  45 .Nm difftime ,
  46 .Nm gmtime ,
  47 .Nm gmtime_r ,
  48 .Nm localtime ,
  49 .Nm localtime_r ,
  50 .Nm mktime ,
  51 .Nm timegm
  52 .Nd transform binary date and time values
  53 .Sh LIBRARY
  54 .Lb libc
  55 .Sh SYNOPSIS
  56 .In time.h
  57 .Vt extern char *tzname[2] ;
  58 .Ft char *
  59 .Fn ctime "const time_t *clock"
  60 .Ft double
  61 .Fn difftime "time_t time1" "time_t time0"
  62 .Ft char *
  63 .Fn asctime "const struct tm *tm"
  64 .Ft struct tm *
  65 .Fn localtime "const time_t *clock"
  66 .Ft struct tm *
  67 .Fn gmtime "const time_t *clock"
  68 .Ft time_t
  69 .Fn mktime "struct tm *tm"
  70 .Ft time_t
  71 .Fn timegm "struct tm *tm"
  72 .Ft char *
  73 .Fn ctime_r "const time_t *clock" "char *buf"
  74 .Ft struct tm *
  75 .Fn localtime_r "const time_t *clock" "struct tm *result"
  76 .Ft struct tm *
  77 .Fn gmtime_r "const time_t *clock" "struct tm *result"
  78 .Ft char *
  79 .Fn asctime_r "const struct tm *tm" "char *buf"
  80 .Sh DESCRIPTION
  81 The functions
  82 .Fn ctime ,
  83 .Fn gmtime
  84 and
  85 .Fn localtime
  86 all take as an argument a time value representing the time in seconds since
  87 the Epoch (00:00:00
  88 .Tn UTC ,
  89 January 1, 1970; see
  90 .Xr time 3 ) .
  91 .Pp
  92 The function
  93 .Fn localtime
  94 converts the time value pointed at by
  95 .Fa clock ,
  96 and returns a pointer to a
  97 .Dq Fa struct tm
  98 (described below) which contains
  99 the broken-out time information for the value after adjusting for the current
 100 time zone (and any other factors such as Daylight Saving Time).
 101 Time zone adjustments are performed as specified by the
 102 .Ev TZ
 103 environment variable (see
 104 .Xr tzset 3 ) .
 105 The function
 106 .Fn localtime
 107 uses
 108 .Xr tzset 3
 109 to initialize time conversion information if
 110 .Xr tzset 3
 111 has not already been called by the process.
 112 .Pp
 113 After filling in the tm structure,
 114 .Fn localtime
 115 sets the
 116 .Fa tm_isdst Ns 'th
 117 element of
 118 .Fa tzname
 119 to a pointer to an
 120 .Tn ASCII
 121 string that's the time zone abbreviation to be
 122 used with
 123 .Fn localtime Ns 's
 124 return value.
 125 .Pp
 126 The function
 127 .Fn gmtime
 128 similarly converts the time value, but without any time zone adjustment,
 129 and returns a pointer to a tm structure (described below).
 130 .Pp
 131 The
 132 .Fn ctime
 133 function
 134 adjusts the time value for the current time zone in the same manner as
 135 .Fn localtime ,
 136 and returns a pointer to a 26-character string of the form:
 137 .Bd -literal -offset indent
 138 Thu Nov 24 18:22:48 1986\en\e0
 139 .Ed
 140 .Pp
 141 All the fields have constant width.
 142 .Pp
 143 The
 144 .Fn ctime_r
 145 function
 146 provides the same functionality as
 147 .Fn ctime
 148 except the caller must provide the output buffer
 149 .Fa buf
 150 to store the result, which must be at least 26 characters long.
 151 The
 152 .Fn localtime_r
 153 and
 154 .Fn gmtime_r
 155 functions
 156 provide the same functionality as
 157 .Fn localtime
 158 and
 159 .Fn gmtime
 160 respectively, except the caller must provide the output buffer
 161 .Fa result .
 162 .Pp
 163 The
 164 .Fn asctime
 165 function
 166 converts the broken down time in the structure
 167 .Fa tm
 168 pointed at by
 169 .Fa *tm
 170 to the form
 171 shown in the example above.
 172 .Pp
 173 The
 174 .Fn asctime_r
 175 function
 176 provides the same functionality as
 177 .Fn asctime
 178 except the caller provide the output buffer
 179 .Fa buf
 180 to store the result, which must be at least 26 characters long.
 181 .Pp
 182 The functions
 183 .Fn mktime
 184 and
 185 .Fn timegm
 186 convert the broken-down time in the structure
 187 pointed to by tm into a time value with the same encoding as that of the
 188 values returned by the
 189 .Xr time 3
 190 function (that is, seconds from the Epoch,
 191 .Tn UTC ) .
 192 The
 193 .Fn mktime
 194 function
 195 interprets the input structure according to the current timezone setting
 196 (see
 197 .Xr tzset 3 ) .
 198 The
 199 .Fn timegm
 200 function
 201 interprets the input structure as representing Universal Coordinated Time
 202 .Pq Tn UTC .
 203 .Pp
 204 The original values of the
 205 .Fa tm_wday
 206 and
 207 .Fa tm_yday
 208 components of the structure are ignored, and the original values of the
 209 other components are not restricted to their normal ranges, and will be
 210 normalized if needed.
 211 For example,
 212 October 40 is changed into November 9,
 213 a
 214 .Fa tm_hour
 215 of \-1 means 1 hour before midnight,
 216 .Fa tm_mday
 217 of 0 means the day preceding the current month, and
 218 .Fa tm_mon
 219 of \-2 means 2 months before January of
 220 .Fa tm_year .
 221 (A positive or zero value for
 222 .Fa tm_isdst
 223 causes
 224 .Fn mktime
 225 to presume initially that summer time (for example, Daylight Saving Time)
 226 is or is not in effect for the specified time, respectively.
 227 A negative value for
 228 .Fa tm_isdst
 229 causes the
 230 .Fn mktime
 231 function to attempt to divine whether summer time is in effect for the
 232 specified time.
 233 The
 234 .Fa tm_isdst
 235 and
 236 .Fa tm_gmtoff
 237 members are forced to zero by
 238 .Fn timegm . )
 239 .Pp
 240 On successful completion, the values of the
 241 .Fa tm_wday
 242 and
 243 .Fa tm_yday
 244 components of the structure are set appropriately, and the other components
 245 are set to represent the specified calendar time, but with their values
 246 forced to their normal ranges; the final value of
 247 .Fa tm_mday
 248 is not set until
 249 .Fa tm_mon
 250 and
 251 .Fa tm_year
 252 are determined.
 253 The
 254 .Fn mktime
 255 function
 256 returns the specified calendar time; if the calendar time cannot be
 257 represented, it returns \-1;
 258 .Pp
 259 The
 260 .Fn difftime
 261 function
 262 returns the difference between two calendar times,
 263 .Pf ( Fa time1
 264 -
 265 .Fa time0 ) ,
 266 expressed in seconds.
 267 .Pp
 268 External declarations as well as the tm structure definition are in the
 269 .Aq Pa time.h
 270 include file.
 271 The tm structure includes at least the following fields:
 272 .Bd -literal -offset indent
 273 int tm_sec;     /\(** seconds (0 - 60) \(**/
 274 int tm_min;     /\(** minutes (0 - 59) \(**/
 275 int tm_hour;    /\(** hours (0 - 23) \(**/
 276 int tm_mday;    /\(** day of month (1 - 31) \(**/
 277 int tm_mon;     /\(** month of year (0 - 11) \(**/
 278 int tm_year;    /\(** year \- 1900 \(**/
 279 int tm_wday;    /\(** day of week (Sunday = 0) \(**/
 280 int tm_yday;    /\(** day of year (0 - 365) \(**/
 281 int tm_isdst;   /\(** is summer time in effect? \(**/
 282 char \(**tm_zone;       /\(** abbreviation of timezone name \(**/
 283 long tm_gmtoff; /\(** offset from UTC in seconds \(**/
 284 .Ed
 285 .Pp
 286 The
 287 field
 288 .Fa tm_isdst
 289 is non-zero if summer time is in effect.
 290 .Pp
 291 The field
 292 .Fa tm_gmtoff
 293 is the offset (in seconds) of the time represented from
 294 .Tn UTC ,
 295 with positive
 296 values indicating east of the Prime Meridian.
 297 .Sh SEE ALSO
 298 .Xr date 1 ,
 299 .Xr gettimeofday 2 ,
 300 .Xr getenv 3 ,
 301 .Xr time 3 ,
 302 .Xr tzset 3 ,
 303 .Xr tzfile 5
 304 .Sh STANDARDS
 305 The
 306 .Fn asctime ,
 307 .Fn ctime ,
 308 .Fn difftime ,
 309 .Fn gmtime ,
 310 .Fn localtime ,
 311 and
 312 .Fn mktime
 313 functions conform to
 314 .St -isoC ,
 315 and conform to
 316 .St -p1003.1-96
 317 provided the selected local timezone does not contain a leap-second table
 318 (see
 319 .Xr zic 8 ) .
 320 .Pp
 321 The
 322 .Fn asctime_r ,
 323 .Fn ctime_r ,
 324 .Fn gmtime_r ,
 325 and
 326 .Fn localtime_r
 327 functions are expected to conform to
 328 .St -p1003.1-96
 329 (again provided the selected local timezone does not contain a leap-second
 330 table).
 331 .Pp
 332 The
 333 .Fn timegm
 334 function is not specified by any standard; its function cannot be
 335 completely emulated using the standard functions described above.
 336 .Sh HISTORY
 337 This manual page is derived from
 338 the time package contributed to Berkeley by
 339 .An Arthur Olson
 340 and which appeared in
 341 .Bx 4.3 .
 342 .Sh BUGS
 343 Except for
 344 .Fn difftime ,
 345 .Fn mktime ,
 346 and the
 347 .Fn \&_r
 348 variants of the other functions,
 349 these functions leaves their result in an internal static object and return
 350 a pointer to that object.
 351 Subsequent calls to these
 352 function will modify the same object.
 353 .Pp
 354 The C Standard provides no mechanism for a program to modify its current
 355 local timezone setting, and the
 356 .Tn POSIX Ns No \&-standard
 357 method is not reentrant.  (However, thread-safe implementations are provided
 358 in the
 359 .Tn POSIX
 360 threaded environment.)
 361 .Pp
 362 The
 363 .Va tm_zone
 364 field of a returned
 365 .Vt tm
 366 structure points to a static array of characters,
 367 which will also be overwritten by any subsequent calls (as well as by
 368 subsequent calls to
 369 .Xr tzset 3
 370 and
 371 .Xr tzsetwall 3 ) .
 372 .Pp
 373 Use of the external variable
 374 .Fa tzname
 375 is discouraged; the
 376 .Fa tm_zone
 377 entry in the tm structure is preferred.