stdtime/FreeBSD/ctime.3

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  30 .\"     From: @(#)ctime.3       8.1 (Berkeley) 6/4/93
  31 .\" $FreeBSD: src/lib/libc/stdtime/ctime.3,v 1.24 2007/01/09 00:28:11 imp Exp $
  32 .\"
  33 .Dd January 2, 1999
  34 .Dt CTIME 3
  35 .Os
  36 .Sh NAME
  37 .Nm asctime ,
  38 .Nm asctime_r ,
  39 .Nm ctime ,
  40 .Nm ctime_r ,
  41 .Nm difftime ,
  42 .Nm gmtime ,
  43 .Nm gmtime_r ,
  44 .Nm localtime ,
  45 .Nm localtime_r ,
  46 .Nm mktime ,
  47 .Nm timegm
  48 .Nd transform binary date and time values
  49 .Sh LIBRARY
  50 .Lb libc
  51 .Sh SYNOPSIS
  52 .In time.h
  53 .Vt extern char *tzname[2] ;
  54 .Ft char *
  55 .Fn asctime "const struct tm *timeptr"
  56 .Ft char *
  57 .Fn asctime_r "const struct tm *restrict timeptr" "char *restrict buf"
  58 .Ft char *
  59 .Fn ctime "const time_t *clock"
  60 .Ft char *
  61 .Fn ctime_r "const time_t *clock" "char *buf"
  62 .Ft double
  63 .Fn difftime "time_t time1" "time_t time0"
  64 .Ft struct tm *
  65 .Fn gmtime "const time_t *clock"
  66 .Ft struct tm *
  67 .Fn gmtime_r "const time_t *clock" "struct tm *result"
  68 .Ft struct tm *
  69 .Fn localtime "const time_t *clock"
  70 .Ft struct tm *
  71 .Fn localtime_r "const time_t *clock" "struct tm *result"
  72 .Ft time_t
  73 .Fn mktime "struct tm *timeptr"
  74 .Ft time_t
  75 .Fn timegm "struct tm *timeptr"
  76 .Sh DESCRIPTION
  77 The functions
  78 .Fn ctime ,
  79 .Fn gmtime ,
  80 and
  81 .Fn localtime
  82 all take as an argument a time value representing the time in seconds since
  83 the Epoch (00:00:00
  84 .Tn UTC ,
  85 January 1, 1970; see
  86 .Xr time 3 ) .
  87 When encountering an error, these functions return
  88 .Dv NULL
  89 and set
  90 .Dv errno
  91 to an appropriate value.
  92 .Pp
  93 The function
  94 .Fn localtime
  95 converts the time value pointed at by
  96 .Fa clock .
  97 It returns a pointer to a
  98 .Dq Fa struct tm
  99 (described below), which contains
 100 the broken-out time information for the value after adjusting for the current
 101 time zone (and any other factors such as Daylight Saving Time).
 102 Time zone adjustments are performed as specified by the
 103 .Ev TZ
 104 environment variable (see
 105 .Xr tzset 3 ) .
 106 The function
 107 .Fn localtime
 108 uses
 109 .Xr tzset 3
 110 to initialize time conversion information, if
 111 .Xr tzset 3
 112 has not already been called by the process.
 113 .Pp
 114 After filling in the tm structure,
 115 .Fn localtime
 116 sets the
 117 .Fa tm_isdst Ns 'th
 118 element of
 119 .Fa tzname
 120 to a pointer to an
 121 .Tn ASCII
 122 string containing the time zone abbreviation to be
 123 used with
 124 .Fn localtime Ns 's
 125 return value.
 126 .Pp
 127 The function
 128 .Fn gmtime
 129 also converts the time value, but makes no time zone adjustment.
 130 It returns a pointer to a tm structure (described below).
 131 .Pp
 132 The
 133 .Fn ctime
 134 function
 135 adjusts the time value for the current time zone, in the same manner as
 136 .Fn localtime .
 137 It returns a pointer to a 26-character string of the form:
 138 .Bd -literal -offset indent
 139 Thu Nov 24 18:22:48 1986\en\e0
 140 .Ed
 141 .Pp
 142 All of the fields have constant width.
 143 .Pp
 144 The
 145 .Fn ctime_r
 146 function
 147 provides the same functionality as
 148 .Fn ctime ,
 149 except that the caller must provide the output buffer
 150 .Fa buf
 151 (which must be at least 26 characters long) to store the result.
 152 The
 153 .Fn localtime_r
 154 and
 155 .Fn gmtime_r
 156 functions
 157 provide the same functionality as
 158 .Fn localtime
 159 and
 160 .Fn gmtime ,
 161 respectively, except the caller must provide the output buffer
 162 .Fa result .
 163 .Pp
 164 The
 165 .Fn asctime
 166 function
 167 converts the broken-out time in the structure
 168 .Fa tm
 169 (pointed at by
 170 .Fa *timeptr )
 171 to the form
 172 shown in the example above.
 173 .Pp
 174 The
 175 .Fn asctime_r
 176 function
 177 provides the same functionality as
 178 .Fn asctime ,
 179 except that the caller provides the output buffer
 180 .Fa buf
 181 (which must be at least 26 characters long) to store the result.
 182 .Pp
 183 The functions
 184 .Fn mktime
 185 and
 186 .Fn timegm
 187 convert the broken-out time
 188 (in the structure pointed to by
 189 .Fa *timeptr )
 190 into a time value with the same encoding as that of the
 191 values returned by the
 192 .Xr time 3
 193 function (that is, seconds from the Epoch,
 194 .Tn UTC ) .
 195 The
 196 .Fn mktime
 197 function
 198 interprets the input structure according to the current timezone setting
 199 (see
 200 .Xr tzset 3 ) .
 201 The
 202 .Fn timegm
 203 function interprets the input structure
 204 as representing Universal Coordinated Time
 205 .Pq Tn UTC .
 206 .Pp
 207 The original values of the
 208 .Fa tm_wday
 209 and
 210 .Fa tm_yday
 211 components of the structure are ignored. The original values of the
 212 other components are not restricted to their normal ranges and will be
 213 normalized, if need be.
 214 For example,
 215 October 40 is changed into November 9,
 216 a
 217 .Fa tm_hour
 218 of \-1 means 1 hour before midnight,
 219 .Fa tm_mday
 220 of 0 means the day preceding the current month, and
 221 .Fa tm_mon
 222 of \-2 means 2 months before January of
 223 .Fa tm_year .
 224 (A positive or zero value for
 225 .Fa tm_isdst
 226 causes
 227 .Fn mktime
 228 to presume initially that summer time (for example, Daylight Saving Time)
 229 is or is not (respectively) in effect for the specified time.
 230 A negative value for
 231 .Fa tm_isdst
 232 causes the
 233 .Fn mktime
 234 function to attempt to divine whether summer time is in effect for the
 235 specified time.
 236 The
 237 .Fa tm_isdst
 238 and
 239 .Fa tm_gmtoff
 240 members are forced to zero by
 241 .Fn timegm . )
 242 .Pp
 243 On successful completion, the values of the
 244 .Fa tm_wday
 245 and
 246 .Fa tm_yday
 247 components of the structure are set appropriately, and the other components
 248 are set to represent the specified calendar time, but with their values
 249 forced to their normal ranges; the final value of
 250 .Fa tm_mday
 251 is not set until
 252 .Fa tm_mon
 253 and
 254 .Fa tm_year
 255 are determined.
 256 The
 257 .Fn mktime
 258 function
 259 returns the specified calendar time; if the calendar time cannot be
 260 represented, it returns \-1;
 261 .Pp
 262 The
 263 .Fn difftime
 264 function
 265 returns the difference between two calendar times,
 266 .Pf ( Fa time1
 267 -
 268 .Fa time0 ) ,
 269 expressed in seconds.
 270 .Pp
 271 External declarations, as well as the tm structure definition,
 272 are contained in the
 273 .In time.h
 274 include file.
 275 The tm structure includes at least the following fields:
 276 .Bd -literal -offset indent
 277 int tm_sec;     /\(** seconds (0 - 60) \(**/
 278 int tm_min;     /\(** minutes (0 - 59) \(**/
 279 int tm_hour;    /\(** hours (0 - 23) \(**/
 280 int tm_mday;    /\(** day of month (1 - 31) \(**/
 281 int tm_mon;     /\(** month of year (0 - 11) \(**/
 282 int tm_year;    /\(** year \- 1900 \(**/
 283 int tm_wday;    /\(** day of week (Sunday = 0) \(**/
 284 int tm_yday;    /\(** day of year (0 - 365) \(**/
 285 int tm_isdst;   /\(** is summer time in effect? \(**/
 286 char \(**tm_zone;       /\(** abbreviation of timezone name \(**/
 287 long tm_gmtoff; /\(** offset from UTC in seconds \(**/
 288 .Ed
 289 .Pp
 290 The
 291 field
 292 .Fa tm_isdst
 293 is non-zero if summer (i.e., Daylight Saving) time is in effect.
 294 .Pp
 295 The field
 296 .Fa tm_gmtoff
 297 is the offset (in seconds) of the time represented from
 298 .Tn UTC ,
 299 with positive
 300 values indicating locations east of the Prime Meridian.
 301 .Sh SEE ALSO
 302 .Xr date 1 ,
 303 .Xr gettimeofday 2 ,
 304 .Xr getenv 3 ,
 305 .Xr time 3 ,
 306 .Xr tzset 3 ,
 307 .Xr tzfile 5
 308 .Sh STANDARDS
 309 The
 310 .Fn asctime ,
 311 .Fn ctime ,
 312 .Fn difftime ,
 313 .Fn gmtime ,
 314 .Fn localtime ,
 315 and
 316 .Fn mktime
 317 functions conform to
 318 .St -isoC ,
 319 and conform to
 320 .St -p1003.1-96
 321 provided the selected local timezone does not contain a leap-second table
 322 (see
 323 .Xr zic 8 ) .
 324 .Pp
 325 The
 326 .Fn asctime_r ,
 327 .Fn ctime_r ,
 328 .Fn gmtime_r ,
 329 and
 330 .Fn localtime_r
 331 functions are expected to conform to
 332 .St -p1003.1-96
 333 (again provided the selected local timezone does not contain a leap-second
 334 table).
 335 .Pp
 336 The
 337 .Fn timegm
 338 function is not specified by any standard; its function cannot be
 339 completely emulated using the standard functions described above.
 340 .Sh HISTORY
 341 This manual page is derived from
 342 the time package contributed to Berkeley by
 343 .An Arthur Olson
 344 and which appeared in
 345 .Bx 4.3 .
 346 .Sh BUGS
 347 Except for
 348 .Fn difftime ,
 349 .Fn mktime ,
 350 and the
 351 .Fn \&_r
 352 variants of the other functions,
 353 these functions leaves their result in an internal static object and return
 354 a pointer to that object.
 355 Subsequent calls to these
 356 function will modify the same object.
 357 .Pp
 358 The C Standard provides no mechanism for a program to modify its current
 359 local timezone setting, and the
 360 .Tn POSIX Ns No \&-standard
 361 method is not reentrant.
 362 (However, thread-safe implementations are provided
 363 in the
 364 .Tn POSIX
 365 threaded environment.)
 366 .Pp
 367 The
 368 .Va tm_zone
 369 field of a returned
 370 .Vt tm
 371 structure points to a static array of characters,
 372 which will also be overwritten by any subsequent calls (as well as by
 373 subsequent calls to
 374 .Xr tzset 3
 375 and
 376 .Xr tzsetwall 3 ) .
 377 .Pp
 378 Use of the external variable
 379 .Fa tzname
 380 is discouraged; the
 381 .Fa tm_zone
 382 entry in the tm structure is preferred.