stdtime/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 1.22 2004/07/02 23:52:12 ru Exp $
  36 .\"
  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 asctime "const struct tm *timeptr"
  60 .Ft char *
  61 .Fn asctime_r "const struct tm *restrict timeptr" "char *restrict buf"
  62 .Ft char *
  63 .Fn ctime "const time_t *clock"
  64 .Ft char *
  65 .Fn ctime_r "const time_t *clock" "char *buf"
  66 .Ft double
  67 .Fn difftime "time_t time1" "time_t time0"
  68 .Ft struct tm *
  69 .Fn gmtime "const time_t *clock"
  70 .Ft struct tm *
  71 .Fn gmtime_r "const time_t *clock" "struct tm *result"
  72 .Ft struct tm *
  73 .Fn localtime "const time_t *clock"
  74 .Ft struct tm *
  75 .Fn localtime_r "const time_t *clock" "struct tm *result"
  76 .Ft time_t
  77 .Fn mktime "struct tm *timeptr"
  78 .Ft time_t
  79 .Fn timegm "struct tm *timeptr"
  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 It 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 containing 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 also converts the time value, but makes no time zone adjustment.
 129 It 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 It 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 of 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 that the caller must provide the output buffer
 149 .Fa buf
 150 (which must be at least 26 characters long) to store the result.
 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-out time in the structure
 167 .Fa tm
 168 (pointed at by
 169 .Fa *timeptr )
 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 that the caller provides the output buffer
 179 .Fa buf
 180 (which must be at least 26 characters long) to store the result.
 181 .Pp
 182 The functions
 183 .Fn mktime
 184 and
 185 .Fn timegm
 186 convert the broken-out time
 187 (in the structure pointed to by
 188 .Fa *timeptr )
 189 into a time value with the same encoding as that of the
 190 values returned by the
 191 .Xr time 3
 192 function (that is, seconds from the Epoch,
 193 .Tn UTC ) .
 194 The
 195 .Fn mktime
 196 function
 197 interprets the input structure according to the current timezone setting
 198 (see
 199 .Xr tzset 3 ) .
 200 The
 201 .Fn timegm
 202 function interprets the input structure
 203 as representing Universal Coordinated Time
 204 .Pq Tn UTC .
 205 .Pp
 206 The original values of the
 207 .Fa tm_wday
 208 and
 209 .Fa tm_yday
 210 components of the structure are ignored. The original values of the
 211 other components are not restricted to their normal ranges and will be
 212 normalized, if need be.
 213 For example,
 214 October 40 is changed into November 9,
 215 a
 216 .Fa tm_hour
 217 of \-1 means 1 hour before midnight,
 218 .Fa tm_mday
 219 of 0 means the day preceding the current month, and
 220 .Fa tm_mon
 221 of \-2 means 2 months before January of
 222 .Fa tm_year .
 223 (A positive or zero value for
 224 .Fa tm_isdst
 225 causes
 226 .Fn mktime
 227 to presume initially that summer time (for example, Daylight Saving Time)
 228 is or is not (respectively) in effect for the specified time.
 229 A negative value for
 230 .Fa tm_isdst
 231 causes the
 232 .Fn mktime
 233 function to attempt to divine whether summer time is in effect for the
 234 specified time.
 235 The
 236 .Fa tm_isdst
 237 and
 238 .Fa tm_gmtoff
 239 members are forced to zero by
 240 .Fn timegm . )
 241 .Pp
 242 On successful completion, the values of the
 243 .Fa tm_wday
 244 and
 245 .Fa tm_yday
 246 components of the structure are set appropriately, and the other components
 247 are set to represent the specified calendar time, but with their values
 248 forced to their normal ranges; the final value of
 249 .Fa tm_mday
 250 is not set until
 251 .Fa tm_mon
 252 and
 253 .Fa tm_year
 254 are determined.
 255 The
 256 .Fn mktime
 257 function
 258 returns the specified calendar time; if the calendar time cannot be
 259 represented, it returns \-1;
 260 .Pp
 261 The
 262 .Fn difftime
 263 function
 264 returns the difference between two calendar times,
 265 .Pf ( Fa time1
 266 -
 267 .Fa time0 ) ,
 268 expressed in seconds.
 269 .Pp
 270 External declarations, as well as the tm structure definition,
 271 are contained in the
 272 .In time.h
 273 include file.
 274 The tm structure includes at least the following fields:
 275 .Bd -literal -offset indent
 276 int tm_sec;     /\(** seconds (0 - 60) \(**/
 277 int tm_min;     /\(** minutes (0 - 59) \(**/
 278 int tm_hour;    /\(** hours (0 - 23) \(**/
 279 int tm_mday;    /\(** day of month (1 - 31) \(**/
 280 int tm_mon;     /\(** month of year (0 - 11) \(**/
 281 int tm_year;    /\(** year \- 1900 \(**/
 282 int tm_wday;    /\(** day of week (Sunday = 0) \(**/
 283 int tm_yday;    /\(** day of year (0 - 365) \(**/
 284 int tm_isdst;   /\(** is summer time in effect? \(**/
 285 char \(**tm_zone;       /\(** abbreviation of timezone name \(**/
 286 long tm_gmtoff; /\(** offset from UTC in seconds \(**/
 287 .Ed
 288 .Pp
 289 The
 290 field
 291 .Fa tm_isdst
 292 is non-zero if summer (i.e., Daylight Saving) time is in effect.
 293 .Pp
 294 The field
 295 .Fa tm_gmtoff
 296 is the offset (in seconds) of the time represented from
 297 .Tn UTC ,
 298 with positive
 299 values indicating locations east of the Prime Meridian.
 300 .Sh SEE ALSO
 301 .Xr date 1 ,
 302 .Xr gettimeofday 2 ,
 303 .Xr getenv 3 ,
 304 .Xr time 3 ,
 305 .Xr tzset 3 ,
 306 .Xr tzfile 5
 307 .Sh STANDARDS
 308 The
 309 .Fn asctime ,
 310 .Fn ctime ,
 311 .Fn difftime ,
 312 .Fn gmtime ,
 313 .Fn localtime ,
 314 and
 315 .Fn mktime
 316 functions conform to
 317 .St -isoC ,
 318 and conform to
 319 .St -p1003.1-96
 320 provided the selected local timezone does not contain a leap-second table
 321 (see
 322 .Xr zic 8 ) .
 323 .Pp
 324 The
 325 .Fn asctime_r ,
 326 .Fn ctime_r ,
 327 .Fn gmtime_r ,
 328 and
 329 .Fn localtime_r
 330 functions are expected to conform to
 331 .St -p1003.1-96
 332 (again provided the selected local timezone does not contain a leap-second
 333 table).
 334 .Pp
 335 The
 336 .Fn timegm
 337 function is not specified by any standard; its function cannot be
 338 completely emulated using the standard functions described above.
 339 .Sh HISTORY
 340 This manual page is derived from
 341 the time package contributed to Berkeley by
 342 .An Arthur Olson
 343 and which appeared in
 344 .Bx 4.3 .
 345 .Sh BUGS
 346 Except for
 347 .Fn difftime ,
 348 .Fn mktime ,
 349 and the
 350 .Fn \&_r
 351 variants of the other functions,
 352 these functions leaves their result in an internal static object and return
 353 a pointer to that object.
 354 Subsequent calls to these
 355 function will modify the same object.
 356 .Pp
 357 The C Standard provides no mechanism for a program to modify its current
 358 local timezone setting, and the
 359 .Tn POSIX Ns No \&-standard
 360 method is not reentrant.
 361 (However, thread-safe implementations are provided
 362 in the
 363 .Tn POSIX
 364 threaded environment.)
 365 .Pp
 366 The
 367 .Va tm_zone
 368 field of a returned
 369 .Vt tm
 370 structure points to a static array of characters,
 371 which will also be overwritten by any subsequent calls (as well as by
 372 subsequent calls to
 373 .Xr tzset 3
 374 and
 375 .Xr tzsetwall 3 ) .
 376 .Pp
 377 Use of the external variable
 378 .Fa tzname
 379 is discouraged; the
 380 .Fa tm_zone
 381 entry in the tm structure is preferred.