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