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 ctime "const time_t *clock"
  56 .Ft double
  57 .Fn difftime "time_t time1" "time_t time0"
  58 .Ft char *
  59 .Fn asctime "const struct tm *tm"
  60 .Ft struct tm *
  61 .Fn localtime "const time_t *clock"
  62 .Ft struct tm *
  63 .Fn gmtime "const time_t *clock"
  64 .Ft time_t
  65 .Fn mktime "struct tm *tm"
  66 .Ft time_t
  67 .Fn timegm "struct tm *tm"
  68 .Ft char *
  69 .Fn ctime_r "const time_t *clock" "char *buf"
  70 .Ft struct tm *
  71 .Fn localtime_r "const time_t *clock" "struct tm *result"
  72 .Ft struct tm *
  73 .Fn gmtime_r "const time_t *clock" "struct tm *result"
  74 .Ft char *
  75 .Fn asctime_r "const struct tm *tm" "char *buf"
  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 and 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 that is 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 similarly converts the time value, but without any time zone adjustment,
 125 and 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 and 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 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 the caller must provide the output buffer
 145 .Fa buf
 146 to store the result, which must be at least 26 characters long.
 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 down time in the structure
 163 .Fa tm
 164 pointed at by
 165 .Fa *tm
 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 the caller provide the output buffer
 175 .Fa buf
 176 to store the result, which must be at least 26 characters long.
 177 .Pp
 178 The functions
 179 .Fn mktime
 180 and
 181 .Fn timegm
 182 convert the broken-down time in the structure
 183 pointed to by tm into a time value with the same encoding as that of the
 184 values returned by the
 185 .Xr time 3
 186 function (that is, seconds from the Epoch,
 187 .Tn UTC ) .
 188 The
 189 .Fn mktime
 190 function
 191 interprets the input structure according to the current timezone setting
 192 (see
 193 .Xr tzset 3 ) .
 194 The
 195 .Fn timegm
 196 function
 197 interprets the input structure as representing Universal Coordinated Time
 198 .Pq Tn UTC .
 199 .Pp
 200 The original values of the
 201 .Fa tm_wday
 202 and
 203 .Fa tm_yday
 204 components of the structure are ignored, and the original values of the
 205 other components are not restricted to their normal ranges, and will be
 206 normalized if needed.
 207 For example,
 208 October 40 is changed into November 9,
 209 a
 210 .Fa tm_hour
 211 of \-1 means 1 hour before midnight,
 212 .Fa tm_mday
 213 of 0 means the day preceding the current month, and
 214 .Fa tm_mon
 215 of \-2 means 2 months before January of
 216 .Fa tm_year .
 217 (A positive or zero value for
 218 .Fa tm_isdst
 219 causes
 220 .Fn mktime
 221 to presume initially that summer time (for example, Daylight Saving Time)
 222 is or is not in effect for the specified time, respectively.
 223 A negative value for
 224 .Fa tm_isdst
 225 causes the
 226 .Fn mktime
 227 function to attempt to divine whether summer time is in effect for the
 228 specified time.
 229 The
 230 .Fa tm_isdst
 231 and
 232 .Fa tm_gmtoff
 233 members are forced to zero by
 234 .Fn timegm . )
 235 .Pp
 236 On successful completion, the values of the
 237 .Fa tm_wday
 238 and
 239 .Fa tm_yday
 240 components of the structure are set appropriately, and the other components
 241 are set to represent the specified calendar time, but with their values
 242 forced to their normal ranges; the final value of
 243 .Fa tm_mday
 244 is not set until
 245 .Fa tm_mon
 246 and
 247 .Fa tm_year
 248 are determined.
 249 The
 250 .Fn mktime
 251 function
 252 returns the specified calendar time; if the calendar time cannot be
 253 represented, it returns \-1;
 254 .Pp
 255 The
 256 .Fn difftime
 257 function
 258 returns the difference between two calendar times,
 259 .Pf ( Fa time1
 260 -
 261 .Fa time0 ) ,
 262 expressed in seconds.
 263 .Pp
 264 External declarations as well as the tm structure definition are in the
 265 .In time.h
 266 include file.
 267 The tm structure includes at least the following fields:
 268 .Bd -literal -offset indent
 269 int tm_sec;     /\(** seconds (0 - 60) \(**/
 270 int tm_min;     /\(** minutes (0 - 59) \(**/
 271 int tm_hour;    /\(** hours (0 - 23) \(**/
 272 int tm_mday;    /\(** day of month (1 - 31) \(**/
 273 int tm_mon;     /\(** month of year (0 - 11) \(**/
 274 int tm_year;    /\(** year \- 1900 \(**/
 275 int tm_wday;    /\(** day of week (Sunday = 0) \(**/
 276 int tm_yday;    /\(** day of year (0 - 365) \(**/
 277 int tm_isdst;   /\(** is summer time in effect? \(**/
 278 char \(**tm_zone;       /\(** abbreviation of timezone name \(**/
 279 long tm_gmtoff; /\(** offset from UTC in seconds \(**/
 280 .Ed
 281 .Pp
 282 The
 283 field
 284 .Fa tm_isdst
 285 is non-zero if summer time is in effect.
 286 .Pp
 287 The field
 288 .Fa tm_gmtoff
 289 is the offset (in seconds) of the time represented from
 290 .Tn UTC ,
 291 with positive
 292 values indicating east of the Prime Meridian.
 293 .Sh SEE ALSO
 294 .Xr date 1 ,
 295 .Xr gettimeofday 2 ,
 296 .Xr getenv 3 ,
 297 .Xr time 3 ,
 298 .Xr tzset 3 ,
 299 .Xr tzfile 5
 300 .Sh STANDARDS
 301 The
 302 .Fn asctime ,
 303 .Fn ctime ,
 304 .Fn difftime ,
 305 .Fn gmtime ,
 306 .Fn localtime ,
 307 and
 308 .Fn mktime
 309 functions conform to
 310 .St -isoC ,
 311 and conform to
 312 .St -p1003.1-96
 313 provided the selected local timezone does not contain a leap-second table
 314 (see
 315 .Xr zic 8 ) .
 316 .Pp
 317 The
 318 .Fn asctime_r ,
 319 .Fn ctime_r ,
 320 .Fn gmtime_r ,
 321 and
 322 .Fn localtime_r
 323 functions are expected to conform to
 324 .St -p1003.1-96
 325 (again provided the selected local timezone does not contain a leap-second
 326 table).
 327 .Pp
 328 The
 329 .Fn timegm
 330 function is not specified by any standard; its function cannot be
 331 completely emulated using the standard functions described above.
 332 .Sh HISTORY
 333 This manual page is derived from
 334 the time package contributed to Berkeley by
 335 .An Arthur Olson
 336 and which appeared in
 337 .Bx 4.3 .
 338 .Sh BUGS
 339 Except for
 340 .Fn difftime ,
 341 .Fn mktime ,
 342 and the
 343 .Fn \&_r
 344 variants of the other functions,
 345 these functions leaves their result in an internal static object and return
 346 a pointer to that object.
 347 Subsequent calls to these
 348 function will modify the same object.
 349 .Pp
 350 The C Standard provides no mechanism for a program to modify its current
 351 local timezone setting, and the
 352 .Tn POSIX Ns No \&-standard
 353 method is not reentrant.
 354 (However, thread-safe implementations are provided
 355 in the
 356 .Tn POSIX
 357 threaded environment.)
 358 .Pp
 359 The
 360 .Va tm_zone
 361 field of a returned
 362 .Vt tm
 363 structure points to a static array of characters,
 364 which will also be overwritten by any subsequent calls (as well as by
 365 subsequent calls to
 366 .Xr tzset 3
 367 and
 368 .Xr tzsetwall 3 ) .
 369 .Pp
 370 Use of the external variable
 371 .Fa tzname
 372 is discouraged; the
 373 .Fa tm_zone
 374 entry in the tm structure is preferred.