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 1.11.2.6 2001/10/02 11:36:10 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 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 .Fn ctime_r
 144 provides the same functionality as
 145 .Fn ctime
 146 except the caller must provide the output buffer
 147 .Fa buf
 148 to store the result, which must be at least 26 characters long.
 149 .Fn localtime_r
 150 and
 151 .Fn gmtime_r
 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 .Fn asctime_r
 170 provides the same functionality as
 171 .Fn asctime
 172 except the caller provide the output buffer
 173 .Fa buf
 174 to store the result, which must be at least 26 characters long.
 175 .Pp
 176 The functions
 177 .Fn mktime
 178 and
 179 .Fn timegm
 180 convert the broken-down time in the structure
 181 pointed to by tm into a time value with the same encoding as that of the
 182 values returned by the
 183 .Xr time 3
 184 function (that is, seconds from the Epoch,
 185 .Tn UTC ) .
 186 .Fn mktime
 187 interprets the input structure according to the current timezone setting
 188 (see
 189 .Xr tzset 3 ) .
 190 .Fn timegm
 191 interprets the input structure as representing Universal Coordinated Time
 192 .Pq Tn UTC .
 193 .Pp
 194 The original values of the
 195 .Fa tm_wday
 196 and
 197 .Fa tm_yday
 198 components of the structure are ignored, and the original values of the
 199 other components are not restricted to their normal ranges, and will be
 200 normalized if needed.
 201 For example,
 202 October 40 is changed into November 9,
 203 a
 204 .Fa tm_hour
 205 of \-1 means 1 hour before midnight,
 206 .Fa tm_mday
 207 of 0 means the day preceding the current month, and
 208 .Fa tm_mon
 209 of \-2 means 2 months before January of
 210 .Fa tm_year .
 211 (A positive or zero value for
 212 .Fa tm_isdst
 213 causes
 214 .Fn mktime
 215 to presume initially that summer time (for example, Daylight Saving Time)
 216 is or is not in effect for the specified time, respectively.
 217 A negative value for
 218 .Fa tm_isdst
 219 causes the
 220 .Fn mktime
 221 function to attempt to divine whether summer time is in effect for the
 222 specified time.
 223 The
 224 .Fa tm_isdst
 225 and
 226 .Fa tm_gmtoff
 227 members are forced to zero by
 228 .Fn timegm . )
 229 .Pp
 230 On successful completion, the values of the
 231 .Fa tm_wday
 232 and
 233 .Fa tm_yday
 234 components of the structure are set appropriately, and the other components
 235 are set to represent the specified calendar time, but with their values
 236 forced to their normal ranges; the final value of
 237 .Fa tm_mday
 238 is not set until
 239 .Fa tm_mon
 240 and
 241 .Fa tm_year
 242 are determined.
 243 .Fn Mktime
 244 returns the specified calendar time; if the calendar time cannot be
 245 represented, it returns \-1;
 246 .Pp
 247 The
 248 .Fn difftime
 249 function
 250 returns the difference between two calendar times,
 251 .Pf ( Fa time1
 252 -
 253 .Fa time0 ) ,
 254 expressed in seconds.
 255 .Pp
 256 External declarations as well as the tm structure definition are in the
 257 .Aq Pa time.h
 258 include file.
 259 The tm structure includes at least the following fields:
 260 .Bd -literal -offset indent
 261 int tm_sec;     /\(** seconds (0 - 60) \(**/
 262 int tm_min;     /\(** minutes (0 - 59) \(**/
 263 int tm_hour;    /\(** hours (0 - 23) \(**/
 264 int tm_mday;    /\(** day of month (1 - 31) \(**/
 265 int tm_mon;     /\(** month of year (0 - 11) \(**/
 266 int tm_year;    /\(** year \- 1900 \(**/
 267 int tm_wday;    /\(** day of week (Sunday = 0) \(**/
 268 int tm_yday;    /\(** day of year (0 - 365) \(**/
 269 int tm_isdst;   /\(** is summer time in effect? \(**/
 270 char \(**tm_zone;       /\(** abbreviation of timezone name \(**/
 271 long tm_gmtoff; /\(** offset from UTC in seconds \(**/
 272 .Ed
 273 .Pp
 274 The
 275 field
 276 .Fa tm_isdst
 277 is non-zero if summer time is in effect.
 278 .Pp
 279 The field
 280 .Fa tm_gmtoff
 281 is the offset (in seconds) of the time represented from
 282 .Tn UTC ,
 283 with positive
 284 values indicating east of the Prime Meridian.
 285 .Sh SEE ALSO
 286 .Xr date 1 ,
 287 .Xr gettimeofday 2 ,
 288 .Xr getenv 3 ,
 289 .Xr time 3 ,
 290 .Xr tzset 3 ,
 291 .Xr tzfile 5
 292 .Sh STANDARDS
 293 The
 294 .Fn asctime ,
 295 .Fn ctime ,
 296 .Fn difftime ,
 297 .Fn gmtime ,
 298 .Fn localtime ,
 299 and
 300 .Fn mktime
 301 functions conform to
 302 .St -isoC ,
 303 and conform to
 304 .St -p1003.1-96
 305 provided the selected local timezone does not contain a leap-second table
 306 (see
 307 .Xr zic 8 ) .
 308 .Pp
 309 The
 310 .Fn asctime_r ,
 311 .Fn ctime_r ,
 312 .Fn gmtime_r ,
 313 and
 314 .Fn localtime_r
 315 functions are expected to conform to
 316 .St -p1003.1-96
 317 (again provided the selected local timezone does not contain a leap-second
 318 table).
 319 .Pp
 320 The
 321 .Fn timegm
 322 function is not specified by any standard; its function cannot be
 323 completely emulated using the standard functions described above.
 324 .Sh HISTORY
 325 This manual page is derived from
 326 the time package contributed to Berkeley by
 327 .An Arthur Olson
 328 and which appeared in
 329 .Bx 4.3 .
 330 .Sh BUGS
 331 Except for
 332 .Fn difftime ,
 333 .Fn mktime ,
 334 and the
 335 .Fn \&_r
 336 variants of the other functions,
 337 these functions leaves their result in an internal static object and return
 338 a pointer to that object.
 339 Subsequent calls to these
 340 function will modify the same object.
 341 .Pp
 342 The C Standard provides no mechanism for a program to modify its current
 343 local timezone setting, and the
 344 .Tn POSIX Ns No \&-standard
 345 method is not reentrant.  (However, thread-safe implementations are provided
 346 in the
 347 .Tn POSIX
 348 threaded environment.)
 349 .Pp
 350 The
 351 .Fa tm_zone
 352 field of a returned tm structure points to a static array of characters,
 353 which will also be overwritten by any subsequent calls (as well as by
 354 subsequent calls to
 355 .Xr tzset 3
 356 and
 357 .Xr tzsetwall 3 ) .
 358 .Pp
 359 Use of the external variable
 360 .Fa tzname
 361 is discouraged; the
 362 .Fa tm_zone
 363 entry in the tm structure is preferred.