man3/ctime.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\" Modified Sat Jul 24 19:49:27 1993 by Rik Faith (faith@cs.unc.edu)
  28 .\" Modified Fri Apr 26 12:38:55 MET DST 1996 by Martin Schulze (joey@linux.de)
  29 .\" Modified 2001-11-13, aeb
  30 .\" Modified 2001-12-13, joey, aeb
  31 .\" Modified 2004-11-16, mtk
  32 .\"
  33 .TH CTIME 3  2004-11-16 "" "Linux Programmer's Manual"
  34 .SH NAME
  35 asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
  36 localtime_r \- transform date and time to broken-down time or ASCII
  37 .SH SYNOPSIS
  38 .nf
  39 .B #include <time.h>
  40 .sp
  41 .BI "char *asctime(const struct tm *" tm );
  42 .br
  43 .BI "char *asctime_r(const struct tm *" tm ", char *" buf );
  44 .sp
  45 .BI "char *ctime(const time_t *" timep );
  46 .br
  47 .BI "char *ctime_r(const time_t *" timep ", char *" buf );
  48 .sp
  49 .BI "struct tm *gmtime(const time_t *" timep );
  50 .br
  51 .BI "struct tm *gmtime_r(const time_t *" timep ", struct tm *" result );
  52 .sp
  53 .BI "struct tm *localtime(const time_t *" timep );
  54 .br
  55 .BI "struct tm *localtime_r(const time_t *" timep ", struct tm *" result );
  56 .sp
  57 .BI "time_t mktime(struct tm *" tm );
  58 .fi
  59 .SH DESCRIPTION
  60 The
  61 .BR ctime (),
  62 .BR gmtime ()
  63 and
  64 .BR localtime ()
  65 functions all take
  66 an argument of data type \fItime_t\fP which represents calendar time.
  67 When interpreted as an absolute time value, it represents the number of
  68 seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal
  69 Time (UTC).
  70 .PP
  71 The
  72 .BR asctime ()
  73 and
  74 .BR mktime ()
  75 functions both take an argument
  76 representing broken-down time which is a representation
  77 separated into year, month, day, etc.
  78 .PP
  79 Broken-down time is stored
  80 in the structure \fItm\fP which is defined in \fI<time.h>\fP as follows:
  81 .sp
  82 .RS
  83 .nf
  84 struct tm {
  85     int tm_sec;         /* seconds */
  86     int tm_min;         /* minutes */
  87     int tm_hour;        /* hours */
  88     int tm_mday;        /* day of the month */
  89     int tm_mon;         /* month */
  90     int tm_year;        /* year */
  91     int tm_wday;        /* day of the week */
  92     int tm_yday;        /* day in the year */
  93     int tm_isdst;       /* daylight saving time */
  94 };
  95 .fi
  96 .RE
  97 .PP
  98 The members of the \fItm\fP structure are:
  99 .TP
 100 .I tm_sec
 101 The number of seconds after the minute, normally in the range 0 to 59,
 102 but can be up to 60 to allow for leap seconds.
 103 .TP
 104 .I tm_min
 105 The number of minutes after the hour, in the range 0 to 59.
 106 .TP
 107 .I tm_hour
 108 The number of hours past midnight, in the range 0 to 23.
 109 .TP
 110 .I tm_mday
 111 The day of the month, in the range 1 to 31.
 112 .TP
 113 .I tm_mon
 114 The number of months since January, in the range 0 to 11.
 115 .TP
 116 .I tm_year
 117 The number of years since 1900.
 118 .TP
 119 .I tm_wday
 120 The number of days since Sunday, in the range 0 to 6.
 121 .TP
 122 .I tm_yday
 123 The number of days since January 1, in the range 0 to 365.
 124 .TP
 125 .I tm_isdst
 126 A flag that indicates whether daylight saving time is in effect at the
 127 time described.
 128 The value is positive if daylight saving time is in
 129 effect, zero if it is not, and negative if the information is not
 130 available.
 131 .PP
 132 The call
 133 .BI ctime( t )
 134 is equivalent to
 135 .BI asctime(localtime( t )) \fR.
 136 It converts the calendar time \fIt\fP into a string of the form
 137 .sp
 138 .RS
 139 "Wed Jun 30 21:49:08 1993\\n"
 140 .RE
 141 .sp
 142 The abbreviations for the days of the week are `Sun', `Mon', `Tue', `Wed',
 143 `Thu', `Fri', and `Sat'.
 144 The abbreviations for the months are `Jan',
 145 `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and
 146 `Dec'.
 147 The return value points to a statically allocated string which
 148 might be overwritten by subsequent calls to any of the date and time
 149 functions.
 150 The function also sets the external variable \fItzname\fP (see
 151 .BR tzset (3))
 152 with information about the current time zone.
 153 The re-entrant version
 154 .BR ctime_r ()
 155 does the same, but stores the
 156 string in a user-supplied buffer of length at least 26.
 157 It need not
 158 set \fItzname\fP.
 159 .PP
 160 The
 161 .BR gmtime ()
 162 function converts the calendar time \fItimep\fP to
 163 broken-down time representation, expressed in Coordinated Universal Time
 164 (UTC).
 165 It may return NULL when the year does not fit into an integer.
 166 The return value points to a statically allocated struct which might be
 167 overwritten by subsequent calls to any of the date and time functions.
 168 The
 169 .BR gmtime_r ()
 170 function does the same, but stores the data in a
 171 user-supplied struct.
 172 .PP
 173 The
 174 .BR localtime ()
 175 function converts the calendar time \fItimep\fP to
 176 broken-time representation, expressed relative to the user's specified
 177 time zone.
 178 The function acts as if it called
 179 .BR tzset (3)
 180 and sets the external variables \fItzname\fP with
 181 information about the current time zone, \fItimezone\fP with the difference
 182 between Coordinated Universal Time (UTC) and local standard time in
 183 seconds, and \fIdaylight\fP to a non-zero value if daylight savings
 184 time rules apply during some part of the year.
 185 The return value points to a statically allocated struct which might be
 186 overwritten by subsequent calls to any of the date and time functions.
 187 The
 188 .BR localtime_r ()
 189 function does the same, but stores the data in a
 190 user-supplied struct.
 191 It need not set \fItzname\fP.
 192 .PP
 193 The
 194 .BR asctime ()
 195 function converts the broken-down time value
 196 \fItm\fP into a string with the same format as
 197 .BR ctime ().
 198 The return value points to a statically allocated string which might be
 199 overwritten by subsequent calls to any of the date and time functions.
 200 The
 201 .BR asctime_r ()
 202 function does the same, but stores the string in
 203 a user-supplied buffer of length at least 26.
 204 .PP
 205 The
 206 .BR mktime ()
 207 function converts a broken-down time structure, expressed
 208 as local time, to calendar time representation.
 209 The function ignores
 210 the specified contents of the structure members \fItm_wday\fP and \fItm_yday\fP
 211 and recomputes them from the other information in the broken-down time
 212 structure.
 213 If structure members are outside their legal interval, they will be
 214 normalized (so that, for example, 40 October is changed into 9 November).
 215 Calling
 216 .BR mktime ()
 217 also sets the external variable \fItzname\fP with
 218 information about the current time zone.
 219 If the specified broken-down
 220 time cannot be represented as calendar time (seconds since the epoch),
 221 .BR mktime ()
 222 returns a value of (time_t)(\-1) and does not alter the
 223 \fItm_wday\fP and \fItm_yday\fP members of the broken-down time structure.
 224 .SH "RETURN VALUE"
 225 Each of these functions returns the value described, or NULL
 226 (\-1 in case of
 227 .BR mktime ())
 228 in case an error was detected.
 229 .SH "CONFORMING TO"
 230 POSIX.1-2001.
 231 C89 and C99 specify
 232 .BR asctime (),
 233 .BR ctime (),
 234 .BR gmtime (),
 235 .BR localtime (),
 236 and
 237 .BR mktime ()
 238 .SH NOTES
 239 The four functions
 240 .BR asctime (),
 241 .BR ctime (),
 242 .BR gmtime ()
 243 and
 244 .BR localtime ()
 245 return a pointer to static data and hence are not thread-safe.
 246 Thread-safe versions
 247 .BR asctime_r (),
 248 .BR ctime_r (),
 249 .BR gmtime_r ()
 250 and
 251 .BR localtime_r ()
 252 are specified by SUSv2, and available since libc 5.2.5.
 253 .LP
 254 In many implementations, including
 255 .IR glibc ,
 256 a 0 in
 257 .I tm_mday
 258 is interpreted as meaning the last day of the preceding month.
 259 .LP
 260 The glibc version of \fIstruct tm\fP has additional fields
 261 .sp
 262 .RS
 263 .nf
 264 long tm_gmtoff;           /* Seconds east of UTC */
 265 const char *tm_zone;      /* Timezone abbreviation */
 266 .fi
 267 .RE
 268 .sp
 269 defined when
 270 .B _BSD_SOURCE
 271 was set before including
 272 .IR <time.h> .
 273 This is a BSD extension, present in 4.3BSD-Reno.
 274 .SH "SEE ALSO"
 275 .BR date (1),
 276 .BR gettimeofday (2),
 277 .BR time (2),
 278 .BR utime (2),
 279 .BR clock (3),
 280 .BR difftime (3),
 281 .BR strftime (3),
 282 .BR strptime (3),
 283 .BR timegm (3),
 284 .BR tzset (3),
 285 .BR time (7)