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 \fBctime\fP(), \fBgmtime\fP() and \fBlocaltime\fP() functions all take
  61 an argument of data type \fItime_t\fP which represents calendar time.
  62 When interpreted as an absolute time value, it represents the number of
  63 seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal
  64 Time (UTC).
  65 .PP
  66 The \fBasctime\fP() and \fBmktime\fP() functions both take an argument
  67 representing broken-down time which is a representation
  68 separated into year, month, day, etc.
  69 .PP
  70 Broken-down time is stored
  71 in the structure \fItm\fP which is defined in \fI<time.h>\fP as follows:
  72 .sp
  73 .RS
  74 .nf
  75 struct tm {
  76     int tm_sec;         /* seconds */
  77     int tm_min;         /* minutes */
  78     int tm_hour;        /* hours */
  79     int tm_mday;        /* day of the month */
  80     int tm_mon;         /* month */
  81     int tm_year;        /* year */
  82     int tm_wday;        /* day of the week */
  83     int tm_yday;        /* day in the year */
  84     int tm_isdst;       /* daylight saving time */
  85 };
  86 .fi
  87 .RE
  88 .PP
  89 The members of the \fItm\fP structure are:
  90 .TP
  91 .I tm_sec
  92 The number of seconds after the minute, normally in the range 0 to 59,
  93 but can be up to 60 to allow for leap seconds.
  94 .TP
  95 .I tm_min
  96 The number of minutes after the hour, in the range 0 to 59.
  97 .TP
  98 .I tm_hour
  99 The number of hours past midnight, in the range 0 to 23.
 100 .TP
 101 .I tm_mday
 102 The day of the month, in the range 1 to 31.
 103 .TP
 104 .I tm_mon
 105 The number of months since January, in the range 0 to 11.
 106 .TP
 107 .I tm_year
 108 The number of years since 1900.
 109 .TP
 110 .I tm_wday
 111 The number of days since Sunday, in the range 0 to 6.
 112 .TP
 113 .I tm_yday
 114 The number of days since January 1, in the range 0 to 365.
 115 .TP
 116 .I tm_isdst
 117 A flag that indicates whether daylight saving time is in effect at the
 118 time described.  The value is positive if daylight saving time is in
 119 effect, zero if it is not, and negative if the information is not
 120 available.
 121 .PP
 122 The call
 123 .BI ctime( t )
 124 is equivalent to
 125 .BI asctime(localtime( t )) \fR.
 126 It converts the calendar time \fIt\fP into a string of the form
 127 .sp
 128 .RS
 129 "Wed Jun 30 21:49:08 1993\\n"
 130 .RE
 131 .sp
 132 The abbreviations for the days of the week are `Sun', `Mon', `Tue', `Wed',
 133 `Thu', `Fri', and `Sat'.  The abbreviations for the months are `Jan',
 134 `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and
 135 `Dec'.  The return value points to a statically allocated string which
 136 might be overwritten by subsequent calls to any of the date and time
 137 functions.  The function also sets the external variable \fItzname\fP (see
 138 .BR tzset (3))
 139 with information about the current time zone.
 140 The re-entrant version \fBctime_r\fP() does the same, but stores the
 141 string in a user-supplied buffer of length at least 26. It need not
 142 set \fItzname\fP.
 143 .PP
 144 The \fBgmtime\fP() function converts the calendar time \fItimep\fP to
 145 broken-down time representation, expressed in Coordinated Universal Time
 146 (UTC). It may return NULL when the year does not fit into an integer.
 147 The return value points to a statically allocated struct which might be
 148 overwritten by subsequent calls to any of the date and time functions.
 149 The \fBgmtime_r\fP() function does the same, but stores the data in a
 150 user-supplied struct.
 151 .PP
 152 The \fBlocaltime\fP() function converts the calendar time \fItimep\fP to
 153 broken-time representation, expressed relative to the user's specified
 154 time zone.    The function acts as if it called
 155 .BR tzset (3)
 156 and sets the external variables \fItzname\fP with
 157 information about the current time zone, \fItimezone\fP with the difference
 158 between Coordinated Universal Time (UTC) and local standard time in
 159 seconds, and \fIdaylight\fP to a non-zero value if daylight savings
 160 time rules apply during some part of the year.
 161 The return value points to a statically allocated struct which might be
 162 overwritten by subsequent calls to any of the date and time functions.
 163 The \fBlocaltime_r\fP() function does the same, but stores the data in a
 164 user-supplied struct. It need not set \fItzname\fP.
 165 .PP
 166 The \fBasctime\fP() function converts the broken-down time value
 167 \fItm\fP into a string with the same format as \fBctime\fP().
 168 The return value points to a statically allocated string which might be
 169 overwritten by subsequent calls to any of the date and time functions.
 170 The \fBasctime_r\fP() function does the same, but stores the string in
 171 a user-supplied buffer of length at least 26.
 172 .PP
 173 The \fBmktime\fP() function converts a broken-down time structure, expressed
 174 as local time, to calendar time representation.  The function ignores
 175 the specified contents of the structure members \fItm_wday\fP and \fItm_yday\fP
 176 and recomputes them from the other information in the broken-down time
 177 structure.
 178 If structure members are outside their legal interval, they will be
 179 normalized (so that, e.g., 40 October is changed into 9 November).
 180 Calling \fBmktime\fP() also sets the external variable \fItzname\fP with
 181 information about the current time zone.  If the specified broken-down
 182 time cannot be represented as calendar time (seconds since the epoch),
 183 \fBmktime\fP() returns a value of (time_t)(\-1) and does not alter the
 184 \fItm_wday\fP and \fItm_yday\fP members of the broken-down time structure.
 185 .SH "RETURN VALUE"
 186 Each of these functions returns the value described, or NULL
 187 (\-1 in case of \fBmktime\fP()) in case an error was detected.
 188 .SH NOTES
 189 The four functions
 190 .BR asctime (),
 191 .BR ctime (),
 192 .BR gmtime ()
 193 and
 194 .BR localtime ()
 195 return a pointer to static data and hence are not thread-safe.
 196 Thread-safe versions
 197 .BR asctime_r (),
 198 .BR ctime_r (),
 199 .BR gmtime_r ()
 200 and
 201 .BR localtime_r ()
 202 are specified by SUSv2, and available since libc 5.2.5.
 203 .LP
 204 In many implementations, including
 205 .IR glibc ,
 206 a 0 in
 207 .I tm_mday
 208 is interpreted as meaning the last day of the preceding month.
 209 .LP
 210 The glibc version of struct tm has additional fields
 211 .sp
 212 .RS
 213 .nf
 214 long tm_gmtoff;           /* Seconds east of UTC */
 215 const char *tm_zone;      /* Timezone abbreviation */
 216 .fi
 217 .RE
 218 .sp
 219 defined when _BSD_SOURCE was set before including
 220 .IR <time.h> .
 221 This is a BSD extension, present in 4.3BSD-Reno.
 222 .SH "CONFORMING TO"
 223 SVr4, POSIX.1-2001, 4.3BSD, C89, C99.
 224 .SH "SEE ALSO"
 225 .BR date (1),
 226 .BR gettimeofday (2),
 227 .BR time (2),
 228 .BR utime (2),
 229 .BR clock (3),
 230 .BR difftime (3),
 231 .BR strftime (3),
 232 .BR strptime (3),
 233 .BR tzset (3),
 234 .BR time (7)