GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


man:ctime_r

CTIME(3) Linux Programmer's Manual CTIME(3)

NAME

     asctime,   ctime,   gmtime,   localtime,  mktime,  asctime_r,  ctime_r,
     gmtime_r, localtime_r - transform date and time to broken-down time  or
     ASCII

SYNOPSIS

     #include <time.h>
     char *asctime(const struct tm *tm);
     char *asctime_r(const struct tm *tm, char *buf);
     char *ctime(const time_t *timep);
     char *ctime_r(const time_t *timep, char *buf);
     struct tm *gmtime(const time_t *timep);
     struct tm *gmtime_r(const time_t *timep, struct tm *result);
     struct tm *localtime(const time_t *timep);
     struct tm *localtime_r(const time_t *timep, struct tm *result);
     time_t mktime(struct tm *tm);
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
     asctime_r(), ctime_r(), gmtime_r(), localtime_r():
            _POSIX_C_SOURCE
                || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

     The ctime(), gmtime() and localtime() functions all take an argument of
     data  type time_t, which represents calendar time.  When interpreted as
     an absolute time value, it represents the  number  of  seconds  elapsed
     since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
     The asctime() and mktime() functions both take an argument representing
     broken-down time, which is a representation separated into year, month,
     day, and so on.
     Broken-down  time  is  stored  in the structure tm, which is defined in
     <time.h> as follows:
         struct tm {
             int tm_sec;    /* Seconds (0-60) */
             int tm_min;    /* Minutes (0-59) */
             int tm_hour;   /* Hours (0-23) */
             int tm_mday;   /* Day of the month (1-31) */
             int tm_mon;    /* Month (0-11) */
             int tm_year;   /* Year - 1900 */
             int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
             int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
             int tm_isdst;  /* Daylight saving time */ };
     The members of the tm structure are:
     tm_sec    The number of seconds after the minute, normally in the range
               0 to 59, but can be up to 60 to allow for leap seconds.
     tm_min    The number of minutes after the hour, in the range 0 to 59.
     tm_hour   The number of hours past midnight, in the range 0 to 23.
     tm_mday   The day of the month, in the range 1 to 31.
     tm_mon    The number of months since January, in the range 0 to 11.
     tm_year   The number of years since 1900.
     tm_wday   The number of days since Sunday, in the range 0 to 6.
     tm_yday   The number of days since January 1, in the range 0 to 365.
     tm_isdst  A  flag  that  indicates  whether  daylight saving time is in
               effect at the time described.  The value is positive if  day-
               light  saving time is in effect, zero if it is not, and nega-
               tive if the information is not available.
     The call ctime(t) is equivalent to asctime(localtime(t)).  It  converts
     the calendar time t into a null-terminated string of the form
         "Wed Jun 30 21:49:08 1993\n" ,in
     The  abbreviations  for  the  days of the week are "Sun", "Mon", "Tue",
     "Wed", "Thu", "Fri", and "Sat".  The abbreviations for the  months  are
     "Jan",  "Feb",  "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct",
     "Nov", and "Dec".  The return value points to  a  statically  allocated
     string  which  might  be  overwritten by subsequent calls to any of the
     date and time functions.  The function also sets the external variables
     tzname,  timezone,  and  daylight (see tzset(3)) with information about
     the current timezone.  The reentrant version ctime_r() does  the  same,
     but  stores the string in a user-supplied buffer which should have room
     for at least 26 bytes.  It need not set tzname, timezone, and daylight.
     The  gmtime()  function converts the calendar time timep to broken-down
     time representation, expressed in Coordinated Universal Time (UTC).  It
     may return NULL when the year does not fit into an integer.  The return
     value points to a statically allocated struct which might be  overwrit-
     ten  by  subsequent  calls  to any of the date and time functions.  The
     gmtime_r() function does the same, but stores the data in  a  user-sup-
     plied struct.
     The  localtime()  function  converts the calendar time timep to broken-
     down time representation, expressed relative to  the  user's  specified
     timezone.   The  function  acts  as  if it called tzset(3) and sets the
     external variables tzname with information about the current  timezone,
     timezone  with  the difference between Coordinated Universal Time (UTC)
     and local standard time in seconds, and daylight to a nonzero value  if
     daylight  savings  time  rules apply during some part of the year.  The
     return value points to a statically allocated  struct  which  might  be
     overwritten  by subsequent calls to any of the date and time functions.
     The localtime_r() function does the same, but  stores  the  data  in  a
     user-supplied  struct.  It need not set tzname, timezone, and daylight.
     The asctime() function converts the broken-down time value  tm  into  a
     null-terminated  string  with  the  same format as ctime().  The return
     value points to a statically allocated string which might be  overwrit-
     ten  by  subsequent  calls  to any of the date and time functions.  The
     asctime_r() function does the same, but stores the string  in  a  user-
     supplied buffer which should have room for at least 26 bytes.
     The  mktime() function converts a broken-down time structure, expressed
     as local time, to calendar time representation.  The  function  ignores
     the  values  supplied  by the caller in the tm_wday and tm_yday fields.
     The value specified in the tm_isdst field informs mktime()  whether  or
     not  daylight  saving  time (DST) is in effect for the time supplied in
     the tm structure: a positive value means DST is in effect;  zero  means
     that  DST  is  not  in effect; and a negative value means that mktime()
     should (use timezone information and system databases  to)  attempt  to
     determine whether DST is in effect at the specified time.
     The  mktime()  function modifies the fields of the tm structure as fol-
     lows: tm_wday and tm_yday are set to values determined  from  the  con-
     tents of the other fields; if structure members are outside their valid
     interval, they will be normalized (so that, for example, 40 October  is
     changed  into  9  November); tm_isdst is set (regardless of its initial
     value) to a positive value or to 0, respectively, to  indicate  whether
     DST  is  or  is  not in effect at the specified time.  Calling mktime()
     also sets the external variable tzname with information about the  cur-
     rent timezone.
     If  the  specified  broken-down  time cannot be represented as calendar
     time (seconds since the Epoch), mktime() returns (time_t) -1  and  does
     not alter the members of the broken-down time structure.

RETURN VALUE

     On success, gmtime() and localtime() return a pointer to a struct tm.
     On  success,  gmtime_r()  and  localtime_r()  return the address of the
     structure pointed to by result.
     On success, asctime() and ctime() return a pointer to a string.
     On success, asctime_r() and ctime_r() return a pointer  to  the  string
     pointed to by buf.
     On  success,  mktime()  returns  the  calendar  time (seconds since the
     Epoch), expressed as a value of type time_t.
     On error, mktime() returns the value (time_t) -1.  The remaining  func-
     tions  return  NULL  on  error.  On error, errno is set to indicate the
     cause of the error.

ERRORS

     EOVERFLOW
            The result cannot be represented.

ATTRIBUTES

     For  an  explanation  of  the  terms  used   in   this   section,   see
     attributes(7).
     allbox; lbw14 lb lbw31 l l l.  Interface Attribute Value T{ asctime()
     T}   Thread safety   MT-Unsafe race:asctime locale T{ asctime_r()
     T}   Thread safety   MT-Safe locale T{ ctime() T}   Thread safety  T{
     MT-Unsafe race:tmbuf
     race:asctime env locale T} T{ ctime_r(), gmtime_r(), localtime_r(),
     mktime() T}   Thread safety  MT-Safe env locale T{ gmtime(), local-
     time() T}   Thread safety  MT-Unsafe race:tmbuf env locale

CONFORMING TO

     POSIX.1-2001.  C89 and C99 specify asctime(), ctime(), gmtime(), local-
     time(),  and  mktime().   POSIX.1-2008  marks  asctime(),  asctime_r(),
     ctime(), and ctime_r() as obsolete, recommending the use of strftime(3)
     instead.

NOTES

     The  four functions asctime(), ctime(), gmtime() and localtime() return
     a pointer to static data and hence are not  thread-safe.   The  thread-
     safe  versions,  asctime_r(),  ctime_r(), gmtime_r() and localtime_r(),
     are specified by SUSv2.
     POSIX.1-2001 says: "The asctime(), ctime(), gmtime(),  and  localtime()
     functions  shall  return values in one of two static objects: a broken-
     down time structure and an array of type char.  Execution of any of the
     functions  may  overwrite  the  information returned in either of these
     objects by any of the other functions."  This can occur  in  the  glibc
     implementation.
     In many implementations, including glibc, a 0 in tm_mday is interpreted
     as meaning the last day of the preceding month.
     The glibc version of struct tm has additional fields
         const char *tm_zone;      /* Timezone abbreviation */
     defined when _BSD_SOURCE was set before including <time.h>.  This is  a
     BSD extension, present in 4.3BSD-Reno.
     According  to POSIX.1-2004, localtime() is required to behave as though
     tzset(3) was called, while localtime_r() does not  have  this  require-
     ment.   For  portable  code,  tzset(3)  should  be called before local-
     time_r().

SEE ALSO

     date(1), gettimeofday(2),  time(2),  utime(2),  clock(3),  difftime(3),
     strftime(3), strptime(3), timegm(3), tzset(3), time(7)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.
                                2017-09-15                          CTIME(3)
/data/webs/external/dokuwiki/data/pages/man/ctime_r.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

Was this page helpful?-10+1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki