Manpages - ctime.3

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

  #include <time.h>

  char *asctime(const struct tm *tm);
  char *asctime_r(const struct tm *restrict tm, char *restrict buf);

  char *ctime(const time_t *timep);
  char *ctime_r(const time_t *restrict timep, char *restrict buf);

  struct tm *gmtime(const time_t *timep);
  struct tm *gmtime_r(const time_t *restrict timep,
   struct tm *restrict result);

  struct tm *localtime(const time_t *timep);
  struct tm *localtime_r(const time_t *restrict timep,
   struct tm *restrict 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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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 daylight saving time is in effect, zero if it is not, and negative 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"

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 overwritten 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-supplied 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 overwritten 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 follows: tm_wday and tm_yday are set to values determined from the contents 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 current 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.

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 functions return NULL on error. On error, errno is set to indicate the error.

EOVERFLOW

The result cannot be represented.

For an explanation of the terms used in this section, see *attributes*(7).

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

POSIX doesn’t specify the parameters of *ctime_r*() to be restrict; that is specific to glibc.

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

  long tm_gmtoff;           /* Seconds east of UTC */
  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.1-2001, *localtime*() is required to behave as though *tzset*(3) was called, while *localtime_r*() does not have this requirement. For portable code, *tzset*(3) should be called before *localtime_r*().

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

This page is part of release 5.13 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/.