NAME asctime, ctime, gmtime, localtime, mktime, asctimer, ctimer,
gmtimer, localtimer - transform date and time to broken-down time or ASCII SYNOPSIS
#include
char *asctime(const struct tm *tm); char *asctimer(const struct tm *tm, char *buf); char *ctime(const timet *timep); char *ctimer(const timet *timep, char *buf); struct tm *gmtime(const timet *timep); struct tm *gmtimer(const timet *timep, struct tm *result); struct tm *localtime(const timet *timep); struct tm *localtimer(const timet *timep, struct tm *result); timet mktime(struct tm *tm); Feature Test Macro Requirements for glibc (see featuretestmacros(7)): asctimer(), ctimer(), gmtimer(), localtimer(): POSIXCSOURCE >= 1 || XOPENSOURCE || BSDSOURCE || SVIDSOURCE || POSIXSOURCE DESCRIPTION The ctime(), gmtime() and localtime() functions all take an argument of data type timet 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
as follows: struct tm { int tmsec; /* seconds */ int tmmin; /* minutes */ int tmhour; /* hours */ int tmmday; /* day of the month */ int tmmon; /* month */ int tmyear; /* year */ int tmwday; /* day of the week */ int tmyday; /* day in the year */ int tmisdst; /* daylight saving time */ }; The members of the tm structure are: tmsec 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. tmmin The number of minutes after the hour, in the range 0 to 59. tmhour The number of hours past midnight, in the range 0 to 23. tmmday The day of the month, in the range 1 to 31. tmmon The number of months since January, in the range 0 to 11. tmyear The number of years since 1900. tmwday The number of days since Sunday, in the range 0 to 6. tmyday The number of days since January 1, in the range 0 to 365. tmisdst 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" 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 ctimer() 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
gmtimer() 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 localtimer() 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
asctimer() 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 tmwday and tmyday fields. The value specified in the tmisdst 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: tmwday and tmyday 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); tmisdst 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 (timet) -1 and does
not alter the members of the broken-down time structure. RETURN VALUE
Each of these functions returns the value described, or NULL (-1 in case of mktime()) in case an error was detected. CONFORMING TO
POSIX.1-2001. C89 and C99 specify asctime(), ctime(), gmtime(), local‐
time(), and mktime(). POSIX.1-2008 marks asctime(), asctimer(), ctime(), and ctimer() 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. Thread-safe versions asctimer(), ctimer(), gmtimer() and localtimer() are spec‐ ified by SUSv2, and available since libc 5.2.5.
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 tmmday is interpreted as meaning the last day of the preceding month. The glibc version of struct tm has additional fields long tmgmtoff; /* Seconds east of UTC */ const char *tmzone; /* Timezone abbreviation */ defined when BSDSOURCE was set before including
. 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 localtimer() does not have this require‐ ment. For portable code tzset(3) should be called before local‐ timer(). 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
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2010-02-25 CTIME(3)