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CTIME(3) Linux Programmer’s Manual CTIME(3)
NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, local-
time_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);
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 00:00:00 on January 1, 1970,
Coordinated Universal Time (UTC).
The asctime() and mktime() functions both take an argument representing broken-down
time which is a representation separated into year, month, day, etc.
Broken-down time is stored in the structure tm which is defined in <time.h> as fol-
lows:
struct tm {
int tm_sec; /* seconds */
int tm_min; /* minutes */
int tm_hour; /* hours */
int tm_mday; /* day of the month */
int tm_mon; /* month */
int tm_year; /* year */
int tm_wday; /* day of the week */
int tm_yday; /* day in the year */
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 61 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 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
variable tzname (see tzset(3)) with information about the current time zone. The
re-entrant version ctime_r() does the same, but stores the string in a user-sup-
plied buffer of length at least 26. It need not set tzname.
The gmtime() function converts the calendar time timep to broken-down time repre-
sentation, 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-time represen-
tation, expressed relative to the user’s specified time zone. The function acts
as if it called tzset(3) and sets the external variables tzname with information
about the current time zone, timezone with the difference between Coordinated Uni-
versal Time (UTC) and local standard time in seconds, and daylight to a non-zero
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.
The asctime() function converts the broken-down time value tm into a 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 of length at least 26.
The mktime() function converts a broken-down time structure, expressed as local
time, to calendar time representation. The function ignores the specified contents
of the structure members tm_wday and tm_yday and recomputes them from the other
information in the broken-down time structure. If structure members are outside
their legal interval, they will be normalized (so that, e.g., 40 October is changed
into 9 November). Calling mktime() also sets the external variable tzname with
information about the current time zone. If the specified broken-down time cannot
be represented as calendar time (seconds since the epoch), mktime() returns a value
of (time_t)(-1) and does not alter the tm_wday and tm_yday 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.
NOTES
The four functions asctime(), ctime(), gmtime() and localtime() return a pointer to
static data and hence are not thread-safe. Thread-safe versions asctime_r(),
ctime_r(), gmtime_r() and localtime_r() are specified by SUSv2, and available since
libc 5.2.5.
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 exten-
sion, present in 4.3BSD-Reno.
CONFORMING TO
SVID 3, POSIX, BSD 4.3, ISO 9899
SEE ALSO
date(1), gettimeofday(2), newctime(3), time(2), utime(2), clock(3), difftime(3),
strftime(3), strptime(3), tzset(3)
2001-12-13 CTIME(3)
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