MKTIME(3C) Standard C Library Functions MKTIME(3C)


NAME


mktime, timegm - convert a tm structure to a calendar time

SYNOPSIS


#include <time.h>

time_t mktime(struct tm *timeptr);

time_t timegm(struct tm *timeptr);


DESCRIPTION


The mktime() function converts the time represented by the tm structure
pointed to by timeptr into a calendar time (the number of seconds since
00:00:00 UTC, January 1, 1970).


The tm structure contains the following members:

int tm_sec; /* seconds after the minute [0, 60] */
int tm_min; /* minutes after the hour [0, 59] */
int tm_hour; /* hour since midnight [0, 23] */
int tm_mday; /* day of the month [1, 31] */
int tm_mon; /* months since January [0, 11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday [0, 6] */
int tm_yday; /* days since January 1 [0, 365] */
int tm_isdst; /* flag for daylight savings time */


In addition to computing the calendar time, mktime() normalizes the
supplied tm structure. The original values of the tm_wday and tm_yday
components of the structure are ignored, and the original values of the
other components are not restricted to the ranges indicated in the
definition of the structure. On successful completion, the values of the
tm_wday and tm_yday components are set appropriately, and the other
components are set to represent the specified calendar time, but with
their values forced to be within the appropriate ranges. The final value
of tm_mday is not set until tm_mon and tm_year are determined.


The tm_year member must be for year 1901 or later. Calendar times before
20:45:52 UTC, December 13, 1901 or after 03:14:07 UTC, January 19, 2038
cannot be represented. Portable applications should not try to create
dates before 00:00:00 UTC, January 1, 1970 or after 00:00:00 UTC, January
1, 2038.


The original values of the components may be either greater than or less
than the specified range. For example, a tm_hour of -1 means 1 hour
before midnight, tm_mday of 0 means the day preceding the current month,
and tm_mon of -2 means 2 months before January of tm_year.


If tm_isdst is positive, the original values are assumed to be in the
alternate timezone. If it turns out that the alternate timezone is not
valid for the computed calendar time, then the components are adjusted to
the main timezone. Likewise, if tm_isdst is zero, the original values are
assumed to be in the main timezone and are converted to the alternate
timezone if the main timezone is not valid. If tm_isdst is negative,
mktime() attempts to determine whether the alternate timezone is in
effect for the specified time.


Local timezone information is used as if mktime() had called tzset(). See
ctime(3C).


The timegm() function is identical to the mktime() function, except that
the timegm() function ignores both the current time zone and the tm_isdst
member and operates as though the time zone were set to UTC.

RETURN VALUES


If the calendar time can be represented in an object of type time_t, the
mktime() and timegm() functions return the specified calendar time
without changing errno. If the calendar time cannot be represented, the
function returns the value (time_t)-1 and sets errno to indicate the
error.

ERRORS


The mktime() and timegm() functions will fail if:

EOVERFLOW
The date represented by the input tm struct cannot be
represented in a time_t. Note that the errno setting may
change if future revisions to the standards specify a
different value.


USAGE


The mktime() and timegm() functions are MT-Safe in multithreaded
applications, as long as no user-defined function directly modifies one
of the following variables: timezone, altzone, daylight, and tzname. See
ctime(3C).


Note that -1 can be a valid return value for the time that is one second
before the Epoch. The user should clear errno before calling mktime()
and timegm(). If mktime() or timegm() then returns -1, the user should
check errno to determine whether or not an error actually occurred.


The mktime() and timegm() functions assume Gregorian dates. Times before
the adoption of the Gregorian calendar will not match historial records.

EXAMPLES


Example 1: Sample code using mktime().




What day of the week is July 4, 2001?


#include <stdio.h>
#include <time.h>
static char *const wday[] = {
"Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "-unknown-"
};
struct tm time_str;
/*...*/
time_str.tm_year = 2001 - 1900;
time_str.tm_mon = 7 - 1;
time_str.tm_mday = 4;
time_str.tm_hour = 0;
time_str.tm_min = 0;
time_str.tm_sec = 1;
time_str.tm_isdst = -1;
if (mktime(&time_str)== -1)
time_str.tm_wday=7;
printf("%s\n", wday[time_str.tm_wday]);


BUGS


The zoneinfo timezone data files do not transition past Tue Jan 19
03:14:07 2038 UTC. Therefore for 64-bit applications using zoneinfo
timezones, calculations beyond this date may not use the correct offset
from standard time, and could return incorrect values. This affects the
64-bit versions of mktime() and timegm().

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------------+
|Interface Stability | Standard |
+--------------------+-------------------------+
|MT-Level | MT-Safe with exceptions |
+--------------------+-------------------------+

SEE ALSO


ctime(3C), getenv(3C), TIMEZONE(4), attributes(5), standards(5)


March 14, 2016 MKTIME(3C)