Tribblix: manual page: mktime.3c

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
historical 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