ATMI C Function Reference
gp_mktime()
—Converts a tm
structure to a calendar time.
#include <time.h>
time_t gp_mktime (struct tm *timeptr);
gp_mktime()
converts the time represented by the tm
structure pointed to by timeptr
into a calendar time (the number of seconds since 00:00:00 Universal Coordinated Time—UTC, January 1, 1970).
The tm
structure has the following format:
struct tm {
int tm_sec; /* seconds after the minute [0, 61] */
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, gp_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 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 time zone. If it turns out that the alternate time zone is not valid for the computed calendar time, then the components are adjusted to the main time zone. Likewise, if tm_isdst
is zero, the original values are assumed to be in the main time zone and are converted to the alternate time zone if the main time zone is not valid. If tm_isdst
is negative, the correct time zone is determined and the components are not adjusted.
Local time zone information is used as if gp_mktime()
had called tzset()
.
gp_mktime()
returns the specified calendar time. If the calendar time cannot be represented, the function returns the value (time_t
)-1.
A thread in a multithreaded application may issue a call to gp_mktime()
while running in any context state, including TPINVALIDCONTEXT
.
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 (gp_mktime(time_str) == -1)
time_str.tm_wday=7;
printf("%s\en", wday[time_str.tm_wday]);
tm_year
of the tm structure must be for year 1970 or later. Calendar times before 00:00:00 UTC, January 1, 1970 or after 03:14:07 UTC, January 19, 2038 cannot be represented.
On systems where the C compilation system already provides the ANSI C mktime()
function, gp_mktime()
simply calls mktime()
to do the conversion. Otherwise, the conversion is provided directly in gp_mktime()
.
In the latter case, the TZ
environment variable must be set. Note that in many installations, TZ
is set to the correct value by default when the user logs on. The default value for TZ
is GMT0
. The format for TZ
is the following:
stdoffset[dst[offset],[start[time],e
nd[time]]]
Three or more bytes that designate the standard time zone (std
) and daylight savings time time zone (dst
). Only std
is required. If dst
is missing, then daylight savings time does not apply in this locale. Uppercase and lowercase letters are allowed. Any characters except a leading colon (:), digits, a comma (,), a minus (-) or a plus (+) are allowed.
Indicates the value one must add to the local time to arrive at Coordinated Universal Time. The offset
has the following form: hh
[:mm
[:ss
]]. The minutes (mm
) and seconds (ss
) are optional. The hour (hh
) is required and may be a single digit. The offset
following std
is required. If no offset
follows dst
, daylight savings time is assumed to be one hour ahead of standard time. One or more digits may be used; the value is always interpreted as a decimal number. The hour must be between 0 and 24, and the minutes (and seconds) if present, between 0 and 59. Out of range values may cause unpredictable behavior. If preceded by a "-", the time zone is east of the Prime Meridian; otherwise it is west (which may be indicated by an optional preceding "+" sign).
Implementation specific defaults are used for start
and end
if these optional fields are not given.
The time
has the same format as offset
except that no leading sign ("-" or "+") is allowed. The default, if time
is not specified, is 02:00:00.
ctime
(3c), getenv
(3c), timezone
(4) in a UNIX system reference manual