PURPOSE

gp_mktime - converts a tm structure to a calendar time

SYNOPSIS


#include <time.h>
time_t gp_mktime (struct tm *timeptr);

DESCRIPTION

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 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 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, the correct timezone is determined and the components are not adjusted.

Local timezone 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.

EXAMPLE

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\n", wday[time_str.tm_wday]);

NOTICES

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.

PORTABILITY

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 later 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, if not set, is GMT0. The format for TZ is the following. stdoffset[dst[offset],[start[time],end[time]]]

std and dst: Three or more bytes that are the designation for the standard (std) and daylight savings time (dst) timezones. Only std is required, if dst is missing, then daylight savings time does not apply in this locale. Upper- and lower-case letters are allowed. Any characters except a leading colon (:), digits, a comma (,), a minus (-) or a plus (+) are allowed.
offset: Indicates the value one must add to the local time to arrive at Coordinated Universal Time. The offset has the 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 timezone is east of the Prime Meridian; otherwise it is west (which may be indicated by an optional preceding ``+'' sign).
start/time,end/time: Indicates when to change to and back from daylight savings time, where start/time describes when the change from standard time to daylight savings time occurs, and end/time describes when the change back happens. Each time field describes when, in current local time, the change is made. The formats of start and end are one of the following: Jn The Julian day n (1 < n < 365). Leap days are not counted. That is, in all years, February 28 is day 59 and March 1 is day 60. It is impossible to refer to the occasional February 29. n The zero-based Julian day (0 < n < 365). Leap days are counted, and it is possible to refer to February 29.
Mm.n.d: The dth day, (0 < d < 6) of week n of month m of the year (1 < n < 5, 1 < m < 12), where week 5 means ``the last d-day in month m'' which may occur in either the fourth or the fifth week). Week 1 is the first week in which the dth day occurs. Day zero is Sunday.

Implementation specific defaults are used for start and end if these optional fields are not given.