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November 1987

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21 Nov '87

I got the latest, November 9, 1987 draft C standard in the mail yesterday. The good news: there's a footnote telling how to get a broken-down wall clock time converted into "elapsed seconds since the epoch" (or whatever the implementation is choosing to use) *without* having to know whether the time being converted is Daylight Time or not: you're supposed to drop a -1 into the "tm_isdst" field. The bad news: this mechanism is described in a footnote, and, unfortunately, footnotes are not part of the Standard (as is explained at the start of the Standard). (There's other stuff relegated to footnotes that might better be made part of the Standard, but that's a different soapbox.) Giving a choice of lobbying to get the footnote's content moved into the Standard or dropping mktime entirely, I'd choose the second alternative, but I realize I'm getting crabbier as I get older. I've attached a transcribed version of the latest wording. --ado 4.12 DATE AND TIME <time.h> 4.12.1 Components of time The header <time.h> declares one macro, three types, and several functions for manipulating time. Many functions deal with a calendar time that represents the current date (according to the Gregorian calendar) and time. Some functions deal with local time, which is the calendar time expressed for some specific time zone, and with Daylight Saving Time, which is a temporary change in the algorithm for determining local time. The local time zone and Daylight Saving Time are implementation defined. The macro defined is CLK_TCK which is the number per second of the value returned by the clock function. The types declared are clock_t and time_t which are arithmetic types capable of representing times, and struct tm which holds the components of a calendar time, called the broken-down time. The structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges are expressed in the comments. int tm_sec; /* seconds after the minute - [0,59] */ int tm_min; /* minutes after the hour - [0,59] */ int tm_hour; /* hours 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; /* Daylight Saving Time flag */ The value of tm_isdst is positive if Daylight Saving Time is in effect, zero if Daylight Saving Time is not in effect, and negative if the information is not available. 4.12.2 Time manipulation functions 4.12.2.1 The clock function [SECTION OMITTED] 4.12.2.2 The difftime function [SECTION OMITTED] 4.12.2.3 The mktime function Synopsis #include <time.h> time_t mktime(struct tm *timeptr); Description The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by timeptr into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the 101 other components are not restricted to the ranges indicated above. On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday is not set until tm_mon and tm_year are determined. Returns The mktime function returns the specified calendar time encoded as a value of type time_t. 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 const 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]); 4.12.2.4 The time function Synopsis #include <time.h> time_t time(time_t *timer); Description The time function determines current calendar time. The encoding of the value is unspecified. Returns The time function returns the implementation's best approximation to the current calendar time. The value (time_t)-1 is returned if the calendar time is not available. If timer is not a null pointer, the return value is also assigned to the object it points to. ____________ 101. A positive or zero value for tm_isdst causes the mktime function initially to presume that Daylight Saving Time, respectively, is or is not in effect for the specified time. A negative value for tm_isdst causes the mktime function to attempt to determine whether Daylight Saving Time is in effect for the specified time.

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