Renormalizes local calendar time expressed as a struct tm object and also converts it to time since epoch as a time_t object. arg->tm_wday and arg->tm_yday are ignored. The values in arg are not checked for being out of range.
A negative value of arg->tm_isdst causes mktime to attempt to determine if Daylight Saving Time was in effect in the specified time.
If the conversion to time_t is successful, the arg object is modified. All fields of arg are updated to fit their proper ranges. arg->tm_wday and arg->tm_yday are recalculated using information available in other fields.
The time since epoch as a time_t object on success, or -1 if arg cannot be represented as a time_t object (POSIX also requires EOVERFLOW to be stored in errno in this case).
[edit] NotesIf the struct tm object was obtained from POSIX strptime or equivalent function, the value of tm_isdst is indeterminate, and needs to be set explicitly before calling mktime.
#define _POSIX_C_SOURCE 200112L // for setenv on gcc
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
setenv("TZ", "/usr/share/zoneinfo/America/New_York", 1); // POSIX-specific
struct tm tm = *localtime(&(time_t){time(NULL)});
printf("Today is %s", asctime(&tm));
printf("(DST is %s)\n", tm.tm_isdst ? "in effect" : "not in effect");
tm.tm_mon -= 100; // tm_mon is now outside its normal range
mktime(&tm); // tm_isdst is not set to -1; today's DST status is used
printf("100 months ago was %s", asctime(&tm));
printf("(DST was %s)\n", tm.tm_isdst ? "in effect" : "not in effect");
}
Possible output:
Today is Fri Apr 22 11:53:36 2016 (DST is in effect) 100 months ago was Sat Dec 22 10:53:36 2007 (DST was not in effect)[edit] References
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