mktime
| Defined in header
<time.h>
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Renormalizes local calendar time expressed as a struct tm object and also converts it to time since epoch as a time_t object. time->tm_wday and time->tm_yday are ignored. The values in time are not checked for being out of range.
A negative value of time->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 time object is modified. All fields of time are updated to fit their proper ranges. time->tm_wday and time->tm_yday are recalculated using information available in other fields.
Contents |
[edit] Parameters
| time | - | pointer to a tm object specifying local calendar time to convert |
[edit] Return value
time since epoch as a time_t object on success or -1 if time cannot be represented as a time_t object (POSIX also requires EOVERFLOW to be stored in errno in this case).
[edit] Notes
If 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.
[edit] Example
#define _POSIX_C_SOURCE 200112L // for setenv on gcc #include <stdlib.h> #include <stdio.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_dst 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"); }
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
- C11 standard (ISO/IEC 9899:2011):
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- 7.27.2.3 The mktime function (p: 390-391)
- C99 standard (ISO/IEC 9899:1999):
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- 7.23.2.3 The mktime function (p: 340-341)
- C89/C90 standard (ISO/IEC 9899:1990):
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- 4.12.2.3 The mktime function
[edit] See also
| (C11)
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converts time since epoch to calendar time expressed as local time (function) |
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C++ documentation for mktime
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