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Showing content from https://en.cppreference.com/w/cpp/algorithm/../algorithm/../../cpp/../c/chrono/localtime.html below:

localtime, localtime_r, localtime_s - cppreference.com

struct tm* localtime  ( const time_t* timer ); (1) struct tm* localtime_r( const time_t* timer, struct tm* buf ); (2) (since C23) struct tm* localtime_s( const time_t* restrict timer, struct tm* restrict buf ); (3) (since C11) 1)

Converts given time since epoch (a

time_t

value pointed to by

timer

) into calendar time, expressed in local time, in the

struct tm

format. The result is stored in static storage and a pointer to that static storage is returned.

2) Same as (1), except that the function uses user-provided storage buf for the result.

3)

Same as

(1)

, except that the function uses user-provided storage

buf

for the result and that the following errors are detected at runtime and call the currently installed

constraint handler

function:

As with all bounds-checked functions, localtime_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <time.h>.
[edit] Parameters timer - pointer to a time_t object to convert buf - pointer to a struct tm object to store the result [edit] Return value 1)

pointer to a static internal

tm

object on success, or null pointer otherwise. The structure may be shared between

gmtime

,

localtime

, and

ctime

and may be overwritten on each invocation.

2,3) copy of the buf pointer, or null pointer on error (which may be a runtime constraint violation or a failure to convert the specified time to local calendar time).

[edit] Notes

The function localtime may not be thread-safe. The Microsoft CRT implementation is thread-safe.

POSIX requires that localtime and localtime_r set errno to EOVERFLOW if it fails because the argument is too large.

POSIX specifies that the timezone information is determined by localtime and localtime_r as if by calling tzset, which reads the environment variable TZ.

The implementation of localtime_s in Microsoft CRT is incompatible with the C standard since it has reversed parameter order and returns errno_t.

[edit] Example 
#define __STDC_WANT_LIB_EXT1__ 1
#define _XOPEN_SOURCE // for putenv
#include <stdio.h>
#include <stdlib.h>   // for putenv
#include <time.h>
 
int main(void)
{
    time_t t = time(NULL);
    printf("UTC:       %s", asctime(gmtime(&t)));
    printf("local:     %s", asctime(localtime(&t)));
    // POSIX-specific
    putenv("TZ=Asia/Singapore");
    printf("Singapore: %s", asctime(localtime(&t)));
 
#ifdef __STDC_LIB_EXT1__
    struct tm buf;
    char str[26];
    asctime_s(str, sizeof str, gmtime_s(&t, &buf));
    printf("UTC:       %s", str);
    asctime_s(str, sizeof str, localtime_s(&t, &buf));
    printf("local:     %s", str);
#endif
}

Possible output:

UTC:       Fri Sep 15 14:22:05 2017
local:     Fri Sep 15 14:22:05 2017
Singapore: Fri Sep 15 22:22:05 2017
UTC:       Fri Sep 15 14:22:05 2017
local:     Fri Sep 15 14:22:05 2017
[edit] References
[edit] See also

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