# localtime, localtime_r, localtime_s

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< c‎ | chrono

Defined in header `<time.h>` |  |
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:

  - `timer` or `buf` is a null pointer
  As with all bounds-checked functions, `localtime_s` 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>`.

### Parameters

timer | - | pointer to a time_t object to convert
buf | - | pointer to a struct tm object to store the result

### 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)

### Notes

The function `localtime` may not be 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.

### Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #define _XOPEN_SOURCE // for putenv
    #include <time.h>
    #include <stdio.h>
    #include <stdlib.h>   // for putenv

    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

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.27.3.4 The localtime function (p: 288)

  - K.3.8.2.4 The localtime_s function (p: 455)

- C11 standard (ISO/IEC 9899:2011):

  - 7.27.3.4 The localtime function (p: 394)

  - K.3.8.2.4 The localtime_s function (p: 627)

- C99 standard (ISO/IEC 9899:1999):

  - 7.23.3.4 The localtime function (p: 343)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.12.3.4 The localtime function

### See also

gmtime gmtime_r gmtime_s (C23)(C11) | converts time since epoch to calendar time expressed as Coordinated Universal Time (UTC) (function)

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