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localtime, localtime_r, localtime_s

From cppreference.com
< 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 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>.

Contents

[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.

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

  • C23 standard (ISO/IEC 9899:2023):
  • 7.27.3.4 The localtime function (p: TBD)
  • K.3.8.2.4 The localtime_s function (p: TBD)
  • 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

[edit] See also

converts time since epoch to calendar time expressed as Coordinated Universal Time (UTC)
(function) [edit]
C++ documentation for localtime