gmtime, gmtime_r, gmtime_s

< c‎ | chrono
Defined in header <time.h>
struct tm *gmtime  ( const time_t *timer );
struct tm *gmtime_r( const time_t *timer, struct tm *buf );
(2) (since C23)
struct tm *gmtime_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 Coordinated Universal Time (UTC) 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, gmtime_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>.


[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 UTC)

[edit] Notes

gmtime may not be thread-safe.

POSIX requires that gmtime and gmtime_r set errno to EOVERFLOW if they fail because the argument is too large.

The implementation of gmtime_s in Microsoft CRT is incompatible with the C standard since it has reversed parameter order.

[edit] Example

#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
    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);

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

  • C17 standard (ISO/IEC 9899:2018):
  • The gmtime function (p: 288)
  • K. The gmtime_s function (p: 454-455)
  • C11 standard (ISO/IEC 9899:2011):
  • The gmtime function (p: 393-394)
  • K. The gmtime_s function (p: 626-627)
  • C99 standard (ISO/IEC 9899:1999):
  • The gmtime function (p: 343)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • The gmtime function

[edit] See also

converts time since epoch to calendar time expressed as local time
(function) [edit]