gmtime, gmtime_r, gmtime_s

Defined in header `<time.h>`
`struct tm gmtime ( const time_t timer );`	(1)
`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>.

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 UTC)

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.

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
    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):

7.27.3.3 The gmtime function (p: 288)
K.3.8.2.3 The gmtime_s function (p: 454-455)

C11 standard (ISO/IEC 9899:2011):

7.27.3.3 The gmtime function (p: 393-394)
K.3.8.2.3 The gmtime_s function (p: 626-627)

C99 standard (ISO/IEC 9899:1999):

7.23.3.3 The gmtime function (p: 343)

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

4.12.3.3 The gmtime function

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