gmtime, gmtime_s
Defined in header <time.h>
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||
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|
(1) | |
|
(2) | (since C11) |
1) Converts given time since epoch (a time_t
value pointed to by time
) 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 result
for the result and that the following errors are detected at runtime and call the currently installed constraint handler function:
time
orresult
is a null pointer
As with all bounds-checked functions, gmtime_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
.
Parameters
time | - | pointer to a time_t object to convert |
result | - | 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) copy of the result
pointer, or null pointer on error (which may be a runtime constraint violation or a failure to convert the specified time to UTC)
Notes
This function gmtime
may not be thread-safe.
POSIX requires that this function sets errno
to EOVERFLOW
if it fails because the argument is too large.
POSIX defines a thread-safe alternative gmtime_r, which is similar to the C11 function gmtime_s
, except that it does not check the validity of its input parameters.
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
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
See also
(C11) |
converts time since epoch to calendar time expressed as local time (function) |
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