Difference between revisions of "c/chrono/localtime"
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[http://pubs.opengroup.org/onlinepubs/9699919799/functions/localtime.html POSIX specifies] that the timezone information is determined by {{tt|localtime}} and {{tt|localtime_r}} as if by calling [http://pubs.opengroup.org/onlinepubs/9699919799/functions/tzset.html tzset], which reads the environment variable {{c|TZ}}. | [http://pubs.opengroup.org/onlinepubs/9699919799/functions/localtime.html POSIX specifies] that the timezone information is determined by {{tt|localtime}} and {{tt|localtime_r}} as if by calling [http://pubs.opengroup.org/onlinepubs/9699919799/functions/tzset.html tzset], which reads the environment variable {{c|TZ}}. | ||
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| + | The implementation of {{tt|localtime_s}} in [https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s?view=vs-2019 Microsoft CRT] is incompatible with the C standard since it has reversed parameter order. | ||
===Example=== | ===Example=== | ||
Revision as of 06:10, 31 December 2019
| Defined in header <time.h>
|
||
| (1) | ||
| (2) | (since C23) | |
| (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:
-
timeorbufis a null pointer
-
- As with all bounds-checked functions,
localtime_sis 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 |
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
This 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.
Example
Run this code
#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
References
- 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
| (C23)(C11) |
converts time since epoch to calendar time expressed as Coordinated Universal Time (UTC) (function) |
| C++ documentation for localtime
| |
