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# asctime, asctime_s

From cppreference.com
< c‎ | chrono

Defined in header `<time.h>` |  |
char* asctime( const struct tm* time_ptr ); | (1) |
errno_t asctime_s(char *buf, rsize_t bufsz, const struct tm *time_ptr); | (2) | (since C11)

1) Converts given calendar time tm to a textual representation of the following fixed 25-character form: Www Mmm dd hh:mm:ss yyyy\n

- `Www` - three-letter English abbreviated day of the week from time_ptr->tm_wday, one of `Mon`, `Tue`, `Wed`, `Thu`, `Fri`, `Sat`, `Sun`.
- `Mmm` - three-letter English abbreviated month name from time_ptr->tm_mon, one of `Jan`, `Feb`, `Mar`, `Apr`, `May`, `Jun`, `Jul`, `Aug`, `Sep`, `Oct`, `Nov`, `Dec`.
- `dd` - 2-digit day of the month from timeptr->tm_mday as if printed by sprintf using %2d
- `hh` - 2-digit hour from timeptr->tm_hour as if printed by sprintf using %.2d
- `mm` - 2-digit minute from timeptr->tm_min as if printed by sprintf using %.2d
- `ss` - 2-digit second from timeptr->tm_sec as if printed by sprintf using %.2d
- `yyyy` - 4-digit year from timeptr->tm_year + 1900 as if printed by sprintf using %4d

The behavior is undefined if any member of *time_ptr is outside its normal range
The behavior is undefined if the calendar year indicated by time_ptr->tm_year has more than 4 digits or is less than the year 1000.
The function does not support localization, and the newline character cannot be removed.
The function modifies static storage and is not thread-safe.
2) Same as (1), except that the message is copied into user-provided storage `buf`, which is guaranteed to be null-terminated, and the following errors are detected at runtime and call the currently installed constraint handler function:

  - `buf` or `time_ptr` is a null pointer
  - `bufsz` is less than 26 or greater than RSIZE_MAX
  - not all members of *time_ptr are within their normal ranges
  - the year indicated by time_ptr->tm_year is less than 0 or greater than 9999
  As all bounds-checked functions, `asctime_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`.

### [edit] Parameters

time_ptr | - | pointer to a tm object specifying the time to print
buf | - | pointer to a user-supplied buffer at least 26 bytes in length
bufsz | - | size of the user-supplied buffer

### [edit] Return value

1) pointer to a static null-terminated character string holding the textual representation of date and time as described above. The string may be shared between `asctime` and ctime, and may be overwritten on each invocation of any of those functions.
2) zero on success, non-zero on failure, in which case buf[0] is set to zero (unless `buf` is a null pointer or `bufsz` is zero or greater than RSIZE_MAX).

### [edit] Notes

This function returns a pointer to static data and is not thread-safe. POSIX marks this function obsolete and recommends strftime instead. The C standard also recommends strftime instead of `asctime` and `asctime_s` because `strftime` is more flexible and locale-sensitive.

POSIX limits undefined behaviors only to when the output string would be longer than 25 characters, when `timeptr->tm_wday` or `timeptr->tm_mon` are not within the expected ranges, or when `timeptr->tm_year` exceeds INT_MAX-1990.

Some implementations handle timeptr->tm_mday==0 as meaning the last day of the preceding month.

### [edit] Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #include <time.h>
    #include <stdio.h>

    int main(void)
    {
        struct tm tm = *localtime(&(time_t){time(NULL)});
        printf("%s", asctime(&tm));

    #ifdef __STDC_LIB_EXT1__
        char str[26];
        asctime_s(str, sizeof str, &tm);
        printf("%s", str);
    #endif
    }

Possible output:

    Tue May 26 21:51:50 2015
    Tue May 26 21:51:50 2015

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.27.2.1 The asctime function (p: 392-393)

  - K.3.8.2.1 The asctime_s function (p: 624-625)

- C99 standard (ISO/IEC 9899:1999):

  - 7.23.3.1 The asctime function (p: 341-342)

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

  - 4.12.3.1 The asctime function

### [edit] See also

ctime ctime_s (C11) | converts a `time_t` object to a textual representation (function) [edit]
strftime | converts a `tm` object to custom textual representation (function) [edit]
C++ documentation for asctime

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