asctime, asctime_s

From cppreference.com

Defined in header `<time.h>`
	(1)
char * asctime ( const struct tm * time_ptr ) ;			(until C23)
[ [ deprecated ] ] char * asctime ( const struct tm * time_ptr ) ;			(since C23)
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.

This function is deprecated and should not be used in new code.

(since C23)

2) Same as (1) , except that the message is written 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 with 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> .

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

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 ).

Notes

asctime 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.

Example

Run this code

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <time.h>
 
int main(void)
{
    struct tm tm = *localtime(&(time_t){time(NULL)});
    printf("%s", asctime(&tm)); // note implicit trailing '\n'
 
#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

References

C17 standard (ISO/IEC 9899:2018):

7.27.2.1 The asctime function (p: 287)

K.3.8.2.1 The asctime_s function (p: 453-454)

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

Compiler support
Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index

Functions
Time manipulation
difftime
time
clock
timespec_get (C11)
timespec_getres (C23)
Format conversions
asctime asctime_s (deprecated in C23) (C11)
ctime ctime_s (deprecated in C23) (C11)
strftime
wcsftime (C95)
gmtime gmtime_r gmtime_s (C23) (C11)
localtime localtime_r localtime_s (C23) (C11)
mktime
Constants
CLOCKS_PER_SEC
Types
tm
time_t
clock_t
timespec (C11)

ctime ctime_s (deprecated in C23) (C11)	converts a time_t object to a textual representation (function)
strftime	converts a tm object to custom textual representation (function)
C++ documentation for asctime