Namespaces
Variants

asctime, asctime_s

From cppreference.net
C
Date and time utilities
Functions
Time manipulation
(C11)
(C23)
Format conversions
asctime asctime_s
(deprecated in C23) (C11)
(deprecated in C23) (C11)
(C95)
(C23) (C11)
(C23) (C11)
Constants
Types
(C11)
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> .

Contents

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

See also

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