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ctime, ctime_s

Defined in header `<time.h>`
char* ctime( const time_t* time );	(1)
errno_t ctime_s(char buffer, rsize_t bufsz, const time_t time);	(2)	(since C11)

1) Converts given time since epoch to a calendar local time and then to a textual representation, as if by calling asctime(localtime(time)).

2) Same as (1), except that the function is equivalent to asctime_s(buffer, bufsz, localtime_s(time, &(struct tm){0})), and the following errors are detected at runtime and call the currently installed constraint handler function:

buffer or time is a null pointer
bufsz is less than 26 or greater than RSIZE_MAX

As with all bounds-checked functions, ctime_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.

The resulting string has the following format:

Www Mmm dd hh:mm:ss yyyy\n

Www - the day of the week (one of Mon, Tue, Wed, Thu, Fri, Sat, Sun).
Mmm - the month (one of Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec).
dd - the day of the month
hh - hours
mm - minutes
ss - seconds
yyyy - years

The function does not support localization.

Parameters

time	-	pointer to a `time_t` object specifying the time to print
buffer	-	pointer to an element of a char array of size at least `bufsz`
bufsz	-	max number of bytes to output, typically the size of the buffer pointed to by `buffer`

Return value

1) pointer to a static null-terminated character string holding the textual representation of date and time. 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 (in which case the string representatino of time has been written out to the array pointed to by buffer), or non-zero on failure (in which case, the terminating null character is always written to buffer[0] unless buffer is a null pointer or bufsz is zero or greater than RSIZE_MAX.

Notes

ctime returns a pointer to static data and is not thread-safe. In addition, it modifies the static tm object which may be shared with gmtime and localtime. POSIX marks this function obsolete and recommends strftime instead. The C standard also recommends strftime instead of ctime and ctime_s because strftime is more flexible and locale-sensitive.

The behavior of ctime may be undefined for the values of time_t that result in the string longer than 25 characters (e.g. year 10000).

Example

#define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
#include <stdio.h>
 
int main(void)
{
    time_t result = time(NULL);
    printf("%s", ctime(&result));
 
#ifdef __STDC_LIB_EXT1__
    char str[26];
    ctime_s(str,sizeof str,&result);
    printf("%s", str);
#endif
}

Output:

Tue May 26 21:51:03 2015
Tue May 26 21:51:03 2015

References

C11 standard (ISO/IEC 9899:2011):

7.27.3.2 The ctime function (p: 393)
K.3.8.2.2 The ctime_s function (p: 626)

C99 standard (ISO/IEC 9899:1999):

7.23.3.2 The ctime function (p: 342)

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

4.12.3.2 The ctime function