# ctime

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           ctime, ctime_r — convert a time value to a date and time string
    


## SYNOPSIS



           #include <time.h>
    
           char *ctime(const time_t *_clock_);
           char *ctime_r(const time_t *_clock_ , char *_buf_);
    


## DESCRIPTION



           For _ctime_(): The functionality described on this reference page is
           aligned with the ISO C standard. Any conflict between the
           requirements described here and the ISO C standard is
           unintentional. This volume of POSIX.1‐2017 defers to the ISO C
           standard.
    
           The _ctime_() function shall convert the time pointed to by _clock_ ,
           representing time in seconds since the Epoch, to local time in the
           form of a string. It shall be equivalent to:
    
               asctime(localtime(clock))
    
           The _asctime_(), _ctime_(), _gmtime_(), and _localtime_() functions shall
           return values in one of two static objects: a broken-down time
           structure and an array of **char**.  Execution of any of the functions
           may overwrite the information returned in either of these objects
           by any of the other functions.
    
           The _ctime_() function need not be thread-safe.
    
           The _ctime_r_() function shall convert the calendar time pointed to
           by _clock_ to local time in exactly the same form as _ctime_() and put
           the string into the array pointed to by _buf_ (which shall be at
           least 26 bytes in size) and return _buf_.
    
           Unlike _ctime_(), the _ctime_r_() function is not required to set
           _tzname_.  If _ctime_r_() sets _tzname_ , it shall also set _daylight_ and
           _timezone_.  If _ctime_r_() does not set _tzname_ , it shall not set
           _daylight_ and shall not set _timezone_.
    


## RETURN VALUE



           The _ctime_() function shall return the pointer returned by
           _asctime_() with that broken-down time as an argument.
    
           Upon successful completion, _ctime_r_() shall return a pointer to
           the string pointed to by _buf_.  When an error is encountered, a
           null pointer shall be returned.
    


## ERRORS



           No errors are defined.
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           These functions are included only for compatibility with older
           implementations. They have undefined behavior if the resulting
           string would be too long, so the use of these functions should be
           discouraged.  On implementations that do not detect output string
           length overflow, it is possible to overflow the output buffers in
           such a way as to cause applications to fail, or possible system
           security violations. Also, these functions do not support
           localized date and time formats. To avoid these problems,
           applications should use _strftime_() to generate strings from
           broken-down times.
    
           Values for the broken-down time structure can be obtained by
           calling _gmtime_() or _localtime_().
    
           The _ctime_r_() function is thread-safe and shall return values in a
           user-supplied buffer instead of possibly using a static data area
           that may be overwritten by each call.
    
           Attempts to use _ctime_() or _ctime_r_() for times before the Epoch or
           for times beyond the year 9999 produce undefined results. Refer to
           asctime(3p).
    


## RATIONALE



           The standard developers decided to mark the _ctime_() and _ctime_r_()
           functions obsolescent even though they are in the ISO C standard
           due to the possibility of buffer overflow. The ISO C standard also
           provides the _strftime_() function which can be used to avoid these
           problems.
    


## FUTURE DIRECTIONS



           These functions may be removed in a future version.
    


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