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ctime, ctime_r — convert a time value to a date and time string
char *ctime(const time_t *clock);
char *ctime_r(const time_t *clock, char *buf);
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:
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
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.
No errors are defined.
The following sections are informative.
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().
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.
These functions may be removed in a future version.
asctime(), clock(), difftime(),
gmtime(), localtime(), mktime(),
strftime(), strptime(), time(),
The Base Definitions volume of POSIX.1‐2017,
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1-2017, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018
Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html .
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