@node ctime_r
@section @code{ctime_r}
@findex ctime_r

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/ctime_r.html}

Future POSIX removal:@* @url{https://www.austingroupbugs.net/view.php?id=1410}

Gnulib module: extensions

Portability problems fixed by Gnulib:
@itemize
@item
This function has an incompatible declaration on some platforms:
Solaris 11.4 (when @code{_POSIX_PTHREAD_SEMANTICS} is not defined).
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
This function is missing on some platforms:
mingw, MSVC 14.
@item
POSIX says this function is obsolescent and it is planned to be
removed in a future version.
Use the functions @code{localtime_r} and @code{strftime}
(or even @code{sprintf}) instead.
@item
This function may put more than 26 bytes into the argument buffer if an
invalid year is passed.
@end itemize

@code{ctime_r} takes a pre-allocated buffer and length of the buffer,
and returns @code{NULL} on errors.
The input buffer should be at least 26 bytes in size.  The output
string is locale-independent.  However, years can have more than 4
digits if @code{time_t} is sufficiently wide, so the length of the
required output buffer is not easy to determine.  Increasing the
buffer size when @code{ctime_r} returns @code{NULL} is not necessarily
sufficient.  The @code{NULL} return value could mean some other error
condition, which will not go away by increasing the buffer size.

A more flexible function is @code{strftime}.  However, note that it is
locale dependent.
