@node ctime
@section ctime
@findex ctime

The @code{ctime} function need not be reentrant, and consequently is
not required to be thread safe.  Implementations of @code{ctime}
typically write the time stamp into static buffer.  If two threads
call @code{ctime} at roughly the same time, you might end up with the
wrong date in one of the threads, or some undefined string.  There is
a re-entrant interface @code{ctime_r}, that take a pre-allocated
buffer and length of the buffer, and return @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} return @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.