Description

   The asctime and asctime_r functions convert the contents of tm
   into a 26-character string and returns a pointer to the string.

   The difference between asctime_r and asctime is that the former
   puts the result into a user-specified buffer. The latter puts
   the result into thread-specific static memory allocated by the
   C RTL, which can be overwritten by subsequent calls to
   ctime or asctime; you must make a copy if you want to save it.

   On success, asctime returns a pointer to the string; asctime_r
   returns its second argument. On failure, these functions return
   the NULL pointer.

   See the localtime function for a list of the members in tm.

                                  NOTE

      Generally speaking, UTC-based time functions can affect in-
      memory time-zone information, which is processwide data.
      However, if the system time zone remains the same during
      the execution of the application (which is the common case)
      and the cache of timezone files is enabled (which is the
      default), then the _r variant of the time functions asctime_
      r, ctime_r, gmtime_r and localtime_r, is both thread-safe
      and AST-reentrant.

      If, however, the system time zone can change during the
      execution of the application or the cache of timezone files
      is not enabled, then both variants of the UTC-based time
      functions belong to the third class of functions, which are
      neither thread-safe nor AST-reentrant.