Activates / Deactivates synchronization with cstdio functions.
  When sync parameter is true, all operations performed on a same file either using iostream or cstdio streams are guaranteed to be properly synchronized.
  Otherwise, if parameter is false, iostream streams are allowed to operate independently of the cstdio streams. Performance may improve in this mode but synchronization is not guaranteed.
  By default iostream and cstdio streams are synchronized.
  This is a static member function: a call to it affects all streams.

Parameters.

sync

Bool parater indicating the need of synchronization between iostream and cstdio streams.

Return Value.
  Returns the previous value of the synchronization mode.

See also.
  ios_base class