The file objects in the library can operate as unbuffered, line-buffered or fully (i.e. block) buffered. When your stdout is connected to a terminal, it is line

The file objects in the library can operate as unbuffered, line-buffered or fully (i.e. block) buffered. When your stdout is connected to a terminal, it is line-buffered and flushes on newline. But when you redirect to a file, it becomes block buffered.