(7.9.3) The characteristics of file buffering:

Output streams, with the exception of the standard error stream (stderr), are by default-buffered if the output refers to a file, and line-buffered if the output refers to a terminal. The standard error output stream (stderr) is by default unbuffered.

A buffered output stream saves many characters, and then writes the characters as a block. An unbuffered output stream queues information for immediate writing on the destination file or terminal immediately. Line-buffered output queues each line of output until the line is complete (a newline character is requested).