An input stream is an external source of bytes or characters that may come from a file, a console or terminal, or even another program. An output stream is an external destination for bytes or characters that might be written to a file, a console or terminal, or another program. A random-access stream is both a source and a destination for bytes and characters, and is usually only associated with a file.

A stream must be opened before use. An open stream is identified by a FILE *, which is declared in <stdio.h>. A stream has a format, which governs how some characters might be transformed. A stream also has orientation, which pertains to its handling of multibyte characters.

A stream should be closed after use.