Onward through the list of less-commonly used Lisp features.  The peek-char function has uses similar to ungetc() in the C standard.  The Lisp peek-char function returns the next character from the input stream, if any, but does not advance the file pointer, so a subsequent read will still pick up that character.  This is most commonly used in selecting dispatching functions in a token parser.

Note that the C function is allowed to push back a character that was not read, in effect modifying the input stream as seen by the file handle, while the Lisp function does not provide a way to modify the stream.