auto-fill-function

auto-fill-function is a variable defined in `C source code'.
Its value is

nil

• Automatically becomes buffer-local when set.
• This variable may be risky if used as a file-local variable.
• This variable is safe as a file local variable if its value
  satisfies the predicate `null'.

Documentation:
Function called (if non-nil) to perform auto-fill.
It is called after self-inserting any character specified in
the `auto-fill-chars' table.
NOTE: This variable is not a hook;
its value may not be a list of functions.