Auto Fill mode is a minor mode that fills lines automatically as text is inserted. See Auto Fill in The GNU Emacs Manual. This section describes some variables used by Auto Fill mode. For a description of functions that you can call explicitly to fill and justify existing text, see Filling.
Auto Fill mode also enables the functions that change the margins and justification style to refill portions of the text. See Margins for Filling.
The value of this buffer-local variable should be a function (of no
arguments) to be called after self-inserting a character from the table
auto-fill-chars, see below. It may be nil, in which case
nothing special is done in that case.
The value of auto-fill-function is do-auto-fill when Auto
Fill mode is enabled. That is a function whose sole purpose is to
implement the usual strategy for breaking a line.
This variable specifies the function to use for
auto-fill-function, if and when Auto Fill is turned on. Major
modes can set buffer-local values for this variable to alter how Auto
Fill works.
A char table of characters which invoke auto-fill-function when
self-inserted—space and newline in most language environments. They
have an entry t in the table.
This variable, if non-nil, means to fill lines automatically
within comments only. More precisely, this means that if a comment
syntax was defined for the current buffer, then self-inserting a
character outside of a comment will not call auto-fill-function.