c-indent-comment-alist

c-indent-comment-alist is a variable defined in `cc-vars.el'.
Its value is

set-from-style

Documentation:
*Specifies how M-x indent-for-comment calculates the comment start column.
This is an association list that contains entries of the form:

(LINE-TYPE . INDENT-SPEC)

LINE-TYPE specifies a type of line as described below, and INDENT-SPEC
says what M-x indent-for-comment should do when used on that type of line.

The recognized values for LINE-TYPE are:

empty-line -- The line is empty.
anchored-comment -- The line contains a comment that starts in column 0.
end-block -- The line contains a solitary block closing brace.
cpp-end-block -- The line contains a preprocessor directive that
closes a block, i.e. either "#endif" or "#else".
other -- The line does not match any other entry
currently on the list.

An INDENT-SPEC is a cons cell of the form:

(ACTION . VALUE)

ACTION says how M-x indent-for-comment should align the comment, and
VALUE is interpreted depending on ACTION. ACTION can be any of the
following:

space -- Put VALUE spaces between the end of the line and the start
of the comment.
column -- Start the comment at the column VALUE. If the line is
longer than that, the comment is preceded by a single
space. If VALUE is nil, `comment-column' is used.
align -- Align the comment with one on the previous line, if there
is any. If the line is too long, the comment is preceded
by a single space. If there isn't a comment start on the
previous line, the behavior is specified by VALUE, which
in turn is interpreted as an INDENT-SPEC.

If a LINE-TYPE is missing, then M-x indent-for-comment indents the comment
according to `comment-column'.

Note that a non-nil value on `c-indent-comments-syntactically-p'
overrides this variable, so empty lines are indented syntactically
in that case, i.e. as if M-x c-indent-command was used instead.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.