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The comment column, the column at which Emacs tries to place
comments, is stored in the variable comment-column. You can
set it to a number explicitly. Alternatively, the command C-x ;
(comment-set-column) sets the comment column to the column
point is at. C-u C-x ; sets the comment column to match the
last comment before point in the buffer, and then does a M-; to
align the current line's comment under the previous one.
The variable comment-column is per-buffer: setting the variable
in the normal fashion affects only the current buffer, but there is a
default value which you can change with setq-default.
See Locals. Many major modes initialize this variable for the
current buffer.
The comment commands recognize comments based on the regular
expression that is the value of the variable comment-start-skip.
Make sure this regexp does not match the null string. It may match more
than the comment starting delimiter in the strictest sense of the word;
for example, in C mode the value of the variable is
"/\\*+ *\\|//+ *", which matches extra stars and spaces
after the ‘/*’ itself, and accepts C++ style comments also.
(Note that ‘\\’ is needed in Lisp syntax to include a ‘\’ in
the string, which is needed to deny the first star its special meaning
in regexp syntax. See Regexp Backslash.)
When a comment command makes a new comment, it inserts the value of
comment-start to begin it. The value of comment-end is
inserted after point, so that it will follow the text that you will
insert into the comment. When comment-end is non-empty, it
should start with a space. For example, in C mode,
comment-start has the value "/* " and
comment-end has the value " */".
The variable comment-padding specifies how many spaces
comment-region should insert on each line between the comment
delimiter and the line's original text. The default is 1, to insert
one space. nil means 0. Alternatively, comment-padding
can hold the actual string to insert.
The variable comment-multi-line controls how C-M-j
(indent-new-comment-line) behaves when used inside a comment.
Specifically, when comment-multi-line is nil, the
command inserts a comment terminator, begins a new line, and finally
inserts a comment starter. Otherwise it does not insert the
terminator and starter, so it effectively continues the current
comment across multiple lines. In languages that allow multi-line
comments, the choice of value for this variable is a matter of taste.
The default for this variable depends on the major mode.
The variable comment-indent-function should contain a function
that will be called to compute the indentation for a newly inserted
comment or for aligning an existing comment. It is set differently by
various major modes. The function is called with no arguments, but with
point at the beginning of the comment, or at the end of a line if a new
comment is to be inserted. It should return the column in which the
comment ought to start. For example, in Lisp mode, the indent hook
function bases its decision on how many semicolons begin an existing
comment, and on the code in the preceding lines.