Variable: linum-format
linum-format is a customizable variable defined in linum.el.gz.
Value
dynamic
Documentation
Format used to display line numbers.
Either a format string like "%7d", dynamic to adapt the width
as needed, or a function that is called with a line number as its
argument and should evaluate to a string to be shown on that line.
See also linum-before-numbering-hook.
Source Code
;; Defined in /usr/src/emacs/lisp/obsolete/linum.el.gz
(defcustom linum-format 'dynamic
"Format used to display line numbers.
Either a format string like \"%7d\", `dynamic' to adapt the width
as needed, or a function that is called with a line number as its
argument and should evaluate to a string to be shown on that line.
See also `linum-before-numbering-hook'."
:group 'linum
:type '(choice (string :tag "Format string")
(const :tag "Dynamic width" dynamic)
(function :tag "Function")))