`M-x make-local-variable RET VAR RET'
Make variable VAR have a local value in the current buffer.
`M-x kill-local-variable RET VAR RET'
Make variable VAR use its global value in the current buffer.
`M-x make-variable-buffer-local RET VAR RET'
Mark variable VAR so that setting it will make it local to the
buffer that is current at that time.
Almost any variable can be made "local" to a specific Emacs buffer.
This means that its value in that buffer is independent of its value in
other buffers. A few variables are always local in every buffer.
Every other Emacs variable has a "global" value which is in effect in
all buffers that have not made the variable local.
`M-x make-local-variable' reads the name of a variable and makes it
local to the current buffer. Further changes in this buffer will not
affect others, and further changes in the global value will not affect
`M-x make-variable-buffer-local' reads the name of a variable and
changes the future behavior of the variable so that it will become local
automatically when it is set. More precisely, once a variable has been
marked in this way, the usual ways of setting the variable automatically
do `make-local-variable' first. We call such variables "per-buffer"
Major modes (Note: Major Modes.) always make variables local to the
buffer before setting the variables. This is why changing major modes
in one buffer has no effect on other buffers. Minor modes also work by
setting variables--normally, each minor mode has one controlling
variable which is non-`nil' when the mode is enabled (Note: Minor
Modes.). For most minor modes, the controlling variable is per
Emacs contains a number of variables that are always per-buffer.
These include `abbrev-mode', `auto-fill-function', `case-fold-search',
`comment-column', `ctl-arrow', `fill-column', `fill-prefix',
`indent-tabs-mode', `left-margin', `mode-line-format', `overwrite-mode',
`selective-display-ellipses', `selective-display', `tab-width', and
`truncate-lines'. Some other variables are always local in every
buffer, but they are used for internal purposes.
A few variables cannot be local to a buffer because they are always
local to each display instead (Note: Multiple Displays). If you try
to make one of these variables buffer-local, you'll get an error
`M-x kill-local-variable' reads the name of a variable and makes it
cease to be local to the current buffer. The global value of the
variable henceforth is in effect in this buffer. Setting the major mode
kills all the local variables of the buffer except for a few variables
specially marked as "permanent locals".
To set the global value of a variable, regardless of whether the
variable has a local value in the current buffer, you can use the Lisp
construct `setq-default'. This construct is used just like `setq', but
it sets variables' global values instead of their local values (if
any). When the current buffer does have a local value, the new global
value may not be visible until you switch to another buffer. Here is
(setq-default fill-column 75)
`setq-default' is the only way to set the global value of a variable
that has been marked with `make-variable-buffer-local'.
Lisp programs can use `default-value' to look at a variable's
default value. This function takes a symbol as argument and returns its
default value. The argument is evaluated; usually you must quote it
explicitly. For example, here's how to obtain the default value of
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