The difference between setq and setq-default in Emacs Lisp

Question

I have a question about Emacs Lisp. What is the difference between setq and setq-default?

Tutorials say setq takes effect in the local buffer while setq-default affects all buffers.

For example, if I wrote (setq a-var a-vars-value) in init.el, I found after starting Emacs and opening a new buffer, the a-var is also there and its value is a-vars-value. I thought it was not supposed to be there. It seems there is no difference between setq and setq-default.

Is there something wrong with my understanding?

For example:

1. I wrote (setq hello 123) in the init.el file, and I run emacs abuffer in the shell, then I input "hello C-x C-e", it shows "123". The same happens when I run this in all new buffers.

2. I wrote (setq tab-width 4) in the init.el file. When I run tab-width C-x C-e, it shows "8" (Current mode is 'Text'). However, when I use (setq-default tab-width 4), it show "4". I can't explain this phenomenon.

Answer 1

Some variables in Emacs are "buffer-local", meaning that each buffer is allowed to have a separate value for that variable that overrides the global default. tab-width is a good example of a buffer-local variable.

If a variable is buffer-local, then setq sets its local value in the current buffer and setq-default sets the global default value.

If a variable is not buffer-local, then setq and setq-default do the same thing.

In your case 2, (setq tab-width 4) set the buffer-local value of tab-width to 4 in the current buffer, leaving the global default value of tab-width still at 8, so when you evaluated tab-width in a different buffer that had no local value, you saw that 8. Then, when you set the default value to 4, that buffer picked it up, since it still had no local value.