How to force the window splitting direction when starting slime?

Question

When I use emacs in a separate Window (under X), if I start slime the window is split horizontally if the window's width is smaller than its height, and vertically otherwise. Is it possible to force the direction of the splitting and make it independent of the dimensions of the emacs window? For example, I would like to have slime always open under my source code buffer. I cannot find anything about this in the documentation.

Answer 1

According to the doc-string of the variable split-window-preferred-function, the value should be a "[f]unction called by 'display-buffer' routines to split a window." The default value is split-window-sensibly, which has a semi-lengthy doc-string that talks about (in part) the variables split-height-threshold and split-width-threshold; and, those variables in turn have doc-strings. In a nutshell, (setq split-width-threshold nil) will result in split-window-sensibly never splitting a window horizontally. For more information, on these functions or variables, type C-h f or C-h v to describe functions/variables -- or M-x describe-function and M-x describe-variable.

Answer 2

Just for reference as lawlist's answer did not work for me. I instead had to use:

  (setq split-height-threshold nil)
  (setq split-width-threshold 0)

Answer 3

The splitting behavior can be controlled by setting the variable display-buffer-alist.

Please refer https://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Action-Functions.html

In particular, note display-buffer-below-selected.

An easier to achieve the same is using the package shackle (my personal choice)

For example, the below code aligns all helm buffers to the bottom of the screen, covering 40% of the screen

(setq shackle-rules '(("\\`\\*helm.*?\\*\\'" :align 'below :size 0.4)))

You could do the same for buffers with a given mode, try something like

(setq shackle-rules '((slime-mode :align 'below)))