Install

git clone https://github.com/commercial-emacs/xlsp.git
make -C xlsp install

Usage

Overview
--------
M-x xlsp-mode        toggles for the current buffer
M-x global-xlsp-mode toggles globally
C-M-i                completion (disambiguate then TAB to complete)
M-.                  jump to definition
M-,                  jump back from definition
C-u M-.              prompt jump to definition (TAB to complete)
M-?                  find references

.emacs or init.el configuration
-------------------------------
(add-hook 'c-mode-hook #'xlsp-mode) ; activate for c-mode
-or-
(global-xlsp-mode) ; activate for any mode in xlsp-server-invocations

Being hip to the youth
----------------------
M-x customize-option RET xlsp-completion-menus-p
M-x customize-option RET xlsp-hover-help-p
...
and other activations which it's hard to imagine are tolerable.

"Nothing is happening."
-----------------------
M-x customize-option RET xlsp-server-invocations RET

What the "x" in "xlsp" references
---------------------------------
It is merely a differentiator, just as the "x" in "xemacs" had been
(contrary to the popular misconception that it referenced X11).

Credits
-------
Logic, some transcribed verbatim, was patterned after GNU eglot.
Functions missing in emacs-27 and emacs-28 transcribed from GNU emacs-29.