These functions and variables provide access to the kill ring at a lower level, but are still convenient for use in Lisp programs, because they take care of interaction with window system selections (see Window System Selections).
The function current-kill rotates the yanking pointer, which designates the front of the kill ring, by n places (from newer kills to older ones), and returns the text at that place in the ring.
If the optional second argument do-not-move is non-nil, then current-kill doesn’t alter the yanking pointer; it just returns the nth kill, counting from the current yanking pointer.
If n is zero, indicating a request for the latest kill, current-kill calls the value of interprogram-paste-function (documented below) before consulting the kill ring. If that value is a function and calling it returns a string or a list of several strings, current-kill pushes the strings onto the kill ring and returns the first string. It also sets the yanking pointer to point to the kill-ring entry of the first string returned by interprogram-paste-function, regardless of the value of do-not-move. Otherwise, current-kill does not treat a zero value for n specially: it returns the entry pointed at by the yanking pointer and does not move the yanking pointer.
This function pushes the text string onto the kill ring and makes the yanking pointer point to it. It discards the oldest entry if appropriate. It also invokes the values of interprogram-paste-function (subject to the user option save-interprogram-paste-before-kill) and interprogram-cut-function (see below).
If replace is non-nil, then kill-new replaces the first element of the kill ring with string, rather than pushing string onto the kill ring.
This function appends the text string to the first entry in the kill ring and makes the yanking pointer point to the combined entry. Normally string goes at the end of the entry, but if before-p is non-nil, it goes at the beginning. This function calls kill-new as a subroutine, thus causing the values of interprogram-cut-function and possibly interprogram-paste-function (see below) to be invoked by extension.
This variable provides a way of transferring killed text from other programs, when you are using a window system. Its value should be nil or a function of no arguments.
If the value is a function, current-kill calls it to get the most recent kill. If the function returns a non-nil value, then that value is used as the most recent kill. If it returns nil, then the front of the kill ring is used.
To facilitate support for window systems that support multiple selections, this function may also return a list of strings. In that case, the first string is used as the most recent kill, and all the other strings are pushed onto the kill ring, for easy access by yank-pop.
The normal use of this function is to get the window system’s clipboard as the most recent kill, even if the selection belongs to another application. See Window System Selections. However, if the clipboard contents come from the current Emacs session, this function should return nil.
This variable provides a way of communicating killed text to other programs, when you are using a window system. Its value should be nil or a function of one required argument.
If the value is a function, kill-new and kill-append call it with the new first element of the kill ring as the argument.
The normal use of this function is to put newly killed text in the window system’s clipboard. See Window System Selections.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Low_002dLevel-Kill-Ring.html