The copy-region-as-kill function (see copy-region-as-kill) uses the filter-buffer-substring
function, which in turn uses the delete-and-extract-region
function. It removes the contents of a region and you cannot get them
back.
Unlike the other code discussed here, the
delete-and-extract-region function is not written in Emacs
Lisp; it is written in C and is one of the primitives of the GNU Emacs
system. Since it is very simple, I will digress briefly from Lisp and
describe it here.
Like many of the other Emacs primitives,
delete-and-extract-region is written as an instance of a C
macro, a macro being a template for code. The complete macro looks
like this:
DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
Sdelete_and_extract_region, 2, 2, 0,
doc: /* Delete the text between START and END and return it. */)
(Lisp_Object start, Lisp_Object end)
{
validate_region (&start, &end);
if (XFIXNUM (start) == XFIXNUM (end))
return empty_unibyte_string;
return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
}
Without going into the details of the macro writing process, let me
point out that this macro starts with the word DEFUN. The word
DEFUN was chosen since the code serves the same purpose as
defun does in Lisp. (The DEFUN C macro is defined in
emacs/src/lisp.h.)
The word DEFUN is followed by seven parts inside of
parentheses:
delete-and-extract-region.
Fdelete_and_extract_region. By convention, it starts with
‘F’. Since C does not use hyphens in names, underscores are used
instead.
interactive declaration in a function written in Lisp: a letter
followed, perhaps, by a prompt. The only difference from Lisp is
when the macro is called with no arguments. Then you write a 0
(which is a null string), as in this macro.
If you were to specify arguments, you would place them between
quotation marks. The C macro for goto-char includes
"NGoto char: " in this position to indicate that the function
expects a raw prefix, in this case, a numerical location in a buffer,
and provides a prompt.
lib-src/make-docfile extracts
these comments and uses them to make the documentation.)
In a C macro, the formal parameters come next, with a statement of
what kind of object they are, followed by the body
of the macro. For delete-and-extract-region the body
consists of the following four lines:
validate_region (&start, &end); if (XFIXNUM (start) == XFIXNUM (end)) return empty_unibyte_string; return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
The validate_region function checks whether the values
passed as the beginning and end of the region are the proper type and
are within range. If the beginning and end positions are the same,
then return an empty string.
The del_range_1 function actually deletes the text. It is a
complex function we will not look into. It updates the buffer and
does other things. However, it is worth looking at the two arguments
passed to del_range_1. These are XFIXNUM (start) and
XFIXNUM (end).
As far as the C language is concerned, start and end are
two opaque values that mark the beginning and end of the region to be
deleted. More precisely, and requiring more expert knowledge
to understand, the two values are of type Lisp_Object, which
might be a C pointer, a C integer, or a C struct; C code
ordinarily should not care how Lisp_Object is implemented.
Lisp_Object widths depend on the machine, and are typically 32
or 64 bits. A few of the bits are used to specify the type of
information; the remaining bits are used as content.
‘XFIXNUM’ is a C macro that extracts the relevant integer from the longer collection of bits; the type bits are discarded.
The command in delete-and-extract-region looks like this:
del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);
It deletes the region between the beginning position, start,
and the ending position, end.
From the point of view of the person writing Lisp, Emacs is all very simple; but hidden underneath is a great deal of complexity to make it all work.