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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 (XINT (start) == XINT (end)) return empty_unibyte_string; return del_range_1 (XINT (start), XINT (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 the 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 “real” documentation.)
In a C macro, the formal parameters come next, with a statement of
what kind of object they are, followed by what might be called the ‘body’
of the macro. For delete-and-extract-region
the ‘body’
consists of the following four lines:
validate_region (&start, &end); if (XINT (start) == XINT (end)) return empty_unibyte_string; return del_range_1 (XINT (start), XINT (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
. These are XINT (start)
and
XINT (end)
.
As far as the C language is concerned, start
and end
are
two integers that mark the beginning and end of the region to be
deleted10.
In early versions of Emacs, these two numbers were thirty-two bits long, but the code is slowly being generalized to handle other lengths. Three of the available bits are used to specify the type of information; the remaining bits are used as ‘content’.
‘XINT’ is a C macro that extracts the relevant number from the longer collection of bits; the three other bits are discarded.
The command in delete-and-extract-region
looks like this:
del_range_1 (XINT (start), XINT (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.
More precisely, and requiring more expert knowledge to understand, the two integers are of type ‘Lisp_Object’, which can also be a C union instead of an integer type.
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