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(emacs-lisp-intro.info)Primitive Functions

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An Aside about Primitive Functions
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   All functions are defined in terms of other functions, except for a
few "primitive" functions that are written in the C programming
language.  When you write functions' definitions, you will write them in
Emacs Lisp and use other functions as your building blocks.  Some of the
functions you will use will themselves be written in Emacs Lisp (perhaps
by you) and some will be primitives written in C.  The primitive
functions are used exactly like those written in Emacs Lisp and behave
like them.  They are written in C so we can easily run GNU Emacs on any
computer that has sufficient power and can run C.

   Let me re-emphasize this: when you write code in Emacs Lisp, you do
not distinguish between the use of functions written in C and the use of
functions written in Emacs Lisp.  The difference is irrelevant.  I
mention the distinction only because it is interesting to know.  Indeed,
unless you investigate, you won't know whether an already-written
function is written in Emacs Lisp or C.