Re: How to get buffer byte length (not number of characters)?

[Eli Zaretskii <eliz@gnu.org>]

> From: Joseph Turner <joseph@ushin.org>
> Date: Tue, 20 Aug 2024 00:10:50 -0700
> 
> Hello!
> 
> `buffer-size' returns the number of characters in a buffer:
> 
> (with-temp-buffer
>   (insert "你好")
>   (buffer-size)) ;; 2
> 
> However, the buffer's byte length may be larger:
> 
> (let* ((filename (make-temp-file "buffer-size-test-"))
>        (file (with-temp-file filename (insert "你好"))))
>   (file-attribute-size (file-attributes filename))) ;; 6
> 
> How can I get a buffer's byte length without writing to a file?

This depends on why do you need the byte length of the buffer.

If I interpret your question literally, then this is the answer:

  (position-bytes (point-max))

perhaps preceded by a call to 'widen'.  But that returns the number of
bytes that the buffer's characters take when represented in the
internal Emacs representation of characters, which is not necessarily
useful to Lisp programs.  For example, if you need to know how many
bytes will Emacs write to a file if you save the buffer, or to a
network connection or a sub-process if you send the buffer there, then
you need to consider the encoding process: Emacs always encodes the
buffer text on output to the external world.  If this is what you
want, then you need to use bufferpos-to-filepos, and make sure you
pass the correct coding-system argument to it.

If you need this for something else, please tell the details.