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From: Eli Zaretskii <eliz@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Fcall_process: wrong conversion
Date: Fri, 19 May 2006 13:41:59 +0300
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> From: "Herbert Euler" <herberteuler@hotmail.com>
> Date: Fri, 19 May 2006 10:49:33 +0800
> Cc: emacs-devel@gnu.org
> 
> I'm sorry but how does Emacs decide whether a string
> is unibyte?

See multibyte-string-p.  It boils down to this macro from lisp.h:

    /* Nonzero if STR is a multibyte string.  */
    #define STRING_MULTIBYTE(STR)  \
      (XSTRING (STR)->size_byte >= 0)

In other words, this information is recorded in the string object
itself.