From: Troy Hinckley <troyhinckley@gmail.com>
To: emacs-devel@gnu.org, Matt Armstrong <matt@rfc20.org>
Subject: Re: question about Qnil representation
Date: Wed, 26 Oct 2022 10:22:54 -0600 [thread overview]
Message-ID: <d48212c3-9688-4a0a-8b1a-401b9ab43a2c@Spark> (raw)
In-Reply-To: <87ilk6a7be.fsf@rfc20.org>
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Clearly I am doing a poor job explaining my question.
Let me try again. As I understand it, every Symbol is an instance if Lisp_Symbol. The Lisp_Object is an integer that can represent multiple types with tags defined by Lisp_Type. If the tag is 0, then the type is a symbol. In that case you would treat the rest of the bytes in the Lisp_Object as pointer to a Lisp_Symbol instance. However if the symbol nil is defined as 0, then it is not a pointer to anything. How then do you access that symbol? Is there special case code to handle the null pointer?
I hope that question makes more sense. Let me know where my understanding is wrong.
On Oct 26, 2022, 10:16 AM -0600, Matt Armstrong <matt@rfc20.org>, wrote:
> Troy Hinckley <troyhinckley@gmail.com> writes:
>
> > but 0 would not be a valid pointer.
>
> I am a confused by this. 0 is a valid value for a C pointer. This is
> what NULL is, etc. You must not dereference it, but it is a valid
> value.
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next prev parent reply other threads:[~2022-10-26 16:22 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <5a560265-fa81-42d2-945b-ab85d72df9e6@Spark>
2022-10-26 15:40 ` question about Qnil representation Troy Hinckley
2022-10-26 16:00 ` Eli Zaretskii
2022-10-26 16:04 ` Troy Hinckley
2022-10-26 16:09 ` Eli Zaretskii
2022-10-27 0:45 ` Po Lu
2022-10-26 16:16 ` Matt Armstrong
2022-10-26 16:22 ` Troy Hinckley [this message]
2022-10-26 16:45 ` Eli Zaretskii
2022-10-26 18:46 ` Troy Hinckley
2022-10-26 19:01 ` Stefan Monnier
2022-10-26 19:03 ` Eli Zaretskii
2022-10-27 0:47 ` Po Lu
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