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Re: valid_pointer_p

From: Kim F. Storm
Subject: Re: valid_pointer_p
Date: Sat, 12 Aug 2006 22:19:50 +0200
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.0.50 (gnu/linux)

Eli Zaretskii <address@hidden> writes:

>> From: Andreas Schwab <address@hidden>
>> Cc: address@hidden (Kim F. Storm), address@hidden
>> Date: Sat, 12 Aug 2006 14:36:22 +0200
>> Eli Zaretskii <address@hidden> writes:
>> > ??? Don't we have macros, like INTEGERP, SUBRP, etc. to do that
>> > without dereferencing?  The length of the primitive Lisp types is
>> > known, right?
>> Try to find out the size of a vector object.
>>     /* Vector of Lisp objects, or something resembling it.
>>        XVECTOR (object) points to a struct Lisp_Vector, which contains
>>        the size and contents.  The size field also contains the type
>>        information, if it's not a real vector object.  */
>>     Lisp_Vectorlike,
> I think something like this should work for such cases:
>       struct Lisp_Vector *p = XVECTOR (obj);
>       if (valid_pointer_p (p, offsetof (struct Lisp_Vector, size)
>                                     + sizeof (EMACS_INT))
>                 && valid_pointer_p (p, p->size))
>       return 1;  /* valid */
> (Modulo the complications of storing other information in the leading
> bits of the `size' field of struct Lisp_Vector.)  This is what I meant
> when I wrote:
>     Even if you are right, dereferencing a pointer accesses a region in
>     memory whose length is known in advance, so at most we will need to
>     call valid_pointer_p twice.
> Am I missing something?

No.  My simple approach was simply to assume that if you can access the
size field of a vector, you can access the rest as well.  I don't mind
making a more correct implementation, although I didn't see the need --
after all, this is for a corner case of debugging emacs.

Kim F. Storm <address@hidden> http://www.cua.dk

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