RE: Prototyping printf()

["Sims, Ken" <KSIMS@xxxxxxxxxxxxxxxx>]

Hi Eric, et.al. -

>Date: Fri, 29 Sep 2000 20:35:15 -0400
>From: "Sims, Ken" <KSIMS@SOUTHERNWINE.com>
>Subject: RE: Prototyping printf()
>
>Hi Eric -
>
>>Ah I beg to differ with you. Try this, declare a data structure and
>>make it have one sub field of a pointer data type. Then make a second
>>data structure just like the first. Then assign the address of the
>>pointer in the first data structure to your favorite variable. Then
>>eval DS2 = DS1 and low and behold the pointer in DS2 is valid :-)
>
>Right, but there you are moving the tags and all.  Try this (I'd try it
>before sending this but I've been doing training all day and it's past time
>to go home).
>
>Have a pointer in a data structure.  Overlay the pointer with a character
>field.
>
>Create a second data structure with the same kind of arrangement.
>       
>Set the pointer in the first data structure.
>
>Move the CHARACTER field in the first data structure to the CHARACTER field
>in the second data structure.
>
>Check the pointer in the second data structure.  I will be EXTREMELY
>surprised if it is valid.

I tried what I suggested above and I am extremely surprised.  The pointer
remained valid being copied via the overlaying character fields.

The rules appear to be that you can copy a pointer as a character string so
long as you copy the entire pointer and at all points it is on a 16-byte
boundary so that the tag bits can be set properly.  You can copy additional
characters besides the pointer, but you cannot copy the pointer in pieces,
nor can you copy it to an intermediate character field that is not aligned
on a 16-byte boundary.  If you do, the pointer becomes invalid.

So getting back to the original premise: copying pointers to a string then
back to pointers.  It appears that it would work if at all points you could
guarantee that the pointers would always be intact and always on a 16-byte
boundary.

Ken
Southern Wine and Spirits of Nevada, Inc.
Opinions expressed are my own and do not necessarily represent the views of
my employer or anyone in their right mind.

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