[Date Prev][Date Next][Thread Prev][Thread Next]
- Subject: Re: Getting the callers environment in c
- From: Sean Conner <sean@...>
- Date: Tue, 8 Mar 2011 18:53:12 -0500
It was thus said that the Great Sam Roberts once stated:
> On Tue, Mar 8, 2011 at 11:30 AM, liam mail <firstname.lastname@example.org> wrote:
> > Just a note to say that a function pointer and void pointer have
> > no guarantees to be the same size.
> If that is an issue for the OP, don't use lightuserdata, an alternate
> technique is to use the fn ptr value encoded as a hex string.
> While ANSI allows sizeof(void (*)()) to be > sizeof(void*), its pretty
> unusual (I've never seen it).
You do have to go back to the MS-DOS days (say, mid-80s, early 90s) for
this to be true. I don't recall the name of the model, but it wouldn't
surprise me (because of the nature of the 8088 (and 80286)  used in PCs if
there wasn't a case where a function pointer was larger than a data pointer.
Let's see ...
tiny code, data, stack and heap all fit in one 64k segment
small code in one 64k segment, data, heap and stack in another 64k
??? code in multiple segments, data, heap, stack in one segment
??? code in one 64k segment, data, heap and stack taking
huge code in multiple segments, data in multiple segments, heap
in multiple segments, stack (possibly) in multiple segments
So, depending upon the memory model used by a C compiler on an MS-DOS
system, a void * could be smaller, the same size, or larger than a function
pointer. If you ever come across old C code with stuff like "NEAR" and
"FAR" associated with pointers, it comes from this era.
-spc (Glad to be past that point in time ... )
 The 8086/8088/80186/80286 used a segmented architecture where each
segment was (up to) 64k in size, and there were four segments in use
at any one time, a code segment (CS), a data segment (DS), a stack
segment (SS), and an "extra" segment (ES). Segments could be the
same (CS = DS = ES = SS) or all separate. A program could comprise
more than just four segments; it's just that only a max of four
would be "visible" at any given moment.