🚀 go-pugleaf

[] Regarding Re: Aligning an arbitrary pointer?;
[] barmar@think.com (Barry Margolin) writes:

    bm> In article <PDS.92Jul18154500@lemming.webo.dg.com>
    bm> pds@lemming.webo.dg.com (Paul D. Smith) writes:

    >I'm modifying an existing piece of code which allocates a huge
    >chunk of memory then segments it into various parts, one of which
    >is an array of structs (it's a page table).

    bm> On some systems, casting a pointer to a more restrictive type
    bm> may have the effect of realigning it. [...]

Bleah :).  Maybe true, but certainly not Standard practice.  I would
expect that on at least some systems the compiler will happily allow
you to assign 0x01381 (or some other non-aligned value) to an int*,
say, and then just core with a bus error (or otherwise die) when you
tried to dereference the pointer (that's pretty much what happens on
my RISC system :-)

In fact I'd much rather it did that than quietly modify my pointer by
dropping bits... it would be a *major* pain to find *that* bug! :-)

    >For various reasons I don't want to introduce massive changes;
    >the obvious way to do this properly is allocate the memory
    >individually so I know it's properly aligned, but I don't want to
    >change things that much.  I can't think of *any* other way to do
    >it, actually.

    bm> Why not just declare a structure that contains the array and
    bm> the preceding stuff?  The compiler will guarantee that the
    bm> array is suitably aligned.

The preceding stuff is of dynamic size; it's read in from a file where
the file contains the total size and interesting offsets.  The page
table size has to be dynamic as well.

Anyway, I guess the consensus is that you can't do this in standard C,
so that's all I wanted to know!  Thanks all...
--

                                                                paul
-----
 ------------------------------------------------------------------
| Paul D. Smith                          |    paul_smith@dg.com    |
| Data General Corp.                     | pds@lemming.webo.dg.com |
| Network Systems Development Division   |                         |
| Open Network Systems Development       |   "Pretty Damn S..."    |
 ------------------------------------------------------------------