Re: RFFR

When designing data structures, it helps to put the largest ones first,
padding where necessary to keep natural alignment, and assuming that
pointers are 8 bytes.  If you do that, you can greatly reduce the
alignment problems.  This is a problem on Alpha processors also.
On Thu, 2004-01-22 at 12:03, spacewalker wrote:
> Note that on sparc processors, packed structures are a problem. I
> remember a long time ago having a program crashing using pcap or libnet
> code (i don't remember which the problem was from). 
> the problem lied in a "long" nonaligned to 4 bytes, placed in a
> structure, and then deferenced. I believed it to be a compiler problem
> (which should detect nonaligned access).
> 
> hope this will help
> On Thu, 22 Jan 2004 13:18:13 +0100
> "Alfredo Andres Omella" <[email protected]> wrote:
> 
> > 
> > IMHO is very important to fix data structures not aligned in memory.
> > 
> > On Solaris8 SPARC an Linux RedHat 7.2 (2.4.7-10) "sizeof(struct
> > libnet_stp_conf_hdr)" is 40 bytes and must be 35 bytes.
> > 
> > Fortunately libnet_stp_conf_hdr is not used within the libnet code but
> > is susceptible to be used by users (hope nobody do it) :).
> > 
> > Do you think is really necessary (the structure, i mean)? :?
> > 
> > If you prefere to keep up with the structure one solution could be to
> > use"__attribute__((packed))" but this would face 2 problems:
> > a) Any user will be forced to use gcc (some people use the Sun
> > compiler, not compatible with "attribute" extensions.)
> > b) Packing structures will penalize the libnet performance.
> > 
> > 
> > Waiting for answers...
> > 
> > 
> > Alfredo
> > 
> > 
> > 
> > 
-- 
    Lawrence MacIntyre     865.574.8696     [email protected]
               Oak Ridge National Laboratory
High Performance Information Infrastructure Technology Group

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