Re: What is the right way to do this simple thing

From	cri@tiac.net (Richard Harter)
Newsgroups	comp.lang.c
Subject	Re: What is the right way to do this simple thing
Date	2012-03-11 16:14 +0000
Message-ID	<4f5ccd50.2020500644@text.giganews.com> (permalink)
References	<4f5ba2ed.1944110152@text.giganews.com> <MPG.29c60b46c869e137989693@news.eternal-september.org>

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On Sun, 11 Mar 2012 01:07:53 -0800, Philip Lantz <prl@canterey.us>
wrote:

>Richard Harter wrote:
>> 
>> Suppose that I have some data that is pointer free.  It isn't
>> necessarily serialized in some format such as JSON but I would like to
>> be able to copy it from one from one machine to another compatible
>> machine and use it without trouble.  Also I don't want to pass around
>> a collection of items and I want everything to be properly aligned.
>> 
>> Naturally I think of using the struct hack and a structure something
>> like this:
>> 
>>    struct package {
>>        message_type type;       /* The type has access info    */
>>        size_t       total_len;  /* Total length of the package */
>>        size_t       data_len;   /* Length of the data          */
>>        char[1]      data;       /* The actual data             */
>>    };
>> 
>> Presumably at some point I will copy said data into an instance of the
>> package which can then be passed around.  Also I really would like to
>> be able to use the data upon receipt without copying it out, e.g.,
>> 
>>    local_data = (data_type *)(pkg->data);
>> 
>> It occurs to me that this might not work there is no guarantee that
>> pkg->data is correctly aligned.  There are a whole bunch of other
>> potential gotchas that I can foresee.  This is particularly troubling
>> to me because my PC has four cores, each with a different DS9000 chip.
>> What's worse my boss is on a "compiler of the week" binge.  Every week
>> we get to test a new compiler.  I really want to get this code
>> bullet-proof.
>> 
>> Help!  What is the right way to do this simple thing?
>
>
>I'm not sure why you're only concerned about the alignment of pkg->data. 
>When passing a structure to a different processor, you also have to be 
>concerned about the size, alignment, padding, and semantics of every 
>field within the structure. The whole idea of passing a struct is 
>futile. You need to convert the struct to a processor-independent byte 
>stream, and convert the byte stream back to a struct on the receiving 
>end.

Did you see the word "compatible"?  Clearly if the processors are not
compatible, i.e., do not have the same size etc then, yes, you need a
processor-independent byte stream.  However you can also have multiple
processors that are the same, multiple cores, and multiple "processes"
running within a single processor.  If you are doing cheap message
passing the cost of serializing/de-serializing is high overhead.

Thread

What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-10 19:26 +0000
  Re: What is the right way to do this simple thing Don Y <this@isnotme.com> - 2012-03-10 12:57 -0700
  Re: What is the right way to do this simple thing ImpalerCore <jadill33@gmail.com> - 2012-03-10 14:12 -0800
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-10 23:31 +0000
      Re: What is the right way to do this simple thing Ian Collins <ian-news@hotmail.com> - 2012-03-11 13:06 +1300
        Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 00:44 +0000
          Re: What is the right way to do this simple thing Ian Collins <ian-news@hotmail.com> - 2012-03-11 14:15 +1300
            Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 04:46 +0000
              Re: What is the right way to do this simple thing Ian Collins <ian-news@hotmail.com> - 2012-03-11 21:27 +1300
                Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 17:23 +0000
                Re: What is the right way to do this simple thing Ian Collins <ian-news@hotmail.com> - 2012-03-12 10:01 +1300
                Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 21:18 +0000
                Re: What is the right way to do this simple thing Ian Collins <ian-news@hotmail.com> - 2012-03-14 10:23 +1300
              Re: What is the right way to do this simple thing Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-03-11 11:59 +0000
                Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 16:51 +0000
                Re: What is the right way to do this simple thing Gene <gene.ressler@gmail.com> - 2012-03-13 07:32 -0700
                Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 21:46 +0000
      Re: What is the right way to do this simple thing ImpalerCore <jadill33@gmail.com> - 2012-03-12 07:28 -0700
        Re: What is the right way to do this simple thing ImpalerCore <jadill33@gmail.com> - 2012-03-12 08:13 -0700
        Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 21:51 +0000
  Re: What is the right way to do this simple thing Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-10 14:21 -0800
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 01:40 +0000
      Re: What is the right way to do this simple thing Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-11 14:11 -0700
        Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 21:56 +0000
          Re: What is the right way to do this simple thing Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-14 14:55 -0700
            Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-15 21:55 +0000
              Re: What is the right way to do this simple thing Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-19 13:33 -0700
  Re: What is the right way to do this simple thing Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-03-10 15:21 -0800
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 00:47 +0000
  Re: What is the right way to do this simple thing Philip Lantz <prl@canterey.us> - 2012-03-11 01:07 -0800
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 16:14 +0000
  Re: What is the right way to do this simple thing Markus Wichmann <nullplan@gmx.net> - 2012-03-11 13:47 +0100
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 16:24 +0000
  Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 21:01 +0000
    Re: What is the right way to do this simple thing Anders Wegge Keller <wegge@wegge.dk> - 2012-03-12 00:05 +0100
      Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 21:57 +0000
    Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-11 23:06 +0000
      Re: What is the right way to do this simple thing Geoff <geoff@invalid.invalid> - 2012-03-11 17:34 -0700
        Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-13 22:00 +0000
          Re: What is the right way to do this simple thing Geoff <geoff@invalid.invalid> - 2012-03-13 18:12 -0700
            Re: What is the right way to do this simple thing cri@tiac.net (Richard Harter) - 2012-03-14 03:38 +0000
              Re: What is the right way to do this simple thing Geoff <geoff@invalid.invalid> - 2012-03-14 00:49 -0700

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