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Groups > comp.lang.c > #16133

Re: partial struct usage

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Richard Damon <news.x.richarddamon@xoxy.net> writes:

> On 9/24/11 1:23 PM, Mark Adler wrote:
>> I have an internal structure with lots of stuff in it for controlling a
>> data input operation. E.g.:
>>
>> struct data {
>> int have;
>> char *next;
>> ... lots more stuff to keep track of the state
>> };
>>
>> I would like to expose to the user (in the interface header file) only
>> those first two elements, so that I can provide a macro (also in the
>> header file) analogous to getc() to pull data that is readily available
>> in the buffer, otherwise to call a function to go get more data. But I
>> want to hide the other stuff in the structure, in order to make sure
>> that users of the library don't end up depending on things that I change
>> later for internal use. And for efficiency I don't want yet another
>> layer of indirection pointing to another structure from within the
>> exposed structure for the other stuff.
>>
>> E.g.
>>
>> struct ex {
>> int have;
>> char *next;
>> };
>>
>> #define getd(d) (((struct ex *)d)->have ? (((struct ex *)d)->have--,
>> *(((struct ex *)d)->next)++) : getdf(d))
>>
>> where d is a struct data *.
>>
>> My question is: does the C standard provide an assurance that this will
>> work? I.e. that two different structures with identical sets of prefix
>> elements will have those prefix elements layed out identically in memory
>> so that casting one structure to the other will permit correct access to
>> those prefix elements?
>>
>> Mark
>>
>
> If you look at the specifications for unions, you will see a
> requirement for access compatibility for structs with equivalent
> initial sequences. While it technically means it only applies in
> unions, to make it work for unions but not elsewhere is hard, so
> unless you are using a "debug" version of the compiler checking for
> this sort of "error", it should work.  [snip unrelated]

Dangerous advice.  As Eric Sosman correctly points out, identical
layout is not enough in the presence of aggressive (and allowed
under the Standard) optimization.

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partial struct usage Mark Adler <madler@alumni.caltech.edu> - 2011-09-24 10:23 -0700
  Re: partial struct usage Richard Damon <news.x.richarddamon@xoxy.net> - 2011-09-24 13:44 -0400
    Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 14:08 -0800
  Re: partial struct usage BGB <cr88192@hotmail.com> - 2011-09-24 11:08 -0700
  Re: partial struct usage Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-09-24 23:36 +0300
    Re: partial struct usage Mark Adler <madler@alumni.caltech.edu> - 2011-09-25 13:23 -0700
      Re: partial struct usage Gene <gene.ressler@gmail.com> - 2011-09-26 01:14 -0700
        Re: partial struct usage Richard Damon <news.x.richarddamon@xoxy.net> - 2011-09-26 12:37 -0400
          Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 16:04 -0800
        Re: partial struct usage Mark Adler <madler@alumni.caltech.edu> - 2011-09-26 18:52 -0700
          Re: partial struct usage Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-27 12:14 +0100
    Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 15:56 -0800
      Re: partial struct usage Shao Miller <sha0.miller@gmail.com> - 2012-01-24 23:30 -0500
        Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 16:11 -0800
          Re: partial struct usage Shao Miller <sha0.miller@gmail.com> - 2012-02-01 00:42 -0500
            Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 09:33 -0800
      Re: partial struct usage Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-12 02:10 +0200
        Re: partial struct usage Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 09:38 -0800
          Re: partial struct usage Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-03-13 18:22 +0200
            Re: partial struct usage blp@cs.stanford.edu (Ben Pfaff) - 2012-03-13 09:55 -0700
  Re: partial struct usage Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-24 17:56 -0400

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