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Re: Structure packing & padding: what sizeof reports? Any caveats?

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Richard Heathfield <rjh@cpax.org.uk> writes:

> On 08/08/16 08:15, Test wrote:
>> I am saving structures and variables to a string. For example:
>> ..
>> struct MixedData
>> {
>>     char Data1;
>>     int num1;
>>     short num2;
>> };
<snip>
>> char mystr[256];
>> struct MixedData whatever;
<snip>
>> memcpy(&mystr[4],&whatever,sizeof(struct MixedData));
<snip>
>> memcpy(&whatever,&mystr[4],sizeof(struct MixedData));

To the OP: I find

  memcpy(dest, &whatever, sizeof whatever);

to be more obviously correct.  There is no need to check that the type
used in the sizeof is that of declared object 'whatever'.  It's a case
of directly writing what you mean rather than something that works.

>> Is above OK
>
> It's horrible, and it isn't guaranteed to work. Your hard-coded 4 is
> one of your problems, since it assumes that there are exactly three
> padding bytes after the Data1 member.

I don't think it does.  &mystr[4] is simply where Test wants to put the
data.  Yes, the 4 is still a hard-coded number, and that might be a
problem for other reasons, but I don't think it's got anything to do
with padding assumptions.

<snip>
-- 
Ben.

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Thread

Structure packing & padding: what sizeof reports? Any caveats? Test <test@.nil.invalid.com> - 2016-08-08 10:15 +0300
  Re: Structure packing & padding: what sizeof reports? Any caveats? Ian Collins <ian-news@hotmail.com> - 2016-08-08 19:36 +1200
    Re: Structure packing & padding: what sizeof reports? Any caveats? Richard Heathfield <rjh@cpax.org.uk> - 2016-08-08 08:52 +0100
  Re: Structure packing & padding: what sizeof reports? Any caveats? Richard Heathfield <rjh@cpax.org.uk> - 2016-08-08 08:51 +0100
    Re: Structure packing & padding: what sizeof reports? Any caveats? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-08-08 11:29 +0100
    Re: Structure packing & padding: what sizeof reports? Any caveats? Keith Thompson <kst-u@mib.org> - 2016-08-08 08:56 -0700
  Re: Structure packing & padding: what sizeof reports? Any caveats? Barry Schwarz <schwarzb@dqel.com> - 2016-08-08 01:37 -0700
  Re: Structure packing & padding: what sizeof reports? Any caveats? James Kuyper <jameskuyper@verizon.net> - 2016-08-08 11:17 -0400

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